Invited contribution to the review book 'Vacuum Science & Technology', Volume 'Three-dimensional structurisation technology
for semiconductor based heterostructure devices and circuitry', 2002, Research Signpost, Kerala, India
Multiple airgap membrane structures
and buried mushroom stripe lasers –
Fabrication by dry and subsequent wet etching
J. Daleiden and H. Hillmer
Institute of Microstructure Technologies and Analytics,
Heinrich-Plett-Strasse 40, D-34132 Kassel, Germany,
email: email@example.com, firstname.lastname@example.org
Real three-dimensional (3D) material structures During the last years miniaturization of micro-
enable enormous perspectives in the functionality electronic circuits has increased exponentially as a
of advanced electronic and optoelectronic III/V function of time. G. Moore based this trend in
semiconductor devices. We review typical fabrica- 1965 on just 5 data pairs. In a semi-logarithmic
tion philosophies and fundamental technological plot this trend is linear and well-known as Moore’s
processes of surface-micromachined III/V semi- law, which still holds in our days. This trend can
conductor devices. Main application fields of be formulated as a function of time for different
compound semiconductor based micromachining properties: such as the number of transistor func-
are demarked from those of Si systems. Consider- tions per chip, or the degree of miniaturization, or
ing fabrication technology, the general principles the calculation speed, or the storage capacity, or it
can be reduced to three fundamental process steps: can be simply formulated in a single covering ex-
deposition of a layered heterostructure on a sub- pression: the complexity. Concerning the rele-
strate, vertical structurization and horizontal un- vance of this trend as a measure for the develop-
dercut by selectively removing sacrificial layers. ment of our information age, it is interesting to
Very useful quality control elements for precise notice that the complexity of micro-optics and mi-
process control are presented. The basic principles crosystem technology has been observed to grow
are applied and illustrated in detail presenting two faster, i.e. superlinear, in a semi-logarithmic dia-
selected optoelectronic examples: (i) fabrication of gram. Compared to micro-electronics and micro-
bent waveguide buried mushroom stripe lasers and optics, microsystem technology is a relatively new
(ii) vertical cavity based tunable single or multi- field, describing a combination of at least two of
membrane devices including airgaps. Finally the the three disciplines micro-electronics, micro-
potential of micromachined III/V compound semi- optics and micro-mechanics.
conductor devices are underlined by some selected
experimental results. Our chapter is devoted to the combination of all
the three disciplines: micro-opto-electro-
mechanical systems (MOEMS). In microsystem
technology, Si is the by far dominating material branes and tooth wheels allow precisely controlled
class. However, it cannot be efficiently used in motions and deformations. Complicated combina-
monolithic systems including active light emitting tions of these basic elements have been shown in
devices, since the radiative recombination rate for Si and allow very complex motions (e.g. Si comb
pure Si is by four orders of magnitude smaller than actuator driven external cavity devices [1,2], digi-
the rate for III/V compound semiconductor materi- tal micromirror devices , high duty factor mi-
als such as GaAs, InP and related ternary, quater- cromechanical pendants of oscillating LC circuits
nary and pentanary compounds. , various free beam optical components [5-13],
microgrippers [14-16], cantilever tips  and nxn
Favorizing monolithic or hybride systems for matrix switches [18-20]. These structures play key
MOEMS? This interesting question is still under roles for micromachined Si sensors and actuators.
intense discussion. Several concepts based on sur- The success of Si is mainly due to a few reasons:
face micromachining, bulk micromachining and crystalline Si has excellent mechanical properties
hybride systems have been successfully demon- and is the cheapest semiconductor material, poly-
strated in the past. Our chapter is devoted to fun- Si and natural Si oxide combine fantastic material
damental fabrication principles of surface micro- properties and extremely low fabrication costs. In
machining for monolithic systems using III/V comparison, the technological difficulties are by
compound semiconductors. We implement the far extended for III/V or II/VI semiconductor sys-
optoelectronic components within a single semi- tems and successful pendants of poly-Si and natu-
conductor material class and use batch processing ral Si oxide are not available for these compounds.
to avoid final micro mounting of individually im- Therefore, compound semiconductor micro-
plemented parts. We demonstrate fundamental machining is more expensive and by far less com-
process principles using two examples and two plex at the moment as Si based micromachining.
heterostructure systems: GaInAsP and AlGaInAs, However, micromachining using compound semi-
both on InP substrates. conductors has consequently and logically devel-
oped into another very successful direction making
In our chapter, the basic principles are applied and full use of the direct band structure and, thus, the
illustrated in detail presenting two selected opto- efficient light generation in III/V or II/VI semi-
electronic examples: first, constricted ridge mesa conductor heterostructures. In our opinion, direct
structures and second, micromachined membrane compound semiconductor systems will always be
based tunable filters. These photonic devices are by far superior to Si concerning radiative recombi-
very attractive for applications in ultra-high capac- nation efficiency. This will even hold, although the
ity 1.55 µm optical communication systems based radiative recombination efficiency can be en-
on dense wavelength division multiplex (DWDM). hanced in element semiconductors using: intrava-
lence band transitions, Brillouin zone foulding in
Si/Ge superlattices, quantum cascades, porous Si,
1. FOUNDATIONS OF SURFACE quantum size effects in Si nano crystals embedded
MICROMACHINING IN III/V in SiO2  or dislocation loops around Boron,
SEMICONDUCTOR ionimplanted in Si . III/V micromachining has
HETEROSTRUCTURES been successfully applied, e.g., in undercut ridge
mesa structures for high-speed edge emitting lasers
 or bent waveguide lasers [24-29]. Based on
cantilevers and suspended membranes tunable mi-
1.1 Material and geometric aspects
crocavity devices such as filters [30-44], vertical
cavity surface emitting lasers (VCSEL’s) [36,45-
Mesa, trenches, grooves, nozzles, pyramides,
51], LEDs  and photodetectors [53-56] using
curved surfaces as well as undercut ridges and
electrostatic or thermal actuation have been im-
stripes are static basic elements in micromachin-
plemented. Other material systems and techniques
ing. Further structures such as cantilevers, mem-
involving extremely short-wavelength light
sources [57,58] are also very attractive, however, ZnS, InAs and others may become important.
not subject of our chapter. Typical material systems for layers B and C are
Suspended membrane structures and cantilevers - AlzGa1-zAs, Ga1-xInxAsyP1-y,
are identified to be very attractive in micro system - AlzGa1-x-zInxAs, Ga1-xInxAs1-vNv
technology, however, this rises questions of me- - AlzGa1-x-zInxAsyP1-y, or AlzGa1-x-zInxAsuSb1-u .
chanical stability. Studying scaling properties of
fundamental physical properties underlines the
need of miniaturization for these concepts. We
studied physical aspects of the efficiency of elec-
trostatic actuation and the tremendous reduction of
the harmful influence of inertia and gravity forces
to avoid the fatigue of material and demonstrated C
this in some detail in Ref. . Summarizing, A
miniaturization plays a key role for mechanical x
stability and lifetime. vertical
1.2 Micromachined fabrication
The micromachined fabrication of these structures
and the general principles can be reduced to three
fundamental process steps. In the following this is horizontal
demonstrated for a three layer heterostructure (Fig.
