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Low Temperature Glass to Glass Wafer Bonding SIMTech



                Low Temperature Glass-to-Glass Wafer Bonding
                          J. Wei, M. L. Nai, C. K. Wong, Z. Sun and L. C. Lee

Abstract – In this paper, results of successful          ture higher than the Au-Si eutectic temperature
anodic bonding between glass wafers at low               of 363 C [4]. When glass layers are used for
temperature are reported. Prior to bonding, a            bonding, the firing temperature has to be above
special technique was used, i.e. an amorphous            400 C [5]. A higher process temperature is still
and hydrogen free silicon film was deposited on          involved in eutectic bonding and glass frit bond-
one of the glass wafers using sputtering tech-           ing. Solder and adhesive bonding can reduce
nique. The effects of bonding temperature and            the bonding temperature and has been em-
voltage were investigated. The bonding tem-              ployed for many applications [6-7]. However,
perature and the voltage applied ranged from             these techniques may generate problems such
     o        o
200 C to 300 C and 200 V to 1000 V, respec-              as outgassing, low positioning accuracy, long-
tively.                                                  term reliability and uncertain bond quality.

As the bonding temperature and bonding voltage           Anodic bonding is being widely used for bonding
increased, both the unbonded area and the size           glass substrate to other conductive materials
of the voids decreased. SEM observations show            due to its good bond quality. It can serve as a
that the two glass wafers are tightly bonded.            hermetic and mechanical connection between
The bond strength is higher than 10 MPa for all          glass- and metal-substrates or a connection be-
the bonding conditions. Furthermore, the bond            tween glass and semiconductor substrates [8],
strength increases with increasing bonding tem-          [9]. In anodic bonding, the substrates are typi-
perature and voltage. The study indicates that           cally heated to a temperature between 400 and
                                                 +            o
high temperature and voltage cause more Na               450 C. A voltage of 400 to 1200 V is usually ap-
ions to neutralize at the negative electrode,            plied to the glass and the other substrates to be
which leads to higher charge density inside the          bonded.
glass wafer. Furthermore, the transition period to
the equilibrium state also becomes shorter. It is        The use of low temperature during bonding can
concluded that the anodic bonding mechanisms             avoid degrading or damaging of pre-fabricated
involve both oxidation of silicon film and the hy-       devices and integrated circuitries. It can also
drogen bonding between hydroxyl groups.                  minimize or eliminate bonding-induced stress
                                                         problems and warpage after cooling. Thus, ma-
Keywords: Glass-to-glass, Anodic bonding, Low            terials with large differences in the thermal ex-
temperature, Strength, Integrity                         pansion coefficient can be bonded together with
                                                         better reliability. Currently, a great deal of wafer
                                                         bonding research is focused on achieving strong
1       BACKGROUND                                       wafer bonding at the lowest possible tempera-
                                                         ture [10-14]. The bond quality generally deterio-
Wafer bonding has increasingly become a key              rates with decreasing bonding temperature. The
technology for materials integration in various          bond strength is low and voids or cavities are
areas of microelectromechanical systems                  prevalent at the interface.
(MEMS), microelectronics, and optoelectronics
[1-2]. It is also widely used for vacuum packag-         Glass-to-glass wafer bonding has not been in-
ing, hermetic sealing and encapsulation.                 vestigated extensively. However, glass-to-glass
                                                         bonding has useful applications in bio-MEMS,
To bond wafers or substrates together, numer-            microfluidic, displays and other areas due to the
ous techniques have been developed. These                unique property of glass materials. In this work,
techniques can be categorized into direct bond-          glass-to-glass wafer bonding with the assistance
ing, intermediate layer bonding, and anodic              of a nano-scale amorphous and hydrogen free
bonding.                                                 silicon film has been performed.

