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									              MEMS Technologies—Bulk Micromachining

15 Mar 2007                                     mems tech bulk.ppt
              MEMS Technologies for Fabrication

There are several standard methods for fabricating MEMS devices that
  employ many of the same methods used for microelectronics.

These methods are:
1. Bulk micromachining—uses chemical etches of materials to produce
   articulated MEMS features that may be very thick.
2. Surface micromachining—uses microelectronic processes to produce
   thin film MEMS structures.
3. LIGA—a specialized method for producing very thick structures with
   high aspect ratios (ratio of lateral size to height).
                     Comparison of MEMS Technologies

Here, comparisons of the 3 MEMS technologies are made relative to
dimension, materials, scalability, assembly and integrability to

    Capability          Bulk                 Surface                 LIGA             Conventional
                   Micromachining        Micromachining                                Machining
Min feature size      few microns            ~ 1 micron           few microns          ~ 25 microns
Device thickness        > 1 mm              few microns             > 1 mm               unlimited
Lateral size            > 1 mm                ~ 1 mm                > 1 mm               unlimited
Materials used     limited to Si-based    microelectronic        Electroplated       virtually unlimited
                                                               metals or injection
                                                                molded plastics
Scalability              limited              scalable               limited              scalable
Assembly reqts     assembly required     assembled in situ     assembly required     assembly required
Integration to         yes (SOI                 yes                    no                    no
microelectronics      processes)
Processing         parallel processing   parallel processing   parallel processing   sequential (serial)
method               at wafer level        at wafer level        at wafer level         processing
  Comparison of MEMS Device Capabilities for each Technology

For each MEMS technology, typical devices exhibit a range of
characteristics and capabilities. These are compared in the table.

 Device Capability    Bulk Micromachining             Surface                 LIGA
Actuation method           electrostatic            electrostatic        electromagnetic
Mass                            large                  small                  large
Capacitance                > 1 pf (10-12 f)            < 1 pf                 > 1 pf
Out-of-plane                    large                  small                  large
Range of motion       restricted to fab plane        3d motion        restricted to fab plane
Large arrays          no (limited by electrical   yes (106 devices         no (limited by
permitted                   connections)           demonstrated)     interconnect & assembly)
Integrated with on-     yes (SOI devices)               yes                     no
                      Bulk Micromachining

Bulk micromachining uses both wet etch and dry etch (plasma etch)
methods on single crystal silicon substrates to build elements of a MEMS
micromachine or microsensor.
These etch methods are similar, in some case identical, to the etch
methods used to fabricate microelectronic devices. But in this case, the
silicon substrate is etched to produce features in the wafer that may
descend a great depth into the substrate, perhaps a the way through its

Recall the character of these two etch methods.
Wet etch—an etch using liquid chemicals to etch a substrate or masked
pattern on a substrate; on amorphous films the etch is isotropic; on
crystalline films the etch may be anisotropic.
Dry etch—an etch using a plasma of ionized gaseous chemicals; the etch
may exhibit either isotropic or anisotropic behavior depending on the
operation of the plasma.
               Isotropic versus Anisotropic Etches

Some definitions:
isotropic—equal in all directions
anisotropic—different in different directions


   substrate or film

 In an isotropic etch, the chemical      In an anisotropic etch, the
 etches at the same rate in all          chemical etches at different
 directions. This can lead to mask       rates in different directions in
 undercutting and loss of small          the substrate or film.
               Bulk Micromachining—Wet Etch

Indicators of a good etch process:
1. Critical dimension—was the correct feature size produced?
2. Sidewall profile—measure of the shape and angle of the sidewall.
3. Etch depth—how deep must the etch continue to make the feature?
4. Etch uniformity—measure of the etch rate across the entire wafer
5. Etch rate—how fast the etch proceeds; this is a through-put issue

The primary materials to be etched in bulk micromachining are single-
crystal Si and amorphous SiO2 or SiN (used as a masking material).
                 Characteristics of Wet Etches

Advantages of wet etches include:
1. Capable of high selectivity
2. Process simplicity (only have to control the chemistry)
3. Batch processing is possible, giving high throughput
4. Simpler equipment (lower cost, simpler maintenance)
5. Availability of crystallographic etches (ODE)

Disadvantages of wet etches
1. Etches are usually isotropic except for ODE, limiting small features
2. Surface tension of liquid chemicals (problems for small features,
3. Safety (large volumes of hazardous chemicals)
                     Control of Wet Etches

Wet etches are controlled by a relatively small number of parameters.
  These include
1. Details of chemical composition / mixture
2. Temperature of the process
3. Agitation of the process
4. Cleanliness / contaminants in the solution
              Composition of Typical Wet Etches

Wet etches are typically composed of several components to increase the
control of the process. These parts include
1. oxidizing agent
2. oxide etchant
3. diluent (typically H2O)
These break the overall etch into a two-step process rather than a single
step. This permits a higher degree of control of the etch by independently
controlling the 2 steps.