1 a) consisting of the substrate A, layer B being
partly or entirely removed and finally the top layer
Step 1: Epitaxy or deposition of the individual lay-
ers (B, C, . . .) on the substrate A (Fig. 1 a): Fig. 1: Three fundamental process steps of surface micro-
In case of mixed material systems, the substrate
may be, e.g., silica glass or a semiconductor sub-
strate (Si, . . .). Polymers may be used for layer B Step 2: Vertical etching (Fig. 1 b):
and dielectrics or a metal for layer C. There exists
a large variety of combinations for mixed systems. Next, the layered structure (A, B, C) is photolitho-
Note that the three fundamental process steps are graphically masked. The uncovered areas are sub-
also characteristic for these systems. sequently dry etched to define the ridge mesa
Typical deposition technologies are for instance: structures. Depending on the process, slightly
evaporation, vapour phase deposition, plasma en- sloped sidewalls, vertical sidewalls or slightly un-
hanced chemical vapour deposition (PECVD), dercut sidewalls can be obtained. The vertical
plasma or magnetron sputtering. Typical epitaxial etching is shown schematically in Fig. 1 b can be
processes are for example: metal-organic vapour obtained, e.g., by gas chopping  or by carefully
phase epitaxy (MOVPE), hydride vapour phase balancing physical and chemical etching compo-
epitaxy, solid source molecular beam epitaxy nents in plasma etching [60-63]. Strongly angled
(MBE), metal-organic molecular beam epitaxy undercut structures can be obtained by pretilting
(MOMBE), gas source molecular beam epitaxy the wafers and subsequent wafer rotation during
GSMBE) and chemical beam epitaxy (CBE). etching. Preferentially an etching process is used
In the following we will emphasize pure semicon- having a very low etching rate of the mask and
ductor heterostructures. Typical substrates are high etching rates for materials B and C (high se-
GaAs and InP. However, also SiC, sapphire, GaSb, lectivity) and at the same time a low selectivity
concerning the etching rates of materials B and C. Note that the crystal orientation is crucial for the
In the schematic diagram the etching process is etching velocity in the xy-plane. Therefore, the
stopped just after reaching the interface between ridges, i.e. the masks, have to be properly oriented
materials A and B. However, in some cases also with respect to the crystalline directions.
the substrate is partly etched during the vertical
structurization step. This may occur intentionally 1.3 Crucial control of strain and of stress
or unintentionally. In the latter case no serious in-
fluence on the third step exists. We believe it is worth to mention that the expres-
sion “strain” is generally used for crystalline mate-
Step 3: Horizontal underetching (Fig. 1 c): rials to describe deviations in lattice constants
among the individual layers. Unstrained, compres-
In the last step, a highly selective wet-chemical sively strained, tensile strained or strain relaxed
etching is used to underetch layer C by a distance layers are used [64-67]. For amorphous het-
d. The relation of the ridge width W to the un- erostructure materials (amorphous dielectrica,
deretching depth d defines the final structure: amorphous Si (a-Si), polymers and others) the ex-
(i) If 2d≥W the central ridge is completely un- pression “stress” is used to describe mechanical
deretched. In this case layer B acts as a sacrifi- tensions. Note that in micromachining literature
cial layer and the remaining central bar of mate- “stress” is used, since that field is dominated by Si
rial C defines a cantilever (see Fig. 1 c). Note and in many cases cantilevers and membranes
that more complicated structures (e.g. sus- consist of a-Si. However for compound semicon-
pended membranes see Figs. 11 and 13) can ductor micromachining, the historic crystallo-
also be implemented, depending on the lateral graphic expression “strain” should be preferred.
structures of the used mask. Therefore, we use “strain” for strained semicon-
(ii) If 2d<W the central ridge is only partly un- ductor layers.
deretched. The remaining central bar of material
C and the remaining central part of material B In the case of amorphous materials, special care
define an undercut ridge mesa (e.g. Fig. 4 a). has to be taken either to avoid stress or to control
Such an undercut ridge mesa can also be ob- stress among the individual layers precisely. This
served in the left and right hand-side outer sec- guarantees in the first case the desired flat cantile-
tions (Fig. 1c). vers or membranes. In the second case this defines
(iii) If 2d is considerably less than the lateral di- the desired bendings.
mensions of the larger mesa, support posts or
ridge posts can be fabricated (see , e.g., Fig. 13 However, also for crystalline semiconductor het-
a). erostructures, in some cases the finally released
layer (here layer C) should include no strain gradi-
Thus, layer B acts as a sacrificial layer in case (i). ents. If unbent cantilevers or membranes are de-
In case (ii) layer B is only partly underetched. In sired prior to actuation, a good choice is a precise
the case of constricted mesa structures, layer B lattice matching to the substrate. If an additional
includes the waveguiding confinement layers, the relaxed buffer layer is applied, lattice matching to
active quantum well (QW) layers and the barrier the last grown monolayers is preferred. However
layers. In this case, the precise final width of mate- also distinct bendings can be desired in other
rial B is crucial for the proper lateral single- or cases. This also requires precise control of total
multi-mode operation of constricted mesa strain and respective strain gradients in the indi-
waveguide laser devices. vidual layers.
2. PROCESS CONTROL USING
QUALITY CONTROL STRUCTURES
E 2d c
σ grad = δ (1)
In the following we have selected typical process 1 −ν l 2
and quality control elements which are batch fab-
ricated parallel to the micromachined structures. where σgrad is the internal stress gradient, dc the
Quality control elements which allow nondestruc- thickness of the layer C, E the elastic modulus of
tive observation and quantitative results, e.g. by the beam, ν Poisson’s ratio, and l the length of the
light microscopy or white light interferometry are cantilever and δ the deflection of the cantilever tip.
preferentially used, since cleaving of the wafers
can be avoided, which decreases yield and in- C
creases production costs. Two main points of in- B
terest are isolated in our chapter: (i) the control of A
strain and stress and (ii) the precise control of lat- δ
2.1 Strain and stress control indicator elements:
Precise control of the strain and the stress in sus- x
pended layers is essential for the successful fabri-
Fig. 2: Schema of cantilever test structures used to measure
cation of micromachined devices. Generally the the internal stain/stress gradient of surface micromachined
overall stress in a layer can be described by a gra- layers (C).
