Chapter 9: Micro Structure Technology and Micromachined Devices

1
Chapter 9Micro Structure Technology
andMicromachined Devices

• Picture shows the interior chip assembly of the
  SA30 Crash Sensor, a microsystem from SensoNor,
  Norway

The course material was developed in INSIGTH II,
a project sponsored by the Leonardo da Vinci
program of the European Union
2
Definitions

• MICRO STRUCTURE TECHNOLOGY can be defined as a
  group of three-dimensional micromachining
  techniques enabling feature dimensions with
  accuracy in the micrometer range.
• MICROMACHINED DEVICES can be defined as devices
  made by Micro Structure Technology.
• These micromachining techniques are mainly based
  upon batch organised microelectronic process
  technology, either directly adapted techniques
  like photolithographics, or modified techniques
  such as anisotropic etching techniques.
• Some micromachining techniques are specially
  developed for this field, e.g., anodic bonding of
  micromachined devices.

3
Example SP80 Pressure Sensor

• Developed at SINTEF (earlier Center for
  Industrial Research), Norway and manufactured by
  Capto as (earlier SensoNor AS, earlier ame),
  Borre, Norway.
• This sensor visualises the main features and
  limitations of micromechanical sensors, and
  points out pressure sensing as a main application
  for these kinds of sensors.

4
SP80 Principal Design

• A piezoresistive integrated pressure sensor with
  the pressure-sensitive diaphragm micromachined in
  a silicon chip by anisotropic etching.
• Ion implanted piezoresistors in a full Wheatstone
  bridge configuration as the electronic sensing
  element.
• Temperature measuring resistor and a heating
  resistor are implanted on the same chip, to
  compensate or thermostat the chip to minimise
  thermal drifts.
• By varying the area and the thickness of the
  diaphragm, pressure ranges from 0.5 Bar full
  scale pressure up to 60 Bar full scale pressure
  can be achieved
• Packaged in a transistor header
• Main application areas are within general
  instrumentation, metrology and aerospace
  application.

5
The SP80 Silicon Chip Set - Drawing

• Consists of diaphragm chip sealed to a support
  chip which is mounted on top of a glass tubing
  acting as a mounting stand as well as a pressure
  port.

6
The SP80 Silicon Chip Set - Picture

• Consists of diaphragm chip sealed to a support
  chip which is mounted on top of a glass tubing
  acting as a mounting stand as well as a pressure
  port.

7
Dimensions and Processing

• The size is 44 mm, thickness approximately 0.3
  mm, the diaphragm area is typical 22 mm and the
  diaphragm thickness is typical 30 micrometers.
• The diaphragm is manufactured by stripping off
  the surface oxide of the silicon wafer by means
  photolithographic technique in the areas we want
  the diaphragm cavity.
• Then the wafer is etched in an anisotropic
  etching solution with the remaining oxide as
  masking film.
• This etching solution attacks the single crystal
  silicon with different speed in the different
  crystal directions.
• The etch is extremely slow in the lt1-1-1gt
  direction The etch is therefore stopped towards
  the (1-1-1) planes.
• The chip material is (1-0-0) silicon
• Therefore, the etch cavity is surrounded by four
  (1-1-1) planes which have an angle of inclination
  of 54.7 degrees relative to the (1-0-0) surface
  plane, rendering a cavity with four sloped walls.

8
SP80 Package

• Cross-sectioned view of the SP80 Pressure Sensor
  packaged in a transistor header.

9
SP80 Package, continued

• Cross-sectioned view of the SP80 Pressure Sensor
  packaged in a transistor header with a top chip
  containing a vacuum reference chamber.

10
SP80 Schematic

• The SP80 schematic consists of 4 ion implanted
  piezoresistors in a full Wheatstone bridge
  configuration as the electronic sensing element.
  In addition, a temperature measuring resistor and
  a heating resistor are implanted on the same
  chip, to compensate or thermostat the chip to
  minimise thermal drifts.

