MEMS and sensors technology

1
MEMS and sensors technology
Recent development of MEMS and sensors technology
is essentially based on micromachinig. This
technology consists of specific design and
fabrication processes, many of which are borrowed
from the integrated circuit industry. The
important feature of micromachining, leading to
manufacturing costs reduction is batch
fabrication, i.e. simultaneous manufacturing of
hundreds or thousans of identical structures.
Gas sensors fabricated on 3 silicon wafer
Schematic process flow in micromachining. It is
repeated until completion of a microstructure.
2
1. Thin Film Deposition Processes

• In technology of MEMS and sensors one of the main
  steps is deposition of thin
• films of materials in interest.
• Deposition technology can be classified in two
  groups
• Depositions with the help of chemical reactions
• Chemical Vapor Deposition (CVD)
• Electrodeposition
• Epitaxy
• Thermal oxidation
• These processes exploit the creation of solid
  materials directly from chemical reactions in gas
  and/or liquid compositions or with the substrate
  material. The solid material is usually not the
  only product formed by the reaction.
• Byproducts can include gases, liquids and
  even other solids.
• Depositions that happen because of a physical
  process
• Physical Vapor Deposition (PVD)
• Casting
• The material deposited is physically moved on
  to the substrate.

2
3
Chemical Vapour Deposition (CVD)
On the substrate placed inside a reactor a solid
material condenses due to chemical reactions
between source gases introduced into the
reactor. The two most important CVD technologies
are the Low Pressure CVD (LPCVD) and Plasma
Enhanced CVD (PECVD). The PECVD process can
operate at lower temperatures (down to 300 C)
thanks to the extra energy supplied to the gas
molecules by the plasma in the reactor. However,
the quality of the films tend to be inferior to
processes running at higher temperatures. The
material is deposited on one side of the wafers.
The LPCVD process produces layers with excellent
uniformity of thickness and material
characteristics. The main problems with the
process are the high deposition temperature
(higher than 600C) and the relatively slow
deposition rate. The material is deposited on
both sides of the substrates (wafers).
3
Typical hot-wall LPCVD reactor
4
Electrodeposition (electroplating)
Essentially restricted to deposition of
electrically conductive materials (metals
copper, gold, nickel). The plating process can be
also electroless external electric field and
conductive surface not required thickness and
uniformity of a deposit difficult to control.
Typical setup for electrodeposition.
Example solutions for electroplating selected
metals
When an electrical potential is applied between a
conducting area on the substrate and a counter
electrode (usually platinum) in the liquid, a
chemical redox process takes place resulting in
the formation of a layer of material on the
substrate and usually some gas generation at the
counter electrode.
4
5
Epitaxial growth
If the substrate is an ordered crystal, it is
possible to grow on it the material with the
same crystallographic orientation, which is known
as epitaxy. Vapor Phase Epitaxy (VPE) is the
most important process in which the
conditions are created to support epitaxial
growth.
Scheme of a typical reactor used in VPE process
In VPE a number of gases are introduced in an
induction heated reactor where only the
substrate is heated. The temperature of the
substrate typically must be at least 50 of the
melting point of the material to be deposited.
The high growth rate allows obtaining layers
exceeding 100 µm in thickness. This is the basic
technology of production electronic c-Si and also
SOI substrates.
5
6
Thermal oxidation

• The substrate can be oxidized in an oxygen rich
  atmosphere.
• In the case of silicon the temperature is raised
  to 800 C - 1100 which gives high
• quality amorphous silicon dioxide.
• The final oxide thickness can be controlled by
  selecting the temperature and
• oxidizing conditions.
• Thermal oxidation of silicon generates
  compressive stress in the silicon dioxide
• film,as SiO2 molecules take more volume than Si
  atoms, and there is also a mismatch between the
  coefficients of thermal expansion of Si and SiO2
  .
• As a result, thermally grown oxide films cause
  bowing of the underlying substrate.