dient part σgrad and a homogeneous part σhom. The
internal stress gradient is the variation of the lat-
eral tension (xy-plane) in z-direction. For instance, 2.1.2 Sensitively rotatable pointer structures –
unintentional variations of composition parameters homogeneous strain/stress
(in z-direction) could be the reason for such a gra-
dient. The homogeneous stress is a volume prop- For the characterization of the homogeneous
erty. Homogeneous or gradient stress (Pa) cause strain/stress, the structure shown in Fig. 3 has been
elongation or contraction of the layers, described used. It consists of two essentially symmetrically
by the strain value (ppm). arranged parts, each indicating two actuator beams
and one indicator beam. One end of each actuator
2.1.1 Multiple-length cantilever structures – beam is fixed to the substrate by a support post and
internal strain/stress gradients the other is connected to the indicator by a narrow
Figure 2 shows quality control structures used to
measure the internal strain/stress gradients of the Y
films consisting of a set of suspended cantilevers z
fixed to the substrate at one end (reference plate). x
After etching the sacrificial layer (layer B accord-
ing to Figs. 1 and Fig 2) the cantilevers (layer C) actuator
are released. An internal strain/stress gradient will
deflect the cantilever beam tip and the deflection Fig. 3: Rotatable pointer structures used to characterize
can be measured as the height of the tip above the the homogeneous strain/stress of surface micromachined
reference plate top level. The stress gradient σgrad
can be calculated from the cantilever tip deflection
δ according to the following formula :
When the actuator beams are released by removing
the sacrificial layer wet-chemically, a homogene-
ous stress causes elongation or contraction. The ridges. In Figure 4 b, the underetching just reaches
indicator beams are not directly touching each d*, hence, the mesa of smaller widths W2 is now
other, but are slightly separated. In the case of un- completely underetched and breaks down. The
strained layers the ends of the two indicator beams wider mesa ridges are still stable. This can be eas-
are facing each other perfectly without any lateral ily observed by optical microscopy from the top
deflection. With increasing strain the lateral de- and is the signal either to stop the wet-chemical
flection grows and, thus, represents a quantitative underetching by deionized water rinsing or to pre-
parameter to measure homogeneous strain. This is cisely look via IR transmission microscopy for the
due to a torque deflecting the indicators. The mag- final desired undercut 2d. In cases, IR microscopy
nitude of the deflection corresponds to the magni- is not used d*=d may be chosen. In summary, 2
tude of the internal homogeneous strain/stress, and simple analytical process control steps: mixing of
its direction corresponds to the type of at least two different wide mesa ridges, differing in
strain/stress, i.e. tensile or compressive width by 2d* combined with IR transmission mi-
strain/stress. A tensile homogeneous strain/stress croscopy allows precise process and quality con-
causes a contraction of the actuator beams. Conse- trol of the above mentioned third micromachining
quently the indicators rotate clockwise. Compres- step.
sive strain/stress forces the indicator to rotate
counter-clockwise. The sensitivity is doubled by
placing pairs of elements opposite to each other. (a)
The homogeneous stress can be determined ac- z
cording to the formula :
W1 W1 W1 W2
σ hom = δ (2)
1 − ν 2lali
where σhom is the internal homogeneous stress, E
the elastic modulus, ν Poisson's ratio, s the dis-
tance between the anchor of the actuator beam and d* d*
the centre of rotation, la the length of the actuator
beam, li the length of the indicator beam, and δ the Fig. 4: Principles of quantitative undercut control by mix-
deflection. ing mesa ridges of different widths Wi in micromachined
2.2 Etch-stop indicator elements for lateral
For telecommunication lasers, stable single-mode
oscillation in lateral direction (y-direction) is
2.2.1 Mixing of mesa ridges of different widths
needed which requires, e.g. a final width W-2d of
about 1.6 µm for 1.55 µm emission wavelength.
For many applications mesa ridges of width W1
Exact values of W-2d depend on the waveguide
have to be undercut by 2d to precisely maintain a
geometry and refractive indices. Thus, a key proc-
residual width W1-2d of layer B. Here we review a
ess is the precisely defined selective wet-chemical
very simple procedure to enable an interruption of
underetching of the mesa at the position of the
the selective wet-chemical underetching when
quaternary material layers (see part d of Fig. 7).
reaching an undercut of 2d* (with 2d* slightly
This is obtained using batch-processing of some
smaller than 2d). Among the mesa ridges of width
5 µm wide mesa ridges among the majority of
W1 a few mesa ridges of width W2 = W1-2d* are
7 µm wide ridges for coarse indicating the passing
positioned (Fig. 2 a). In Figure 4 a, the underetch-
of underetching 2d*. The fine indication to pre-
ing is well below d*, hence, no complete un-
cisely approach 2d is controlled, e.g., by IR mi-
deretching is reached for both types of mesa
For suspended membranes it is crucial to know the (a)
exact moment when the membranes just reach en- l
tire underetching. This is very important for free
motion of the membranes. If unstrained mem-
branes are applied and no bending and buckling of
the membranes can be observed by, e.g., white w
light interferometry, after entire underetching and
full release of the membranes, the above described (b)
indicator elements are very welcome. In that case,
the widths of the mesa ridge is adjusted to the re-
spective diameter of the membrane.
2.2.2 Multiple-width cantilever structures
Accurate control of the underetching as a function Fig. 5: Schema of cantilever structures with different
of time can be achieved by using several cantile- widths. The structures are used to control the underetching
depth d. Layer C shows a strong negative strain/stress gra-
vers in sequence of distinctly increasing widths. dient in (a) and a lower value in (b).
Figure 5 shows schematically a cantilever test-
structure for 4 different width w1, w2, w3 and w4.
In the example the cantilevers are underetched by
a distance d whereby w1 < w2 < 2d < w3 < w4. The 3. FABRICATION OF CONSTRICTED
bending of the two cantilevers w1 and w2 might be
RIDGE MESA STRUCTURES
observed by simple microscope inspection and the
upper (w3) and lower (w2) limit of d is determined.
Longitudinally single-mode oscillation is obtained
For high (negative) strain/stress gradients the
by distributed feedback (DFB) gratings including a
completely underetched cantilevers stick to the
phase shift of about λ/4. This phase shift is either
substrate promptly (Fig. 5a) whereas for low
abrupt and located close to the centre of the reso-
strain/stress gradients the cantilevers have to be
nator, or it is divided into two or three longitudi-
long enough so that the free cantilevers clearly
nally separated abrupt parts, or it is continuously
deviate from the attached cantilevers (Fig. 5b).
distributed in axial direction (chirped DFB grat-
Anisotropy can be monitored by differently rotated
ing). Figure 6 bottom schematically depicts a con-
cantilevers. This method is strongly related to the
tinuously phase shifted chirped DFB laser obtained
“undereching star” used as a very successfull qual-
by distinctly bent waveguides (Fig. 6, main dia-
ity control element in Si micromachining.
gram) [27, 29].