11
Picture of SP80 in Transistor Package

• Comment The Norwegian coin is approximately the
  size of Ø10 mm

12
Main Features of SP80

• Low non-linearity ( lt - 0.1 )
• Negligible hysteresis ( lt - 0.005 of full scale
  output )
• Low long term drift ( typical less than 0.1 per
  year )
• Active thermal compensation by utilising the
  on-the-chip heating resistor.
• Small size.

13
Drawbacks of SP80

• Reference pressure medium must be non-conducting
  and non-corrosive to be compatible with the
  on-chip sensing elements and electronics.
• Safe overload is limited to 3 times rated
  pressure as no mechanical overload stop is
  implemented.
• The devices have no normalised output signal.
  Each device has to be individually calibrated
  when system installed.
• Temperature range is limited (-55 - 125 C) and
  uncompensated thermal sensitivity drift is
  relative high ( -0.2/C).

14
Applications for Micromachined Sensors and
Microsystems

• The biomedical market
• Blood pressure sensors
• The space, defence and avionics markets
• Accelerometers for rocket navigation
• Micro gravity sensor
• Gyroscopes for navigation
• The agriculture electronics market
• Automotive sensors used in tractors, harvesters
  etc.
• The off-shore oil exploitation market
• High pressure measurement in oil wells
• Sea wave sensor
• The automotive market
• Acceleration microsystems for air bag systems
• Tire pressure microsystems
• The data and peripheral market
• Disk drive write and read heads
• The consumer market
• Photo diodes in cameras
• Level measurement in white goods appliances.

15
Top10 Success Factors

• 1. Batch organised processing technology
• 2. Microelectronics manufacturing infrastructure
• 3. Research results from solid state technology
  and other related fields of microelectronics
• 4. Micromachining
• 5. Wafer and chip bonding
• 6. Mechanical material characteristics
• 7. Sensor effects
• 8. Actuator functions
• 9. Integrated electronics
• 10. Combination of features

16
Bottom10 Limiting Factors

• 1. Slow market acceptance
• 2. Low production volumes
• 3. Immature industrial infrastructure
• 4. Poor reliability
• 5. Complex designs and processes
• 6. Immature processing technology
• 7. Immature packaging and interconnection
  technologies
• 8. Limited research resources
• 9. Limited human resources
• 10. High costs

17
Milestones in the Planar Silicon Processing
Technology (and some other related breakthroughs)

• 1890 Punched cards invented
• 1939 Vacuum tubes and mechanical computing
• 1948 The invention of the transistor
• 1959 The invention of the planar silicon
  processing
• 1959 The invention of the integrated circuit
• 1964 Mainframe computing
• 1971 The invention of the microprocessor
• 1981 introduction of personal computers
• 1985 1 Megabit random-access-memory chips
  available
• 1991 64 Megabit random-access-memory chips
  available
• 1994 Internet in widespread use
• 1994 256 Megabit random-access-memory chips
  available
• 1995 Microprocessors with more than 3 million
  transistors available
• 2000 Microprocessors with more than 100 million
  transistors available
• 2005 1 Gigabit random-access-memory chips
  available
• 2006 Digital consumerisation (Video on mobile
  phones etc)
• 2007 The Intel Itanium microprocessor with 1.2
  billions transistors.
• 2008 4 Gigabit random-access-memory chips
  available

18
Manufacturers of Micromechanical Devices

• The industry structure is highly diversified both
  in size, technological basis and organisation
  type.
• Traditional sensor manufacturers have seen
  micromechanical sensors as a natural expansion of
  their technological basis, and have taken up
  research and production of these sensors as a
  part of their activity.
• Semiconductor companies have entered this market
  as an expansion of their integrated circuit
  activity, since they already have most of the
  needed equipment and the appropriate marketing
  channels.
• System companies or original equipment
  manufacturers which see micromechanical devices
  as a way to boost their systems.
• "Start ups", companies having micromechanical
  devices as their main business idea.
• There are of course companies that does not fit
  into any of these types and some are someplace in
  between these types.