This is seen in simple cleanup solutions like piranha:
To etch organic materials (photoresist), simple H2SO4 could be used but
this creates C contaminant released into the solution.
Instead, piranha (mixture of H2SO4:H2O2:H2O) is used to produce a clean
etch result.
                        Silicon Nitric-based Wet Etch

This is an etch using                             0
It can produce surfaces from
a very rough etched surface              75           25
to a highly polished, smooth


etched surface.


                          igh     50


                                                                        NO 3
                         25                                           75

                 ~ 8%

                                   75           50           25                 0
                                                                   ~ 17%
                                        Weight % (CH3COOH)
                                                                                    polishing etches in
                          peaked corners & edges
                                                                                    this region
                          square corners & edges

                         rounded corners & edges
                                                  Nitric Etch Wall Profiles
                                                     120 sec etch with nitric mixture


Trench Depth (um)




                          0   500         1000                1500                                  2000
                                    Scan Position (um)
                                                                                                                                        5 min KOH etch

                                                                               Trench Depth (um)






                                                                                                         0   50          100           150               200
                                                                                                                  Scan Position (um)
            Temperature Behavior of Wet Etches
Chemical reactions are often critically dependent on temperature.
Consider a general chemical reaction:
                aA + bB         cC + dD
The reaction rate of this (forward) reaction (ignoring chemical
equilibrium) is governed by the concentration of the reactants and the
                reaction rate ~ [A]a[B]bexp(-Ea/kBT)


                                                       concentration of B
                                                       concentration of A

                     The Silicon Crystal Lattice

The etches all depend upon chemistry, which happens through the
breaking of interatomic bonds. Not surprisingly, in crystalline materials the
etch rates can be different in different directions. This is the basis for
crystallographic or orientational dependent etches (ODE).
The Si lattice is a diamond lattice. It is composed of two interpenetrating
face-centered cubic (FCC) lattices. Silicon is a group 4 element,
possessing 4 valence electrons in its outermost shell. This means Si
atoms will bond with four nearest-neighbors to form a crystalline solid.

                                        Directions in the Si lattice are
                                        labeled by Miller indices.
                                        The blue arrows are equivalent
                                        directions in the crystal. They are
                                        the (100) directions. Planes
                                        perpendicular are 100 planes.
                                        The red arrows, crossing a body
                                        diagonal, are the (111) directions.
                   Orientational Dependent Etch

A standard wet etch used in bulk micromachining is a potassium hydroxide
etch (KOH).
KOH etches the (100) planes 100x faster than the (111) planes. As a
patterned Si 100 wafer is etched by KOH, a characteristic sidewall is
created at an angle of 54.7°. This can result in V-shaped grooves or
trenches with positive sidewalls.


                         Si (100) wafer

        V-groove                                   trench
View from above of KOH etch on Si (100).

                        Si (100) surface

                        masked region                          o


KOH wet etch of Si (100)

                      Bulk micromachining using KOH for a
                      magnetic microsensor.
On a Si (110) wafer, the same KOH etch produces vertical sidewalls.
Due to the chemical nature of the etch, the resulting sidewalls are
extremely smooth.

                                                   Si (110) wafer
100 µm

  KOH etch of Si 110 surface.

                                Source: Helsinki Univ of Technology
                           Plasma Etch

Plasma etches are also used in bulk micromachining for MEMS devices.

The etch rate in a plasma etch is controlled by a number of parameters
that are under control. These include
Temperature (substrate and chamber)
RF power (also RF frequency chosen for the process)
Gas flow rate
Pumping speed

For example, the combination of gas flow rate into the chamber and
pumping speed determine the length of time a molecule stays in the
chamber. This is called residence time and it can effect the etch process
at the wafer surface.
                   Plasma Physical Characteristics

1. Plasmas are formed by applying a large field (E or EM).

2. They can operate from DC to microwave frequencies. Most popular
   frequencies are 13.56 MHz and 2.45 GHz, many others.