In the following the above mentioned foundations
are applied for two selected examples: (i) fabrica-
tion of buried constricted mesa lasers and (ii) ver-
The GaInAsP/InP semiconductor heterostructure
tical micro resonator based devices, capable of
layers were grown by MOVPE (i), the Al-
micromachined tuning. In both cases, the above
GaInAs/InP structures by MBE (ii).
mentioned process and quality control structures
(i) Using MOVPE, the following layers were
grown on p-InP substrates: a nominally undoped
150 nm InP buffer, the p-sided GaInAsP confine-
ment layers (50 nm up to 80 nm, 5x1017cm-3 p),
the strain-compensated GaInAsP multiple quantum
well (QW) active region (typically 10 QW’s) and
the n-sided GaInAsP confinement layers (50 nm DFB grating, waveguiding calculations were per-
up to 120 nm, 5x1017cm-3 also p-doped!). Then the formed to determine e.g. the DFB grating depth as
growth is stopped to define the DFB grating (grat- well as the distance between the grating and the
ing depth between 25 nm and 90 nm have been active layer (laser structure design). The coupling
implemented, enabling to vary the coupling coeffi- coefficients obtained by theoretical waveguiding
cient K). At this position, the pn-junction is located calculations agree very well with experimental
before starting the regrowth of the grating by values obtained from experimental spectra, which
MOVPE. 2 µm of 2x10 18cm-3 n-InP and 0.5 µm have been simulated by theoretical model calcula-
5x1018cm-3 n-InP (representing layers C) are epi- tions [69-71].
(ii) Using MBE, the following layers were grown
on p-InP substrates: a 150 nm lattice-matched Al-
GaAs buffer, the p-sided AlGaInAs confinement
layers (25 nm up to 80 nm, 5x1017cm-3 p), the ac-
tive strain-compensated AlGaInAs active multiple
quantum well (QW) active region (typically 10
QW’s) and the n-sided AlGaInAs confinement
layers (50 nm up to 120 nm, 5x1017cm-3 also p-
doped!). Then the growth is stopped to define the
DFB grating (grating depths between 20 nm and
60 nm have been implemented, enabling to vary
the coupling coefficient K). At this position, the
pn- junction is located before starting the regrowth
of the grating using MOVPE. 2 µm of 2x10 18cm-3
n-InP and 0.5 µm 5x10 18cm-3 n-InP (being com-
prised in layer C) are epitaxially grown. In some
cases the top layer is replaced by a 200 nm highly Fig. 6: Centre: Schematic view of a bent waveguide to ob-
Be-doped MBE grown n-GaInAs layer (1019cm-3). tain chirped DFB gratings. Top inset: waveguide parts ori-
ented perpendicular to the grating lines (ϑ=0 ) provide the
The number of QW’s and the thickness of the con- smallest grating periods Λ=Λx, and the largest angle occur-
finement layers have been considerably varied in ing in the bending ϑmax provides the largest effective grating
periods Λ=Λx/cosϑmax. Lower inset: cross sectional view of
different structures and new asymmetric structures the structure shown in the centre in axial z-direction. Since
have been applied to enhance the modulation the waveguide includes largest local angles ϑ(z) in the cen-
bandwidths . 26 GHz bandwidth have been tre of the waveguide, the effective grating periods conse-
obtained in our fastest structures, including quently show largest values there. The lower inset addition-
10 QW’s, short p-sided confinement layers ally includes the epitaxially regrown grating (layers C).
(25 nm) and longer n-sided confinement layers
(112 nm). To fabricate buried constricted mesa laser devices
or buried mushroom lasers, constricted ridge mesa
3.2 DFB grating definition and regrowth structures (Fig. 7 a ) have to be implemented. Fig-
ure 6 centre depicts the ridge of layers B (accord-
To define a first-order DFB grating in the top con- ing to Fig. 1). The mushroom cap layer (layer C) is
finement layer (corrugation pitch length close to not displayed, in order to show the DFB grating
Λ=232.5 nm) electron beam lithography and Ar/O2 and the bent waveguide. Note, that this structure is
ion milling are used. MOVPE is applied to regrow created by the undercut as described now and ex-
the grating by a 2 µm n-InP layer (Si, 5⋅1017cm-3) ists during no fabrication step without the cap
and to terminate the structure by a 0.5 µm n-InP layer C!
layer (Si, 2⋅1018cm-3). Prior to the definition of the
3.3 Vertical and horizontal etching The mesa is realized by N2+ dry etching using
H2O bypass . The Si3N4 and the n+-InP layer
To provide lateral optical and carrier confinement, are etched down to a total depth of 2.7 to 3 µm,
the mushroom-stripe laser structure (constricted hence the DFB grating, the confinements layers
mesa) [72-75] shown in Fig. 7 is used. Part a in and the QW active layers are passed during the
Fig. 7 shows the preparation of the metal mask etching process (part c of Fig. 7). Now, the Ti
used for the mesa fabrication by dry etching. A Ti mask is removed carefully by HF in order to pre-
stripe (lateral width 7 µm, thickness 100 nm) serve the Si3N4 layer.
shown in part b of the figure is defined on top of a
sputtered Si3N4 layer (about 80 nm thick). (i) First the micromachined undercut of the
GaInAsP/InP ridge mesa structures is described:
Layer B comprises the QW active layers, the bar-
rier layers and the confinement layers. To obtain
stable single-mode oscillation (lateral direction, y-
direction) a final width W1-2d between 1.4 and
1.7 µm was found to be optimum by detailed theo-
retical and experimental studies. Thus, a key proc-
ess is the precisely defined selective wet-chemical
underetching of the mesa at the position of the
quaternary material layers (see part d of Fig. 7).
This is obtained by two analytical process control
steps: batch-processing of some W2=5 µm wide
mesa ridges among the majority of W1=7 µm wide
ridges combined with IR microscopy. Wet-
chemical etching by 3H2SO4:1H2O2:1H2O at a
temperature of 38°C is used for the undercut of the
ridge mesa. Just at the moment of breaking down
the 5 µm wide ridges, the remaining material layer
B has a width of slightly less than 2 µm. The re-
sidual active layer width is precisely controlled by
IR light transmission microscopy during the etch-
ing. As indicated in the figure, the light is scattered
and reflected at the inclined interfaces, which thus
yields a lateral intensity contrast. Semi-insulating
InP (Si-InP) is used in the next step (part e in Fig.
7) to fill the undercut volumes and to planarize the
mesa structure. Hydride vapour phase epitaxy is
used providing a Fe concentration above 1018cm-3.
The Si3N4 layer is used to preserve the n-InP con-
tact layer on top of the mesa and is removed wet-
chemically by HF after this process step.
A cross sectional view of our mushroom DFB la-
Fig. 7: Various steps of the technological fabrication of ser structure is depicted in Fig. 8 by a scanning
buried mushroom stripe lasers (constricted ridge mesa
electron micrograph where the undercut volume is
partly refilled by semi-insulating InP (SI-InP). The
device shown in the figure originates from a pre-
run where the undercut volume is only partially
cladded on both sides by SI-InP. The small circle
in the centre indicates the location of the GaInAsP and low-cost wavelength definition of the individ-
waveguiding layers (horizontal bright stripes) ual channels of laser arrays for DWDM .
which embed the active GaInAsP QW’s (dark
lines). The five dark horizontal stripes indicate the In the case of bent and differently tilted mesa
position of the 5 QW’s. The right hand side circu- ridges [24-29, 77] the etching rate has to be known
lar inset shows the result of an image processing of or measured as a function of the angle in the xy-
the left inset. plane (anisotropy of the etching rate) prior to the
device design and the definition of the maximum
tilt and bending angles. If all the local tilt angles
are kept below ±6°, the variation of the wet-
chemical selective underetching rate was found to
depend less than 5% on the variation of the angles.