19
Manufacturers

• USA
• Honeywell, Microswitch, SenSym, IC Sensors,
  Motorola, Delco, Foxboro/ICT, Endevco, Kulite,
  Lucas/NovaSensor, Michigan Microsensors
• Japan
• Hitachi, Toshiba, NEC, Yokagawa Hokushin, Toyota
  Motor Company
• Europe
• Germany Infineon, Bosch,
• The Netherlands Philips, Microtel, Xensor
  Integration
• UK Druck
• Switzerland Keller, Kistler
• Finland Vaisala
• Sweden Radi Medical Systems
• Norway SensoNor

20
Research Centers

• USA
• Stanford University, Case Western Reserve
  University, University of Michigan, University of
  California at Berkeley, University of Wisconsin,
  MIT
• Japan
• Tohoku University, Kyoto University, Fudan
  University,
• Europe
• The Netherlands Delft University, Twente
  University
• Belgium IMEC, Catholic Un of Leuven
• Switzerland University of Neuchâtel, CSEM
• Germany Fraunhofer Institute, IFT Munich,
  Fraunhofer Institute, IMT Itzehoe, Techn. Un of
  Berlin
• Denmark Techn. Un of Denmark
• Finland VTT
• Sweden Uppsala University, KTH/Acreo
• Norway SINTEF

21
Batch Processes Adapted from Microelectronics/IC
Technology with no or Minor Modifications  

• Photolithography
• Spin coating
• Etching techniques
• Diffusion of dopants
• Implantation
• Epitaxy
• Chemical vapour deposition (CVD)
• Thin film technology
• Thick film technology

22
Batch Processes Modified from Microelectronics/IC
Technology Processes

• Double-sided photolithography
• Wafer fusion bonding
• LIGA and LIGA-like techniques
• Laser micromachining  

23
Batch Processes Adapted or Modified from Other
Technologies than Microelectronics/IC Technology

• Micro stereo lithography
• Micro electro discharge machining

24
Batch Processes Mainly Developed for
Micromachined Devices

• Bulk micromachining
• Surface micromachining
• Anodic wafer bonding
• Fusion bonding (Direct bonding)These
  technologies will be commented on the following
  slides

25
Bulk Micromachining in Silicon

• Bulk Micromachining in Silicon is here defined as
  three-dimensional micromachining in single
  crystal silicon by means of photolithographic
  etching techniques.
• It is also called Bulk Micromechanics in Silicon
  or Silicon Micromachining
• To understand this technology, some basic insight
  in single crystal silicon is needed

26
Crystal Structure of Single Crystal Silicon

• It is a face-centered cubic structure (diamond
  structure) with two atoms associated with each
  lattice point of the unit cube. One atom is
  located in position with xyz coordinates (0, 0,
  0), the other in position (a/4, a/4, a/4), a
  being the basic unit cell length.

27
Miller Indices for a Plane in a Crystal

• The orientation of of different crystal planes in
  the basic unit cell can be described by the
  Miller indices (hkl) between parentheses with
  each plane defined by a vector description (hx
  ky lz) of the direction perpendicular to that
  plane. This is related to a coordinate system
  oriented in parallel with the side edges of the
  basic cell, with the Miller indices reduced to
  the smallest possible integers with the same
  ratio.

28
Important Crystal Planes in the Silicon Crystal

• (100), (110) and (111) are the three most
  important crystal planes of the silicon crystal
  structure.

29
Silicon as a Mechanical Material
30
Silicon as an Electronic Material
31
Principles of Micromachining in Silicon

• Micromechanics in silicon is here defined as
  three-dimensional micromachining in single
  crystal silicon by means of photolithographic
  etching techniques.
• This definition covers most techniques used to
  make micromechanical sensors, although in some
  cases additive structures such as polysilicon and
  silicon dioxide also have been micromachined by
  selective etching techniques, and in some cases
  mechanical drilling or other machining methods
  are used.