Typical view of a furnace used for wafers
oxidation.
6
7
Evaporation
Evaporation belongs to PVD processes in which the
material is released from a source and
transferred to the substrate. In evaporation the
substrate and evaporation source are placed
inside a vacuum chamber. The source material is
then heated to the point where it evaporates and
the vapours condense on the substrate. Two
methods of heating the source are the most
popular e-beam heating and resistive heating.
Nearly any element (e.g.,Al, Si, Ti, Au),
including many high-melting-point (refractory)
metals and compounds (e.g., Cr, Mo, Ta, Pd, Pt,
Ni/Cr), can be evaporated. Deposited films
consisting of more than one element may not have
the same composition as the source material due
to the differences in evaporation rates of
constituting elements. The compound films may
then be nonstoichiometric.
Schematic view of a thermal evaporation unit with
resistive heating
7
8
Sputtering
In sputtering, a target made of a material to be
deposited is physically bombarded by a flux of
inert-gas ions (usually argon) in a vacuum
chamber at a pressure of 0.110 Pa. Atoms or
molecules from the target are ejected and
deposited onto the substrate. There are several
kinds of sputtering differing by the ion
excitation mechanism.
In direct-current (dc) glow discharge, suitable
only for electrically conducting materials, the
inert-gas ions are accelerated in a dc electric
field between the target and the substrate. In
RF (radio frequency), the target and the wafer
form two parallel plates with RF excitation
applied to the target. In ion-beam deposition
ions are generated in a remote plasma, then
accelerated at the target. Applying external
magnetic field increases the ion density near the
target, thus raising the deposition rates
(magnetron sputterind).
Deposition of thin films in a typical dc
sputtering unit
9
Casting

• In casting the material to be deposited is
  dissolved in a solvent and then applied
• to the substrate by spraying or spinning.
• Once the solvent is evaporated, a thin film of
  the material remains on the substrate.
• This is particularly useful for polymer
  materials, which may be easily dissolved in
• organic solvents, and it is the common method
  used to apply photoresist to
• substrates (in photolithography).
• The thicknesses obtained range from a single
  monolayer of molecules (adhesion promotion) to
  tens of micrometers. In recent years, the casting
  technology has also been applied to form films of
  glass (SOG) materials on substrates.
• Thick (5100 µm) SOG has the ability to uniformly
  coat surfaces and smooth out underlying
  topographical variations, effectively planarizing
  surface features.

The spin casting process used in deposition of
photoresist in photolithography.
9
10
2. Lithography
Lithography involves three sequential steps
Application of photoresist, which is a
photosensitive emulsion layer Optical exposure
to print an image of the mask onto the resist
Immersion in an aqueous developer solution to
dissolve the exposed resist and render
visible the latent image. The mask itself
consists of a patterned opaque chromium (the most
common), emulsion, or iron oxide layer on a
transparent fused-quartz or soda-lime glass
substrate. The pattern layout is generated using
a computer-aided design (CAD) tool and
transferred into the opaque layer at a
specialized mask-making facility, often
by electron-beam or laser-beam writing. A
complete microfabrication process
normally involves several lithographic operations
with different masks.
10
11
Lithography positive and negative resist

• When resist is exposed to a radiation source of a
  specific wavelength (from UV to blue), the
  chemical resistance of the resist to developer
  solution changes.
• If the resist is placed in a developer solution,
  it will etch away one of the two regions (exposed
  or unexposed).
• If the exposed material is etched away by the
  developer and the unexposed region is resilient,
  the material is considered to be a positive
  resist.
• The exact opposite process
• happens in negative resists.