In those cases the weak anisotropy can be ne-
glected. If larger local tilt angles are desired the
anisotropy can be easily compensated by a local
variation of the ridge widths.
4. Fabrication of micromachined
membrane-based tunable filters
4.1 Fundamental principles
Optical microcavity devices such as tunable Fabry-
Perot filters comprise two high reflective distrib-
Fig. 8: Cross-sectional view of the mushroom DFB laser uted Bragg reflectors (DBR’s) separated by an air
structure. The waveguiding and active layers have been
cavity of the thickness L = m· λ/2 (m = 1,2,…). In
wet-chemically undercut and partially refilled. Large left
circle: magnification of the central small circle. Large right our case the microcavity is vertically oriented. The
circle: image processed graph. lower DBR and the substrate represent layer A
(according to Fig.1). The sacrificial layer B finally
(ii) In the case of the AlGaInAs/InP laser struc- defines the air cavity. The upper DBR corresponds
tures, the quaternary layers plus AlInAs buffer are to layer C.
comprised in layer B. All the steps are similar to Fig. 9 schematically shows the top view as well as
those used for the GaInAsP/InP lasers, except the the cross section of the tunable air gap filters. The
selective wet-chemical undercut. For the quater- top DBR is a circular membrane suspended in the
nary Al-containing layers, citric acid was used. In lateral xy-plane by 2, 3 or 4 suspensions (support
some cases CO ion beam etching was used for the beams) of small widths. The suspensions are con-
ridge mesa definition. nected to support posts on the opposite side. The
cross sectional view intersects from the left to the
Note that the Ti mask has to be removed in order right: the left hand side support post, the left hand
to allow IR light passing the heterostructure for side suspensions, the membrane, the right hand
transmission microscopy. This fabrication tech- side suspension, and the right hand side support
nique has been successfully used also for bent and post (section XX’).
tilted waveguide DFB lasers having shown record Continuous tuning of the filter transmission wave-
values in wavelength tunability (3-section DFB length is achieved by electrostatic or thermal ac-
laser) [25, 26], increase of functionality, low-cost tuation of the upper DBR (layer C) and thus
chirped DFB grating fabrication  and precise changing the cavity length L. Depending on L, the
filter can be adjusted to be transparent for only one
of the DWDM channels (different vertical arrows) (i) InP/airgap filters and
while blocking the others. (ii) Si3N4/SiO2/airgap filters.
In the case of electrostatic tuning, the top DBR is
p-doped, the sacrificial layer is intrinsic and the (i) InP/airgap filters
lower DBR is n-doped, forming a p-i-n structure. In the case of InP/airgap filters we made use of an
By reverse biasing this p-i-n diode, the suspended advanced multi-airgap structure achieved by a se-
top DBR moves towards the substrate. The advan- quence of several membrane (C) layers and sacri-
tage of this principle is the high continuous tuning ficial (B) layers on top of a substrate (A):
range obtained by a single control parameter and ABCBCBC…
the low actuation power due to the low leakage This sequence is an alternation of Ga0.53In0.47As
current [36, 37, 39]. (B) and InP (C) lattice matched to the InP sub-
strate (A). In this design the function of the airgaps
(a) top view: is twofold:
suspension - The mDBR· λ/4 (mDBR=1,3,5..) air layers inside the
DBR implicate an ultra-high refractive index con-
X X' trast with respect to the InP layers resulting in a
very high reflectivity.
z - The mcavity· λ/2 (mcavity=1,2,3..) air layer defines
(b) section X-X':
x the air cavity and includes a tuning capability.
L B L-∆L
Fig. 9: (a) Top view and (b) cross section of the tunable air
cavity filter. (b) shows the electromechanical tuning princi-
ple: untuned resonator (left), actuated cavity (right).
In the case of thermal actuation thin-film heaters
[42, 78, 79] are additionally defined. The tuning of
the top DBR is achieved by a heating current and Fig. 10: Scanning electron micrograph of InP cantilevers.
The structures are bend upwards due to a strain gradient
by thermal expansion of the suspensions clamping caused by As incorporation inside the InP.
the mirror membranes. These air cavity structures
also allow a wide continuous tuning with a single
control parameter. MOVPE is used to grow the multilayer structure
lattice-matched to (100) InP substrates: alternating
4.2 Device fabrication λair/4 GaInAs sacrificial layers (defining finally
the position of the air-gaps) and 3λInP/4 InP mem-
The fabrication steps of those MOEMS devices are
brane layers. Special care has to be taken to reach
based on the foundations described in section 2. In
lattice matching very well since compressive strain
the following detailed process descriptions are
and vertical strain gradients are very critical, lead-
emphasized. We will focus on two material sys-
ing to serious and undesired bendings of the un-
deretched membranes. It has been observed that
InP layers grown by MOVPE contain arsenic as an
impurity whenever they are grown after an As con-
taining layer . This results in InP cantilevers sequent reactive ion etching (20sccm CH4, 70sccm
(Fig. 2) that bend upwards due to strain caused by H2, 35mTorr, 200W, 13.56 MHz) is used to etch a
a gradient in the As distribution within the InP depth down to ~5.5 µm. Using a Si 3N4 mask in-
layer (Fig. 10). The homogeneous strain in these stead of a resist mask guarantees higher selectivity
structures is compressive causing InP bridge struc- during the dry-etching and prevents polymer depo-
tures to buckle. This problem can be solved either sition which is highly undesired at the mesa side-
by optimizing the MOVPE growth procedure  walls. This non-selective process provides vertical
or by replacing InP with GaxIn1-xAsyP1-y with a side-walls with high aspect ratio and sharp,
minimum strain/stress gradient and a slight tensile straight edges (Fig. 12). In this case we achieved a
homogeneous strain . selectivity between the III/V material and the
Si3N4 mask of more than 100.
On the masks different membrane geometries are
included as well as strain/stress indicating struc-
tures (Figs. 2 and 3) and indicator elements for
lateral undercut (Fig. 5). For the four-suspension
filter structure (Fig. 11), the membranes have di-
ameters of 40 µm, the suspensions have 10 µm
widths and lengths between 10 µm and 80 µm.
The suspensions are oriented in equivalent <100>
direction. The borders of the square support posts
(150 µm x 150 µm) are parallel to <110> direc-
Fig. 12: Scanning electron micrograph of mesa structures
in InP/GaInAs etched by CH4/H2-RIE.