32
Wet Chemical Etching of Silicon using Alkaline
Etchants

• The fundamental reactions are electrochemical in
  nature.
• Holes are injected from the etching solution into
  the silicon and Si-atoms are ionized to Si.
• Hydroxyl (OH-) from the etching solution reacts
  with Si to hydrated silicon.
• Hydrated silicon reacts with a complexing agent
  in the etching solution to form a soluble
  reaction product.
• The soluble reaction product is dissolved into
  the etching solution and carried away from the
  etching site on the silicon surface into the
  solution.
• All in all, silicon is etched and the reactant
  products are diluted into the etching solution.

33
Isotropic Etching of Silicon

• Typical wet isotropic silicon etches are either
  organic or inorganic acids such as acetic acid
  (CH3COOH) or hydrogenfluorid (HF) or mixtures
  together with water. Often a complexing agent is
  needed transforming the oxidized product into
  soluble species.
• By using selective etching techniques in
  combination with etching time some sort of
  dimensional control of the etched structure can
  be obtained. By using spray etching, agitation or
  light enhanced etching preferred etching
  directions can be obtained.
•  Generally, dimensional accuracy below
  approximately 30 µmeters are very hard to
  achieve, making wet isotropic etching a less
  favourable and less used method for
  micromechanics in silicon compared to anisotropic
  etching.

34
Isotropic Etching of Silicon

• This table shows some popular isotropic etches

35
A Typical Isotropic Etch Cavity

• Isotropic etch cavity in a silicon chip with a
  square masking film opening. The result is an
  underetched etch pit with rounded structures.

36
Anisotropic Etching of Silicon

• An anisotropic etching solution or orientation
  -dependent etching solution will attack the
  various crystal directions in single crystal
  silicon with different speed. Orientation effects
  during this type of preferential etch have been
  attributed to crystallographic properties. One
  explanation is that the atomic bonds in some
  planes are more exposed than in some others. A
  suitable designed etching agent will thus attack
  and strip away certain plane orientations more
  quickly than others.
• Typical wet anisotropic silicon etches are
  organic or inorganic alkaline solutions used at
  elevated temperatures, such as a mixture of
  ethylene diamine, pyrocatechol and water
  (EDP-etch) or potassium hydroxide and water
  (KOH-etch), or tetra-methyl-ammonium-hydroxide
  (TMAH).. Hydrazine-water mixture are also popular
  anisotropic silicon etchants. In the following
  table some examples of anisotropic etchants are
  given, including appropriate masking films.
• These typical anisotropic etching solutions are
  all characterized by an extremely slow etching
  speed in the lt111gt directions of single crystal
  silicon, as shown in the example given in the
  following figure.

37
Anisotropic Etching of Silicon
38
Anisotropic Lateral Etch Rate

• Lateral etch rate as a function of crystal
  direction on (110) silicon wafers for an
  EDP-etch.
• The composition of the etchant was 1l
  ethylene-diamine, 133 ml water, 160 gram
  pyrocatechol and 6 gram pyrazine.
• The dashed (111) directions are all equivalent
  with the (111) direction in single crystal
  silicon.

80 micrometer/hour is around 1.3 micrometer/min
39
Anisotropic Etch Cavity in (100) Silicon

• Anisotropic etch cavity in (100) silicon with a
  square masking film opening oriented in parallel
  with the lt110gt direction. Due to the four-fold
  symmetry of the slow-etching (111) planes,
  sideways etching is stopped giving a cavity with
  four sloped sidewalls. The photography shows such
  an etched cavity.