Direct bonding necessitates stringent cleanli-           2       EXPERIMENTAL
ness and flatness of the surfaces to be bonded
and requires a high annealing temperature to             2.1     Wafer preparation, film deposition
achieve reliable bond quality [3].                               and pre-cleaning

Intermediate layer bonding, for example Au-Si            The glass wafers are the commonly used 4-inch
eutectic bonding, involves a process tempera-            Pyrex 7740 borosilicate glass wafers. The sur-

                                                               Low Temperature Glass-to-Glass Wafer Bonding

face roughness of the glass wafers, Ra, is less          tensile testing machine. For each bonding condi-
than 15 Å, and the flatness is better than 5 m.         tion, five samples were tested. The bond
The thickness of the glass wafer is about 500            strength was calculated based on the average
m. An amorphous and hydrogen free silicon               fracture force. The interface was analyzed with
thin film was deposited onto the glass substrates        scanning electron microscopy (SEM) and Ra-
using a DC magnetron sputtering system                   man spectroscopy. Raman spectra were ob-
(Unaxis LLSEVO). To achieve a hydrogen-free              tained with a Rennishaw Ramanscope using an
film, the base pressure of chamber was pumped            argon laser beam of 514.5 nm as the excitation
down to 5  10 Pa. A high-purity 99.99% silicon
                -5                                       source.
planar target was mounted at a distance of 10
cm from the substrate and Ar was used as the
sputtering gas. The gas flow rate was typically          3        RESULTS & DISCUSSION
100 sccm (standard cubic centimeter per min-
ute), and the pressure was about 0.2 Pa. The             3.1      Surface morphology of the film
target power density was in the range of 1.1 to
1.5 W/cm . By controlling the deposition time, a         The surface topography of the amorphous film
film thickness of about 100 nm was obtained, as          on glass wafer was analyzed by AFM. The sur-
measured by an ellipsometer (J.A. WOOLLAM                face is very smooth and has a low roughness of
Co., HS-190 ).                                           0.86 nm (Ra). All cleaning and depositing proc-
                                                         esses were performed in a clean room to avoid
Prior to alignment, the surfaces of the film             any foreign particle from falling on the bonding
coated glass wafer and bare glass wafer were             surfaces.
cleaned in RCA solutions at a temperature of
60-80 C to remove the surface contaminants               3.2      Bonding current
and impart hydrophilic property. RCA is a solu-
tion with the following chemicals; RCA-1                 Anodic bonding takes place immediately on the
(NH4OH:H2O2:H2O = 0.25:1:5) and RCA2                     application of the external voltage. At the start of
(HCl:H2O2:H2O = 1:1:6). The cleaned wafers               the bonding process, the value of the current is
were rinsed with deionized (DI) water and dried          high but falls rapidly, especially for high bonding
in pure N2.                                              temperatures. The initial peak current corre-
                                                         sponds to the initial transport of sodium and po-
2.2     Experimental setup                               tassium ions from the glass wafer to the cath-
                                                         ode. These ions are consequently neutralized at
After alignment, the stacked glass wafers were           the cathode. Once an equilibrium state is estab-
placed in an EV501 bonding chamber. To avoid             lished, the current remains at a low value. High
wafer contact during vacuumizing, 20-50 mi-              temperatures generate high ion mobility in the
crons thick spacers that were placed at the              glass substrate. As the temperature is raised,
edges of the stacked wafers separated the two            the conductivity of glass increases exponentially.
wafers. During vacuumizing, both wafers were             A rapid build up of the space charge occurs at
heated to the predetermined temperature. On              the interface, giving rise to electrostatic forces,
reaching the predetermined temperature setting,          which brings the two wafers into intimate con-
the two wafers were initially brought into contact       tact. Therefore, high temperatures cause high
under pressure in the central area. Next, the            current (Fig. 1(a)) and result in good bond qual-
spacers were removed to allow the rest of the            ity. At high temperatures, more ions decompose
surface of the glass wafer to come into contact          from Na2O and K2O to migrate to the cathode.
with the film. Anodic bonding was then carried           The equilibrium state is easily obtained and the
out by applying a voltage between the two wa-            transition period is shorter.
fers. The positive potential was applied to the
silicon coated glass wafer with respect to the           A higher applied voltage is expected to increase
glass wafer.                                             the mobility of the Na ions. A higher voltage
                                                         produces a higher electric field. The higher elec-
2.3     Bond strength and interface integrity            tric field will increase the drift velocity of the so-
                                                         dium ions. Higher voltage will also accelerate the
After anodic bonding, the bonded pair was                detachment of the sodium ions from the lattice
checked with scanning acoustic microscopy                matrix, and contribute to the concentration of
(SAM) and the micrographs were analyzed with             free sodium ions. This is the likely reason for the
an image analysis tool. For each bonding condi-          higher current and the longer time required to
tion, three pairs of wafers were bonded. The             establish the equilibrium state (Fig. 1(b)). There-
wafers were diced into 1010 mm pieces for               fore, a higher applied voltage can generate more
tensile strength measurement with an Instron