Transfer of a pattern to a photosensitive
material.
11
12
Lithography subtractive and additive processes
A photosensitive layer is often used as a
temporary mask when etching an underlying layer,
so that the pattern may be transferred to the
underlying layer. Photoresist may also be used
as a template for patterning material deposited
after lithography. The resist is subsequently
etched away, and the material deposited on the
resist is "lifted off".
Pattern transfer from patterned photoresist to
underlying layer by etching (a) and pattern
transfer from patterned photoresist to overlying
layer by lift-off (b).
12
13
Lithography - alignment
In order to make useful devices the patterns for
different lithography steps that belong to a
single structure must be aligned to one another.
The first pattern transferred to a wafer usually
includes a set of alignment marks, which are high
precision features that are used as the reference
when positioning subsequent patterns, to the
first pattern as shown in the figure. By
providing the location of the alignment mark it
is easy for the operator to locate the correct
feature in a short time. Each pattern layer
should have an alignment feature so that it may
be registered to the rest of the layers.
Use of alignment marks to register subsequent
layers
13
14
3. Etching

• In order to form a functional MEMS structure on a
  substrate, it is necessary to etch the thin films
  previously deposited or the substrate itself.
• In general, there are two classes of etching
  processes
• wet etching where the material is dissolved
  when immersed in a chemical solution
• dry etching where the material is sputtered or
  dissolved using reactive ions or a vapor phase
  etchant.

Differences between anisotropic and isotropic
wet and dry etching.
Anisotropic etching in contrast to isotropic
etching means different etch rates in different
directions in the material. The example is the
(111) crystal plane sidewalls that appear when
etching a hole in a (100) silicon wafer in the
potassium hydroxide (KOH).
14
15
Anisotropic etching
The etch front begins at the opening in the mask
and proceeds in the lt100gt direction, which is the
vertical direction in (100)-oriented substrates,
creating a cavity with a flat bottom and slanted
sides. The sides are 111 planes making a 54.7º
angle with respect to the horizontal (100)
surface. If left in the etchant long enough,the
etch ultimately self-limits on four equivalent
but intersecting 111 planes, forming an
inverted pyramid or V-shaped trench.
Anisotropic wet etching of silicon wafer of (100)
crystallographic orientation.
Gas sensor on the silicon membrane
16
Dry etching
The dry etching technology can split in three
separate classes called sputter etching, vapor
phase (chemical) etching and reactive ion etching
(RIE).
In sputter etching the systems used are very
similar in principle to sputtering deposition
systems but the difference is that substrate is
now subjected to the ion bombardment instead of
the target. In vapor phase etching the material
to be etched is dissolved at the surface in a
chemical reaction with the gas molecules (mostly
isotropic process). In RIE,under the influence
of RF power the gas molecules break into ions
which are accelerated towards, and react at, the
surface of the material being etched. The
balance of chemical and physical etching can give
sidewalls that have vertical shapes.
Ions Ar, O2
CF4, SF6
RIE
Illustration of different dry etching processes
16
17
Deep Reactive Ion (DRIE) etching
In this process, etch depths of hundreds of
microns can be achieved with almost vertical
sidewalls. The primary technology is based on
the so-called "Bosch process", where two
different gas compositions are alternated in the
reactor. The first gas composition creates a
polymer on the surface of the substrate, and
the second gas composition etches the substrate.
The polymer is immediately sputtered away by ion
bombardment, but only on the horizontal surfaces
and not the sidewalls. Since the polymer only
dissolves very slowly in the chemical part of the
etching, it builds up on the sidewalls and
protects them from etching.
Profile of a DRIE trench using the Bosch process.
Etching aspect ratio (ratio of height to width)
of 50 to 1 can be achieved.
17
18
Screen printing
A wide variety of materials, including metals,
chemical compopunds and ceramics, can be applied
using screen printing (e.g. in sensor
technology). Screen printing begins with the
production of a stencil, which is a flat,
flexible plate with solid and open areas. The
stencil has a fine-mesh screen as a bottom layer.
Separately, a paste is made of fine particles of
the material of interest, along with an organic
binder and a solvent. A mass of paste is applied
to the stencil, then smeared along with a
squeegee. A layer of paste is forced though the
openings in the stencil, leaving a pattern on the
underlying substrate. Drying evaporates the
solvent. Firing burns off the organic binder and
sinters the remaining metal or ceramic into a
solid.
Illustration of the screen printing process.
18