40
Understanding Anisotropic Underetching

• Anisotropic underetching of mask openings
  nonparallel with lt110gt direction, and anisotropic
  underetching of convex corners.
• (a) is a typical pyramidal pit, bounded by the
  (111) planes, etched into silicon with an
  anisotropic etch through a square hole in an
  oxide mask.
• (b) is a type of pit which is expected from
  anisotropic etch with a slow convex undercut
  rate.
• (c) is the same mask pattern resulting in an
  substantial degree of undercutting using an
  etchant with a fast undercut rate such as EDP.
• In (d), further etching of (c) produces a
  cantilever beam suspended over the pit.
• (e) is an illustration of the general rule for
  anisotropic etch undercutting assuming a
  "sufficiently long" etching time. The reader who
  understands (e) has understood the main
  principles.

41
Selective Etching of Silicon

• There are four different techniques in use
• Calculate the needed etching time on the basis of
  the etching speed of the used etch. This is an
  easy, but inaccurate method, as etching speed
  varies with the chemical condition of the etch
  and upon geometrical factors limiting the
  agitation of the etch. Typical accuracy 20
  micrometer.
• Inspect etch cavity depth in appropriate time
  intervals until needed depth is reached. More
  time consuming than the above method, but
  improved accuracy. Uneven etching depth from
  cavity to cavity due to chemical and geometrical
  factor is still a problem limiting accuracy,
  which is typical 10 micrometer.
• Chemical selective techniques stopping the etch
  when an impurity doped chemical resistive layer
  is reached. Accuracy is typical 3 micrometer.
• Electrochemical selective techniques stopping the
  etch towards a biased p-n junction. This enhances
  passivation very effectively, giving a typical
  accuracy of 1 micrometer.

42
Chemical Selective Etching of Silicon

• Chemical selective etching with EPD-etch as a
  function of boron doping concentration. The boron
  stop layer can be made by diffusion deposition or
  implantation on the opposite side of the wafer
  compared to the etch cavity, which are both well
  known processing techniques.

43
Epitaxial Layer Atop Boron Doped Stop Layer
n-type epitaxial layer
p type boron stop layer
n-type substrate

• The shortcoming of not being able to integrate
  electronics in the boron stop layer can be
  avoided by depositing an epitaxial layer atop the
  stop layer, with doping appropriate as substrate
  material for integrated devices.

44
A Typical Etching Dewar for Wet Chemical Etching
of Silicon
45
Electrochemical Selective Etching of Silicon

• Low-doped material can be passivated, both p-type
  and n-type. This gives more processing
  flexibility, and low-doped silicon can be used as
  substrate material for integrated components such
  as piezoresistors.
• High accuracy of thickness of unetched layer can
  be achieved, typical 1micrometer, by using
  well-controlled implantation and diffusion
  techniques for making the p-n- junction.
• This method makes KOH a useful selective etch,
  avoiding the health dangers of EDP-etch.

46
Surface Micromachining

• Surface micromachining can be defined as a set of
  methods to make three-dimensional surface
  structures, with deposition of thin films as
  additive technique and selective etching of the
  deposited thin films as subtractive techniques.
• In practice, single crystal silicon wafer is the
  dominant substrate material, and chemical vapor
  deposited (CVD) polysilicon is mostly used as the
  material making up the three-dimensional surface
  structures.

47
Surface Micromachining, continued

• A main advantage, compared to bulk
  micromachining, is that it does not need double
  sided processing (back side processing) of the
  wafers.
• The main additive deposition techniques are
  evaporation, sputtering, chemical vapor
  deposition (CVD), and variants of these.
• The main subtractive methods are selective wet
  etching and dry plasma etching.
• Photolitography is used for pattern definition.
• The use of sacrificial layers is important. With
  this method, etching of the sacrificial layers
  underneath non-etched thin film structures can be
  done. In this way several three-dimensional
  surface structures can be made, such as cavities,
  supported microbeams, microstrings, diaphragms,
  lateral mobile microelements etc.