3. For thin film processes, typical characteristics are:

     P ~ 50 mTorr         5 Torr

     ne ~ 109        1012 cm-3

     kBTe ~ 1        10 eV (Te ~ 104 to 105 K)

4. The plasmas are nonequilibrium since Te/Tion > 10

5. The plasmas are weakly ionized with ne/nneutral ~ 10-3 to 10-6
               Plasma Physical Characteristics

These factors result in:

a. highly reactive chemical species in the plasma (if the chosen
   gases exhibit chemistry)

b. gas temperatures near ambient (not necessarily a hot process)

c. surface chemistry that can be modified by ionic & electronic
              Effects of Plasma Parameters

Parameter             Too Small            Too Large

pressure              not enough        too many collisions
                   reactive species     plasma quenches

temperature         chemical etch
                   too slow at low T

RF power           not enough ionic     sample overheating
                    bombardment         & damage

gas flow             too slow, not      too fast, not enough
                   enough reactants     time to get reactants
                   (reactant limited)   to surface
                                        (diffusion limited)
In many instances, the plasma etch rate goes through a maximum as a
given control parameter is varied from a small value to a large value.
In this way it is possible to optimize the etch to fit the desired needs.

                                          maximum         optimized
                                          etch rate   =   parameter
                 Etch Rate

                             Control Parameter
               Si Etch Chemistries
1. CF4 : O2

     - F is a vigorous etchant for Si, etch rate follows [F].

                Si + CF4 + O2         SiF4(g) + CO2(g)

     - Add O2 to scavenge C, improve selectivity with

                CF4 + e-         CF3 + F + 2e-

                CF3 + O + e-          COF2 + F

     - Hard to make the etch anisotropic due to strong
       chemical etch.

2. Cl2

     - less vigorous etchant, easier to produce high

                     Si + 2Cl2        SiCl4
                       SiO2 Etch Chemistries
1. CF4 : H2

     - As before, lots of F is generated, but atomic H scavenges it
       very efficiently to form HF

                     CF4 + H2 + e-        CF3 + HF + 2e-

     - By doing this, the etch rate for Si is decreased to very low levels.

     - The fluorocarbon radicals (CF3+, etc) diffuse to fill the chamber,
       including near the wafer.

     - These radicals can be adsorbed on the wafer.

     - While on the surface, the radicals undergo ionic bombardment
       by the ions.

     - This bombardment generates chemically active species that etch
       the SiO2.

This ion-induced etching gives the required selectivity.
The great advantage of plasma etches is that it is possible to vary the relative
etch rates of the chemical etch and the physical etch due to ionic
bombardment. In this manner, a variety of wall profiles may be obtained.
Also, with anisotropic plasma etches it is possible to make very small
                                     etch mask

(a) isotropic sidewalls     (b) vertical sidewalls    (c) anisotropic sidewalls

     purely chemical             purely ion etch,        combined chemical
     etch, no ion etch           no chemical etch        and ion etch

(a) positive sloped walls    (b) vertical sidewalls   (c) negative sidewalls
                                                          (or reentrant)
             Bulk Micromachining Typical Device Features

     Using the methods of bulk micromachining, it is possible to manufacture a
     variety of mechanical structures for use in a MEMS device.

grooves & trenches                                       cantilevers & bridges
for microfluidic                                         for accelerometers &
devices.                                                 articulated structures

                                                             thin membranes
                                                             for pressure
                                                             sensors & valves
          Bulk Micromachining Process Sequence

The SCREAM process (single-crystal reactive etching and metallization)

Startthis sidewall etch(PECVD deep Si Si release(~substrate. for
Use withmask oxideoxideoxide.0.22, 1-2aµm)plasma 0.3with
Isotropic&the “floor”mask (100) SiO µm)etch into substrate.
 Perform aclean thewafer using anisotropic sputter process
 Remove conformal (SF6) (~ (111).
 Strip maskmetal mask for to orDeep etch
 Pattern a etch depositionfor perform masking etch µm).
 Deposit plasmaSi oxide2nd sidewall using into etch.
 uniform thickness.
Device Example—Aerospace Corp

                          SEM of a cantilever

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