                                                                                      Low Temperature Glass-to-Glass Wafer Bonding

free sodium ions, and subsequently, contributes
to a larger electrostatic force.
   Current (mA)

                                                                                          (a)               (b)               (c)



                       0      200   400     600      800    1000      1200
                                          Time (S)
                  12                                                                       (d)              (e)               (f)
                  10                                       G-G-400V
                                                                                  Fig. 2. Scanning acoustic micrographs of the glass-
                  8                                        G-G-800V               to-glass bonded pairs under different temperatures:
  Current (mA)

                                                                                          o         o              o
                                                           G-G-1000V              (a) 200 C, (b) 250 C and (c) 300 C at voltage of 600
                  6                                                               V and under different voltages: (d) 200 V, (e) 600 V
                                                                                  and (f) 1000 V at temperature of 250 C. Bonding
                                                                                  force of 200 N, bonding time of 10 min and vacuum
                  2                                                               of 1 Pa.
                       0      200   400     600      800     1000      1200
                                                                                  The SAM images were further analysed with
                                          Time (S)
                                                                                  image analysis method. The unbonded area was
Fig. 1. The current-time relationship under different
temperatures at voltage of 600 V (a) and under dif-
                                                                                  measured and plotted in Fig. 3. It can be seen
ferent voltages at temperature of 250 C. Force is 200                             that the unbonded area decreases from 2.2% to
N and vacuum is 1 Pa.                                                             0.25% when the bonding temperature increases
                                                                                            o        o
                                                                                  from 200 C to 250 C. With a further increase of
                                                                                  the bonding temperature to more than 275 C,
The influencing parameters on the bonding qual-                                   the whole wafer area is bonded together. The
ity include the bonding temperature, the applied                                  unbonded area decreases from 1.65% to 0.02%
voltage, the pressure, the bonding time and                                       when the voltage increases from 200 V to 1000
vacuum level. However, the temperature and the                                    V. The interface integrity was also observed by
voltage play the most important role in anodic                                    SEM. No observable gap can be seen from the
bonding process [11]. Therefore, the effects of                                   SEM micrographs, as shown in Fig. 4.
these two parameters are investigated in detail.

3.3                        Interface integrity

The bonding interface integrity was evaluated by
comparing the bonded area with the entire wafer
surface. It was characterized using SAM. Fig. 2
shows the SAM micrographs for the bonded wa-
fers under different temperatures and voltages
at bonding force of 200 N, bonding time of 10                                                                                                                                 2.5
min and vacuum of 1 Pa. At low bonding tem-
peratures (say, 200 C), more and larger voids                                                                                                                                 2.0
                                                                                                                                                          Unbonded area (%)

are found at the interface. With increased bond-
ing temperature, the number of voids and the
void size decrease noticeably. When the bond-                                                                                                                                 1.0
ing temperature is higher than 275 C and the
voltage is higher than 800 volts, no voids are
found. The bonding temperature and the voltage                                                                                                                                0.0
have a significant influence on the voids. The                                                                                                                                   1
unbonded area or voids are mainly attributed to
gas entrapment between the mating surfaces of
the two wafers. No voids arise from particle con-                                 Fig. 3. Unbonded area under different temperatures
                                                                                                                                            Unbonded area (%)

                                                                                  at voltage of 600 V (a) and under different voltages at
tamination, thus indicating that the cleaning pro-                                                     o
                                                                                  temperature of 250 C (b). Bonding force of 200 N,                                    1.50
cedure is effective.                                                              bonding time of 10 min and vacuum of 1 Pa.

                                                                                                        Low Temperature Glass-to-Glass Wafer Bonding

                                                                                                  inside glass. The fracture initiates in the glass,
                                                                                                  then the crack propagates along the glass, and
                                                                                                  sometimes it propagates into the other wafer
                                                                                                  through the interface without damaging the
                                                                                                  bond. It further demonstrates that a good bond
                                                                                                  is formed.