Next, the etch mask is removed wet-chemically
(HF/H2O) providing the pure semiconductor mesa
(Fig. 11b). For the underetching of the InP mem-
branes the GaInAs sacrificial layers are selectively
removed using FeCl3/H2O providing excellent se-
lectivity and good semiconductor/air interfaces
(Fig. 11c). For a temperature of T=21°C, an opti-
mized etching time of 35min was found for the
above described structures. Note that the etching is
Fig. 11: Technological implementation of InP/airgap filters. anisotropic, showing an about 5 times higher etch-
(a) mask definition, (b) mesa definition by dry-etching, (c) ing rate in <100> directions compared to <110>
wet-chemical underetching (sacrificial layer removal), (d) directions. This has been taken into account during
side view of underetched filter membranes.
the definition of the filter orientation. The mem-
branes and suspensions have to be underetched
First, Si3N4 is deposited by PECVD on the epi- completely to enable free vertical motion. How-
taxial structure, then a positive resist mask is used ever, the 4 support posts are also underetched. This
in combination with reactive ion etching (5.1sccm can be avoided by additional masking techniques.
Ar, 3.5sccm CHF3, 50mTorr, 100W, 13.56 MHz)
to define the mask. Figure 11a shows a cut with a
single filter element (400 filters are fabricated on a
10mm x 10mm size) including the black Si3N4
mask. Outside the black area the semiconductor
surface is uncovered, ready for dry-etching. Sub-
After etching the sacrificial layer the sample has to white light interferometry (Zygo LOT). Note the
be dried in a way that the capillary forces due to homogeneous behavior of the four support posts
the liquid inside the “airgaps” are minimized. Oth- and the four stripe-shaped suspensions among
erwise the released parts tend to collapse and stick equivalent elements and lateral positions, respec-
together. CO2 critical point drying avoids the for- tively. The four support posts are covered by a
mation of a liquid meniscus between microstruc- protection layer avoiding underetching. A structure
ture surfaces  and gives excellent results . including 3 spiral suspensions is shown in Fig.
Membranes with extreme aspect ratios have been 13c.
successfully fabricated. InP membranes with di-
mensions of 0.37 µm, 20 µm, 100 µm (thickness,
width, length) with air gaps of 0.38 µm thickness
have been suspended without sticking problems.
Figure 11d depicts a 3D side view of the multi-
membrane filter structure. The 3 membranes of the
top DBR are clearly separated from the 3 mem-
branes of the bottom DBR. The larger air gap in
the centre is the air cavity.
Fig. 14: White light interferometric image (Zygo LOT),
80 µm long suspensions.
Although the all-semiconductor devices require
rather expensive materials and epitaxial processes,
the devices have a potential of high optical
performance. However, the filter is only a low-cost
product if a very large number of devices is batch-
fabricated simultaneously. If in addition cheaper
materials, deposition processes and sacrificial lay-
ers are involved, the total price can be considera-
(ii) Si3N4 / SiO2 /airgap filters
Fig. 13: Scanning electron micrographs: (a) 4 suspensions
per filter, (b) detail of b, (c) 3 suspensions per filter.
The use of Si3N4 / SiO2 DBR’s in combination
Fig. 13a displays a scanning electron microscope with a polymer sacrificial layer enables low-cost
image of such a four-suspension filter structure filters which also have been fabricated and studied.
(resonator details in Fig. 13b). The InP membranes The dielectric DBR’s were deposited by plasma
have a thickness of 3/4λ. The optical quality of the enhanced chemical vapor deposition (PECVD) at
two surfaces of each membrane is defined by the low temperatures. The compositions and layer
quality of the epitaxial heterointerfaces. The sur- thicknesses of the structure have been optimized
face micromachining fabrication process requires and controlled using ellipsometry in combination
no micro-mounting since all is fabricated using a with transfer matrix model calculations. The mate-
batch process. Furthermore we have a monolithic rial stress has been controlled by frequency inter-
implementation in the GaInAsP/InP material sys- lacing during PECVD . The polymer sacrifi-
tem. According to Fig. 14 the bending of the sur- cial layer has been deposited on top of the lower
face membranes can be very well observed using dielectric DBR via spin coating. Subseqently the
top DBR was deposited by low temperature 4.3 Device results
The InP/airgap filters show extremely high per-
For the lateral structurization the above mentioned formance due to the very high reflective DBR’s.
mask set has been used as well. Additionally me- The narrowest full width at half maximum
ander-shape like thin film heaters are defined on (FWHM) with only 2.5 period InP/airgap DBR’s
the suspensions to allow for thermal actuation of and a λ/2 air cavity was 0.4 nm . A maximum
the membranes. In this case the geometric design tuning range of ∆λ=62 nm with an actuation volt-
of the microstructures is of crucial importance. age of 14V and insertion loss of –3dB has been
The mesa structure was realized using a photore- achieved. In this case the suspension length was
sist (AZ1518) mask followed by reactive ion etch- 40 µm.
ing (5.1sccm Ar, 3.5sccm CHF3, 35mTorr, 200W,
13.56 MHz) of the Si3N4 / SiO2 top DBR. The se- The first demonstrator Si3N4 / SiO2 filters (5.5 pe-
lectivity of Si3N4 / SiO2 to the PR was approxi- riods top DBR and 5 periods lower DBR) show a
mately 5. The sacrificial polymer layer acts as an FWHM of 15 nm at the moment. Thermal tuning
etch stop for this process step. Plasma etching of this filter shows ∆λ = 12 nm for 1.7 V (for I =
(Technics Plasma 2.45 GHz barrel reactor,O2 , 1 mA, 40 µm suspension length) .
0.7 mbar, 250 W) is also used to etch both the PR-
mask and the polymer sacrificial layer. Thereby a 4.4 Outlook
special drying technique as for the InP/airgap fil-
ters is not necessary. Figure 15 shows a white light Both concepts, the InP/airgap filter and the
interferometric measurement of a dielectric filter. Si3N4/SiO2 filter are capable to be monolithically
The upper Si3N4 / SiO2 DBR shows bending-up of integrated with GaInAsP-based active layers, re-
the suspensions, whereas the circular filter mem- quired, e.g., for VCSEL’s or photodetectors for
brane remains nearly flat. Controlling the stress we 1.55 µm applications.
are able to form half-symmetric cavities matching
to a single spatial mode. Depending on lateral de-
sign and PECVD stress calibration we are able to 7. CONCLUSIONS
fabricate membrane curvatures between 0.5 mm
and 20 mm, suitable for single spatial mode de- In this chapter we presented typical fundamental
vices at λ = 1550 nm (spot size 10 µm with cavity technological processes required for surface-
lengths between λ and 10λ). micromachined III/V semiconductor devices. The
main application field of compound semiconductor
based micromachining is put in contrast to the
widely applied and highly developed Si based sys-
tems. Considering fabrication technology, the gen-
eral principles were reduced to three fundamental
process steps: deposition of a layered heterostruc-
ture on a substrate, vertical structurization and
horizontal undercut by selectively removing sacri-
ficial layers. The basic principles were applied and
demonstrated by two specially selected optoelec-
tronic examples: (i) bent waveguide buried mush-
room stripe lasers and (ii) vertical cavity based
tunable single or multi-membrane devices includ-
Fig. 15: White light interferometric image (Zygo LOT) of
Si3N4/SiO2 membrane suspended by four arms (80 µm long).
ing airgaps. The potential of these techniques has
been underlined by remarkable experimental de-
vice results. Advances in functionality and capa-
bility of telecommunication devices have been
obtained by the combination of III/V compound  Tien, N.C., Solgaard, O., Kiang, M-H.,
semiconductor photonic devices and 3D MEMS. Daneman, M., Lau, K.Y., and Muller, R.S., 1996,
Sensors and Actuators A52, 76.