48
Micrograph of a Surface Micromachined Structure

• Lateral mobile polysilicon microwheels on a
  silicon substrate fabricated by surface
  micromachining. Each wheels is free to rotate
  around its axis at the center of the stud
  element, which is fixed against the substrate and
  thus keeps the wheel in place. The wheels have
  gear teeth to show a possible gear function. 

49
Process Sequence

• The process sequence for fabricating laterally
  mobile elements, such as the microwheel shown in
  Photo XIII.2, is schematically depicted in Figure
  XIII.12.
• First (a), an oxide film is grown on the silicon
  wafer.
• Then (b), a polycrystalline film is deposited by
  chemical vapor deposition (CVD), and openings are
  defined and etched out using standard
  photolithography (c).
• A second oxide layer is deposited by CVD (d), an
  opening in the oxide is etched using a second
  lithographic mask, and a second polysilicon film
  is deposited and patterned with a third mask (e).
  
• Finally (f), the sacrificial oxide layers are
  removed by selective etching in hydrofluoric acid
  (HF), leaving the first polysilicon film free to
  move laterally, and the second polysilicon film
  as a supporting element fixed to the substrate. 

50
Process Sequence Diagram

• Figure XIII.12 Process sequence for the
  fabrication of laterally mobile structures using
  surface micromachining and sacrificial layer
  technique.

51
Examples of Sensor Elements Using Surface
Micromachining

• Sensor elements can be made by surface
  micromachining by either using thin films with
  sensing effects, such zinc oxide ZnO with
  piezoelectric field, or using mechanical sensing
  properties such as variable air gap elements
  and/or vibrating structures.
• An example of such a sensor, the Berkeley
  Polysilicon Microbridge Integrated Vapor Sensor.
  This sensor has a surface micromachined
  polysilicon microbridge. This sensor uses the
  vibrating structure sensing principle, with
  vibration activation and vibrating sensing by
  means of the capacitance between the bridge and
  the substrate. (Coulomb force activation and
  capacitance change sensing)

52
Anodic Wafer Bonding

• Can be defined as a method of electrostatically
  bonding two dissimilar materials together to form
  a strong, hermetic seal that involves little
  alteration in the shape, size, and dimensions of
  the members making up the joint.
• It is a high yield wafer-to-wafer sealing method
  that makes it possible to obtain hermetic seals.
  The technique was first developed for
  silicon-to-glass anodic wafer bonding, and has
  later been further developed to
  silicon-to-silicon anodic wafer bonding and
  silicon-to-thin film anodic wafer bonding.

53
Anodic Wafer Bonding Schematic View

• Schematic view of silicon-to-silicon anodic
  bonding and silicon-to-glass anodic bonding.

54
Example Digital Micromirror Device (DMD) from
Texas Instruments

• The device is using very advanced surface
  micromachining of thin Al alloys on Si substrates
  containing CMOS drive electronics

55
Picture of the packaged DMDs

• The DMDs are pixel devices
• Here are the VGA (640x480), the SVGA (800x600)
  and the XGA (1024x768) devices shown

56
Principle of Operation for the DMD

• The hinge system of each pixel structure enables
  electronic control mirror position.

57
Picture of Digital Micromirror Device

• The device is packaged in an elastomer connect
  package with a glass window. Here shown mounted
  on a PCB with back end drive electronics

58
The Davis DPX 16 Projector using the TI Digital
Micromirror Device

• XGA resolution (1024 x 768 pixels)
• 2.3 kg weight
• 1000 Lumens brightness

59
The Zeiss Optical Engine for the DP X16 Projector

• Advanced optics
• Small size and low weight

60
The Zeiss Optical Engine for the DP X16
Projector Modelling

• Mechanical modelling using ProEngineer Design
  Tools

61
Example The SA30 Crash Sensor from SensoNor

• This is a good example of the features of
  microsystems please refer to the separate slide
  presentation