                                     (a)                                              (b)

Fig. 4. Typical cross-sections of bonded glass and
glass wafers under different bonding temperatures of
        o              o
(a) 200 C and (b) 300 C, at voltage of 600 V, bond-
ing force of 200 N, bonding time of 10 min and vac-
uum of 1 Pa.

                                                                                                                  (a)                 (b)
3.4                               Bond strength
                                                                                                  Fig. 6. Typical fractures at bonding temperature of
                                                                                                  300 C, voltage of 600 V, bonding force of 200 N,
The bond strength is an important factor for                                                      bonding time of 10 min and vacuum of 1 Pa (a) and
bond quality and reliability. High bond strength                                                  (b) show mating surfaces.
indicates that a good bond has been formed.
Fig. 5 shows the bond strength versus bonding
temperature and voltage. The tensile strength of                                                  The thermal residual stress induced by the low
the bonded pairs is higher than 10 MPa for all                                                    temperature anodic bonding process was
the bonding conditions. The bond strength ob-                                                     measured via the change in the curvature of the
tained in this study is comparable to that of                                                     wafers. As expected, low temperature bonding
Si/glass wafers bonded using higher bonding                                                       largely reduced the induced stress. In this study,
temperatures by other researchers [15-17]. The                                                    all the bonded wafers under different conditions
bond strength increases with an increase in the                                                   were measured, no residual stress was de-
bonding temperature.                                                                              tected.

                            30                                                                    3.5      Bonding mechanisms
      Bond strength (MPa)

                                                                                                  Fig. 7 shows the typical Raman spectra of the
                                                                                                  amorphous silicon film and the interface be-
                            15                                                                    tween the amorphous silicon film and glass wa-
                            10                                                                    fer. For crystalline silicon wafer, the main peak
                                                                                                  occurs at 520 cm . For amorphous silicon
                                                                               (a)                coated glass, an expanded band around 470
                                                                                                  cm can be observed. At the interface between
                                 150             200          250            300        350
                                                  Bonding temperature ( C)
                                                                                                  the amorphous silicon film and glass wafer, the
                                                                                                  band around 446 cm expands significantly.
                                                                                                  This implies that more distortion occurs. The
                            25                                                                                             -1
                                                                                                  band around 800 cm is likely to be due to the
      Bond Strength (MPa)

                            20                                                                    Si-O bonds. Based on the analysis above, it is
                            15                                                                    reasonable to assume that the silicon atoms at
                                                                                                  the interface are not arranged regularly as is the
                                                                                                  case inside the crystalline silicon. The Si-O
                                                                               (b)                bonds that are formed at the interface do no ex-
                             0                                                                    ist solely in the form of crystalline SiO2.
                                 0         200         400    600     800      1000    1200
                                                       Bonding Voltage (V)
                                                                                                  In the present study, an amorphous silicon film
Fig. 5. Bond strength under different temperatures at                                             was deposited on one of the glass wafers to ful-
voltage of 600 V (a) and under different voltages at                                              fill two functions. Firstly, it acts as the barrier
temperature of 250 C and (b) Bonding force of 200
N, bonding time of 10 min and vacuum of 1 Pa.
                                                                                                  layer to prevent sodium and potassium ion shift
                                                                                                  from the film coated glass wafer to the bare
                                                                                                  glass wafer. Secondly, it acts as an intermediate
Fig. 6 shows the typical fracture surfaces after                                                  layer for bonding.
pull-test. It can be seen that the fracture occurs

                                                                                       Low Temperature Glass-to-Glass Wafer Bonding

                                                                                   to hydrogen than silicon. Hydrogen will prefera-
                                                                                   bly react with oxygen, thereby decreasing the
                                                                                   bonding sites for silicon and oxygen. The reac-
    Intensity (arb. units)