 Daneman, M.J., Solgaard, O., Tien, N.C.,
ACKNOWLEDGMENTS Lau, K.Y., and Muller, R.S., 1996, IEEE
Phot.Technol. Lett. 8, 396.
Support by the German DFG, German Telekom  Volland, E., Heerlein, H., and Rangelow,
TOPBUS and BMBF funding is gratefully ac- I.W., 2002, Microelectronic Engineering
knowledged. Parts of the work were performed  Volland, E., Heerlein, H., Kostic, I.,and
under the umbrella of the European TUNVIC and Rangelow, I.W., 2000, Microelectronic Engineer-
MOEMS research project. The authors thank C. ing 57-58, 641.
Prott, A. Tarraf, S. Irmer, F. Römer, V. Rangelov,  Rangelow, I.W., 2000, Vacuum 62, 279.
S. Schüler, M. Strassner, N. Chitica, P. Meissner,  Oesterschulze, E., 1998, Appl. Phys. A 66, 3.
H. Halbritter, F. Riemenschneider, E. Ataro, D.  Lin, L.Y., Goldstein, E.L., and Tkach, R.W.,
Gutermuth, H. Schröter-Hohmann, I. Wensch, H. 1998, IEEE Photon. Technol. Lett. 10, 525.
Burkhard, S. Hansmann, B. Kempf, R. Lösch, W.  Neilson, D.T., 2000, Proc. Techn. Digest OFC
Schlapp, E. Kuphal, R. Göbel, B. Hübner and B. 2000, PD 12-1.
Klepser, for technical support, simulation and  Marxer, C., de Rooij, N. F., 1999, IEEE J.
measurement results and stimulating discussions. Lightwave Technol. 17, 2.
 Pavesi, L., 2000, Nature, 408, 440.
 Homewood, K., 2001, Nature, 410, 192.
REFERENCES  Hillmer, H., Greiner, A., Steinhagen, F.,
Lösch, R., Schlapp, W., Binder, E., Kuhn, T., and
 Uenishi, Y., Tsugai, M., and Meregany, M., Burkhard, H., 1996, SPIE Proc. series, 2693, 352.
1995 J. Micromech. Microeng. 5, 305.  Hillmer, H., Magari, K., and Suzuki, Y., 1993,
 Uenishi, Y., Tsugai, M., and Meregany, M., Photonic Technol. Lett. 5, 10.
1995 Electron. Lett. 31, 965.  Hillmer, H., Zhu, H.-L., Grabmaier, A.,
 Hornbeck, L.J., 1996, IEEE Digest 1996 Sum- Hansmann, S., Burkhard, H., and Magari, K., 1994,
mer Topical Meetings, MOEMS, 96TH8164, 7. Appl. Phys. Lett. 65, 2130.
 Tilmans, H.A.C., 1996, J. Micromech. Micro-  Hillmer, H., Grabmaier, A., Hansmann, S.,
eng. 6, 157. Zhu, H.-L., and Burkhard, H., 1995, IEEE J. of
 Lee, S.S., Lin, L. Y., and Wu, M.C., 1995, Selected Topics in Quantum Electronics, 1, 356.
Appl.Phys. Lett. 67, 2135.  Hillmer, H., Grabmaier, A., Zhu, H.-L.,
 Lee, S.S., Lin, L. Y., Pister, K.S.J., Wu, M.C., Hansmann, S., and Burkhard, H1995, IEEE J.
Lee, H.C., and Grodzinski, P., 1995, IEEE Phot. Lightwave Technol. 13, 1905.
Technol. Lett. 7, 1031.  Hillmer, H., Burkhard, H., Seebald, E., and
 Lin, L. Y., Shen, J. L., Lee, S.S., and Wu, Kiesel, K., 1995, J. Vac. Sci. Technol. B 13, 2853.
M.C., 1996, Optics Letters 21, 155.  Hillmer, H., Grabmaier, A., and Burkhard, H.,
 Lin, L. Y., Shen, J. L., Lee, S.S., Wu, M.C., 1997, IEE Optoelectronics, 144, 256.
and Sergent, A.M., 1996, IEEE Photon. Technol.  Spisser A., Ledantec R., Seassal C., Leclercq
Lett. 8, 101. J.L., Benyattou T., Rondi D., Blondeau R., Guillot
 Lin, L. Y., Shen, J. L., Lee, S.S., and Wu, G., and Viktorovitch P., 1998, IEEE Photonics
M.C., 1997, IEEE Photon. Technol. Lett. 9, 345. Technol. Lett. 10, 1259.
 Toshiyoshi, H., Piyawattnanmetha, W., Chan,  Tayebati P., Wang P.D., Vakhshoori D., and
C.T., and Wu, C. M., 2001, IEEE J. Microelectro- Sacks R.N., IEEE Photonics Technol. Lett., vol.
mech. systems 10, 205. 10, no.3, 394.
 Solgaard, O., Daneman, M., Tien, N.C.,  Vail, E.C., Wu, M.S., Li, G.S.,Eng, L. and
Friedberger, A., Muller, R.S., and Lau, K.Y., Chang-Hasnain, C.J., 1995, Electronics Letters 31,
1995, IEEE Phot.Technol. Lett. 7, 41. 228.
 Jayaraman V., Goodnough T. J., Beam T.l .,  Li, M.Y., Yuen, W., Li, G.S., and Chang-
Ahedo F.M., and Maurice R.A., 2000, IEEE Pho- Hasnain, C.J., 1998, IEEE Photonics Technology
ton. Technol. Lett., vol.12, no.12, 1595. Letters 10, 18.
 Peerlings, J., Dehe, A., Vogt, A., Tilsch, M.,  Vakhshoori, D., Tayebati, P., Chih-Cheng Lu,
Hebeler, C., Langenhan, F., Meissner, P., and Azimi, A., Wang, P., Jiang-Huai Zhou, and Ca-
Hartnagel, H.L., 1997, IEEE Photonics Technolo- noglu, E., 1999, Electronics Letters 35, 1.
gy Letters 9, 1235.  Chitica, N., and Strassner, M., 2001, Appl.
 Azis, M., Pfeiffer, J., Wohlfahrth, M., Luber, Phys. Lett. 78, 3935.
C., Wu, S., and Meissner P., 2000, IEEE Phot.  Chang-Hasnain C.,2000, IEEE Journal on
Technol. Lett. 12, 1522. Selected Topics in Quantum Eletctronics, vol.6,
 Hillmer, H., Daleiden, J., Prott, C., Römer, F., no.6, 978.