                             1800                                                  tion between oxygen and hydrogen will also in-
                                                                                   crease the void size and the unbonded area.
                             1700                                                  Higher bonding temperature and voltage in-
                                                                                   crease the mobility and the drift velocity of both
                             1600                                a-Si              alkali ions and oxygen anion, thus promoting the
                                                                                   oxidation process.
                                 100   300    500    700     900        1100
                                                                                   In Si-to-Si, Si-to-SiO2 or SiO2-to-SiO2 direct
                                             Wavenumber (cm-1)
                                                                                   bonding [3, 18-19], the surface terminated hy-
Fig. 7. Typical Raman spectra of the amorphous sili-                               droxyl groups react to form hydrogen bonds
con film and the interface between the amorphous                                   even at low temperature. In the present study,
silicon film and glass.                                                            Pyrex 7740 glass is used. SiO2 is the major
                                                                                   component of such glass. Prior to stacking and
                                                                                   bonding, both bare glass and amorphous silicon
The reactive phenomenon at the interface is                                        film coated glass wafers are treated to be hy-
largely dependent on the bonding temperature                                       drophilic, the surfaces are terminated with hy-
and voltage since temperature and voltage are                                      droxyl groups. After stacking, the –OH starts to
the major driving forces of ion mobility and                                       dehydrate at a bonding temperature around
atomic diffusion. High bonding temperatures                                            o
                                                                                   200 C, and the hydrogen bonds are replaced by
introduce higher energy, drift more oxygen ions                                    Si-O-Si bonds. Bonding between the hydroxyl
to the interface, enhance the diffusion of oxygen                                  groups on the glass and silicon can occur
into silicon, and finally promote greater reaction                                 through hydrogen bonding between SiO2 and
in the interface. In this work, an amorphous sili-                                 amorphous Si.
con film is applied on the glass wafer. It has
higher surface energy and looser structure than                                    At the range of temperatures used in the study,
those of the crystalline silicon. The higher sur-                                  the oxide layer at the interface is amorphous
face energy of amorphous Si film allows reac-                                      SiOx, where x is less than 2. It is deduced that
tions to occur in the interface with less energy.                                  the anodic bonding mechanisms consist of the
The looser structure permits easy diffusion of                                     oxidation of silicon and hydrogen bonding be-
oxygen into the film even at low temperatures.                                     tween hydroxyl groups. Higher temperature can
Therefore, the bond strength can still be main-                                    promote such reactions and result in better bond
tained at a high value although the bonding tem-                                   quality.
perature is low.

The wafer surfaces were treated to be hydro-                                       4       CONCLUSION
philic prior to anodic bonding process. Hydroxyl
groups terminate the wafer surfaces. When the                                      Glass-to-glass low temperature wafer bonding
two wafers are brought into contact, the contact                                   has been successfully developed. The amor-
force composes of electrostatic force, attraction                                  phous and hydrogen-free silicon nano-film plays
force between the hydroxyl groups and applied                                      an important role in the bonding process. High
force. In the anodic bonding process, a space                                      bond quality, bond strength and bond efficiency,
charge region is formed at the glass side of the                                   has been achieved. The bond strength between
silicon-glass interface, leaving behind relatively                                 glass wafers is higher than 10 MPa, and the un-
immobile oxygen anions. This in turn creates an                                    bonded area at wafer level is less than 2.2%.
equivalent positive charge (image charge) on                                                                                o
                                                                                   With a bonding temperature above 275 C and a
the silicon side of the silicon-glass interface, re-                               voltage above 800 V, void-free interfaces are
sulting in a high electrostatic force between the                                  obtained. Higher bonding temperatures and
two wafers. The oxygen anions drift away from                                      voltages will yield higher bond quality. Residual
         +       +
the Na and K depletion region to the amor-                                         stress is too small to be detected. The bonding
phous silicon surface. Due to the high surface                                     mechanisms are comprised of the oxidation of
energy of this amorphous film, silicon is oxidized                                 silicon film and the hydrogen bonding between
by the oxygen anions, even at the relatively low                                   hydroxyl groups.
bonding temperature. Therefore, the reaction
between oxygen and silicon constitutes one of
the bonding mechanisms.                                                            5       INDUSTRIAL SIGNIFICANCE
Minimizing hydrogen content in the amorphous                                       The bonding technique can be widely used in
Si film is essential. Oxygen has a higher affinity                                 following areas:

                                                              Low Temperature Glass-to-Glass Wafer Bonding

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