Tarraf, A., Irmer, S., Rangelov, V., Schüler, S.,  Sugihwo F., Larson M.C., and Harris J.S., Jr.,
Strassner, M., 2002, SPIE Proc. series 4646. 1998, Appl. Phys. Lett. 72, 10.
 Strassner, M., Daleiden, J., Chitica, N., Ke-  Larson, M.C., and Harris Jr., J.S., 1995, IEEE
iper, D., Stålnacke, B., Greek, D., and Hjort, K., Photonics Technology Letters 7, 1267.
2000, Sensors and Actuators 85, 249.  Riemenschneider R., Peerlings J., Pfeiffer J.,
 Chitica N., Daleiden J., Bentell J., Andre J., Dehe A., Vogt A., Meissner P., Hartnagel H.L.,
Greek S., Pasquariello D., Karlsson M., Gupta R., Chitica N., Daleiden J., Streubel K., Künzel H.,
Hjort K., 1999, Physica Scripta, Vol. T79, 131- Görtz W., 1998, SPIE Photonic West 98, Vol.
 Daleiden, J., Chitica, N., Strassner, M.,  Peerlings J., Riemenschneider R., Naveen
Rondi, D., Goutain, E., Peerlings, J., Pfeiffer, J., Kumar V., Strassner M., Pfeiffer J., Scheuer V.,
Riemenschneider, R., Hjort, K., Dantec, R., Ben- Daleiden J., Mutamba K., Herbst S., Hartnagel
yattou, T., Spisser, A., Leclercq, J.L., and Vik- H.L., Meissner P., 1999, IEEE Photonics Technol-
torovitch, P., 1999, Proc. of the Conf. on InP and ogy Letters, Vol. 11, No.2.
related Materials, ISBN 0-7803-5562-8, p 285.  Strite S., Ünlü M.S., 1995, Electron. Lett.,
 Daleiden, J., Chitica, N., Strassner, M., Prott, vol. 31, no.8, 672.
C., Roemer, F., Tarraf, A.,and Hillmer, H., Summer  Christenson G.L., Tran A.T.T.D., Zhu Z.H.,
School and European Optical Society Topical Lo Y.H., Hong, Mannaerts J.P., and Bhat R., 1997,
Meeting on Semiconductor Microcavity Photonics, IEEE Photonics Technol. Lett. 9, 724.
October 21-25, 2000, Technical Digest, Ascona,  Kipp, L., Skibowski, M., Johnson, R.L.,
Switzerland. Berndt, R., Adelung, R., Harm, S., and Seemann,
 Larson, M.C., Pezeshki, B., and Harris Jr., R., 2001 Nature.
J.S., 1995, IEEE Photonics Technology Letters 7,  Müller, A., Göttert, J., Mohr, J., Rogner, A.,
382 1996, Microsystem Technologies 2, 40.
 Hillmer, H., Prott, C., Strassner, M., Chitica,  Volland, B., Shi, F., Hudek, P., Heerlein, H.,
N.,and Daleiden, J., Proceedings „Optic in comput- and Rangelow, I.W., 1999, J. Vac. Sci. Technol. B
ing“, Hagen Sept.2000, ISSN 1437-8507, 75. 17, 2768.
 Chitica N., Daleiden J., Strassner M., and  Daleiden J., Kiefer R., Klußmann S., Kunzer
Streubel K., 1999, IEEE Photonics Technol. Lett. M., Manz C., Walther M., Braunstein J., and
11, 584. Weimann G., 1999, Microelectronic Engineering
 Viktorovitch P., Leclercq J.L., Goutain E., 45/1, pp 9-14.
Rondi D., 2000, SPIE Microfabrication Sympo-  Daleiden J., Czotscher K., Hoffmann C., Kie-
sium 2000, Santa Clara CA. fer R., Klußmann S., Müller S., Nutsch A., Plet-
 Wu, M.S., Vail, E.C., Li, G.S.,Yuen, W. and schen W., Weisser S., Tränkle G., Braunstein J.,
Chang-Hasnain, C.J., 1995, Electronics Letters 31, and Weimann G., 1998, J.Vac.Sci.Technol. B
1671. 16(4), 1864-1866.
 Larson, M.C., Massengale, A.R., and Harris,
J.S., 1996, Electronics Letters 32, 330.
 Daleiden J., Eisele K., Keller R., Vollrath G.,  Maboudian R., Howe R. T., 1997, J. Vac. Sci.
Fiedler F., Ralston J.D., 1996 , Optical and Quan- Technol. B 15(1).
tum Electronics 28, 527-532.  Tarraf, A., to be published.
 Daleiden J., Eisele K., Sah R.E., Schmidt
K.H.and Ralston J.D., 1995, J.Vac.Sci.Technol. B
13 (5), 2022-2024.
 Hillmer, H.,. Lösch, R., and Schlapp, W.,
1997, Journal of Crystal Growth 174, 1120.
 Hillmer, H., Lösch, R., Schlapp, W., and
Burkhard, H., 1995, Phys. Rev. Rapid Commun.,
 Hillmer, H., Lösch, R., Steinhagen, F.,
Schlapp, W., Pöcker, A., and Burkhard, H., 1995,
Electron. Lett. 31, 1346.
 Hillmer, H., review paper, 1997, Research
Trends 3, 159.
 Ericson F., Greek S., Söderquist J., Schweitz
J.-A.,1997, J. Micromech. Microeng. 7, 30-36.
 Hillmer, H., Hansmann, S., and Burkhard, H.,
1995, Optical Engineering 34, 2985.
 Hillmer, H., Hansmann, S., Burkhard, H.,
Walter, H., Krost, A., and Bimberg, D1994, IEEE
J. Quantum Electron., 10, 2251.
 Hansmann, S., Walter, H., Hillmer, H., and
Burkhard, H1994, IEEE J. Quant. Electron. 30,
 Burkhard, H., and Kuphal, E1985, IEEE J.
Quantum Electron., QW-21, 650.
 Bowers, J.E., Hemenway, B.R., Gnauck,
A.H., Bridges, T.J., Burkhardt, E. G., Milt, D. P.,
and Waynard, S., 1985, Appl. Phys. Lett. 47, 78.
 Liau, Z.L., and Walpole, J.N., 1982, Appl.
Phys. Lett. 40, 568.
 Broberg, B., Koentjoro, S., Furuya, K., and
Suematsu, Y., 1985, Appl. Phys. Lett. 47, 4.
 Kempf, B., Göbel, R., Dinges, H.W., and
Burkhard, H., 1990, Proceedings of 22nd Confer-
ence on Solid State Devices and Materials, Sendai,
Japan, Aug. 22 1990.
 Hillmer, H., "Optoelectronic Multiwavelength
component", US Patent Nr. 5,600,743, Feb. 4,
 Klepser, B., and Hillmer, H., 1998, J. Light-
wave Technol., 16, 1888.
 Hillmer, H., and Klepser, B., 1998, Proceed-
ings of SPIE 3688, 308.
 Chitica, N., Strassner, M., and Daleiden, J.,
2000, Appl. Phys. Lett. 77, 202.