Chapter 2, Microfabrication Electronics Manufacturing

1
Chapter 2, Microfabrication (Electronics
Manufacturing)
2
Learning Objectives

• Be able to describe the basic processes of
  microfabrication
• Be able to explain the principles of
  photolithography.
• Be able to describe the basic mechanisms of the
  additive processes, including relative
  comparisons among them.
• Physical Vapor Deposition (evaporation,
  sputtering)
• Chemical Vapor Deposition
• Be able to describe the basic mechanisms of the
  subtractive processes, including relative
  comparisons among them.
• Wet Etching (isotropic, anisotropic)
• Dry Etching (physical, chemical,
  physical-chemical)
• Be able to describe the process of bonding and
  packaging

3
What is LIGA?

• LIGA is a fabrication process for high aspect
  ratio microstructures consisting of three major
  process steps.
• X-ray lithography (LIlithographie) to generate
  primary microstructures.
• (DXRLdeep X-ray lithography,
  UDXRLultra-deep X-ray lithography)
• Electroplating/Electrodeposition (GGalvanik) to
  produce microstructures in metal.
• Molding (AAbformung) to batch produce secondary
  microstructures in polymers, metals, ceramics

4
Brief History of LIGA

• Electrodeposition and X-ray lithography
• Romankiw et al at IBM, 1975.
• Adding molding
• Ehrfeld et al at KfK, 1982.
• In Germany, LIGA was initially independent from
  semiconductor industry.
• Guckel (U. Wisc) integrated LIGA with existed
  semiconductor industry so that LIGA becomes one
  major micro fabrication process.
• Today, more and more prototypes have been
  fabricated. And some LIGA companies have been
  founded and provided professional services to
  users.

5
Why Is LIGA Interesting?

• Very high aspect ratio structures can be achieved
• Height(typical) 20-500 µm, this can not be
  achieved by state of art silicon surface
  micromachining
• Have more material selectivity in final products
• Could be metal, polymer, and even ceramics.
• Vertical and better surface roughness in sidewall
  
• Vertical slopelt1µm/mm,
• Surface roughness 0.03-0.05 µm(max. peak to
  valley)
• It can be applied in both MEMS and traditional
  precision manufacturing.

6
LIGA ProcessX-ray lithography
7
LIGA ProcessElectroplating
8
LIGA ProcessMolding
9
LIGA Materials

• Polymers
• Used in X-ray lithography period. The materials
  must be able to allow photochemical reaction
  under exposure under X-ray
• Thick PMMA is a popular choice
• Metals
• Used in electrodeposition phase to form a mould
• Nickel is a popular choice.
• Polymer again
• Used in molding period
• The final LIGA product is usually made of polymers

10
LIGA Applications
high aspect ratio microstructures
Micro turbine rotor, KfK
11
LIGA Applications
Spinneret Capillary, IMM
12
Problem Associated with LIGA

• You need to buy a big X-ray source
• expansive
• Need long time to exposure and develop
• Time consuming (several days)
• Other compete technology are rapidly catching up
• LIGA-Like
• Si DRIE technology
• The X-ray lithography process can be replaced to
  reduce the cost.

13
LIGA-Like Process Overview
14
DRIE System
Plasmalab System 100 Modular ICP-RIE Etching
System
Oxford Instruments
15
Overview of DRIE

• Reactive ion etching (RIE) is a dry etching
  method which combines plasma etching and ion beam
  etching principles.
• Choice for most advanced product line
• Its not well suitable for deep etching (gt10µm)
• Inductively coupled plasma (ICP) reactors have
  been introduced for silicon RIE process leading
  to the deep reactive ion etching (DRIE)
  technique.
• Higher plasma density
• Higher etching rate, either for anisotropic and
  isotropic etching
• Higher aspect ratio (AR)
• Reduction of parasitic effects

http//www.ipe.cuhk.edu.hk/
16
DRIE Applications
Use ICP etcher to create deep etching in silicon
substrate
http//www.ipe.cuhk.edu.hk/
17
DRIE Applications

• The silicon structures were part of an innovative
  escape mechanism of the Ulysse Nardin "Freak"
  watch. ( CSEM SA, IMT)

• Silicon parts
• Lighter
• Lower friction coefficient
• Insensitive to magnetic fields

18
Silicon Review

• In a perfect crystal, each of silicons four
  outer electrons form covalent bonds, resulting in
  poor electron mobility (i.e. insulating)
• Doping silicon with impurities alters electron
  mobility (i.e. semiconducting)
• Extra electron (N-type, with phosphorous, for
  example)
• Missing electron (P-type, with boron, for
  example)

19
Silicon Circuits

• Silicon makes the transistors
• Transistors can be made to very large scale
  integration (VLSI) and ultralarge scale
  integration (ULSI)

A Pentium 4 process contains 42 million
transistors
20
Silicon Micromachines

• The other application is micromachines, also
  called the microelectricmechanical system (MEMS),
  which have the potential of making the computer
  obsolete
• The micromachines include
• Fuel cells
• DNA chips

21
Microfabrication

• Silicon crystal structure is regular,
  well-understood, and to a large extent
  controllable.
• It is all about control the size of a transistor
  is 1 ?m, the doping must therefore less than have
  of that
• How to control?

22
Microfabrication Techniques
23
Process of Microfabrication
Single crystal growing
Wafer slicing
Film deposition
Oxidation
Diffusion
Ion implantation
Etching
Lithography
Metallization
Bonding
Packaging
Testing
24
Crystal Growing
10 ?m/s

• Silicon occurs naturally in the forms of silicon
  dioxide and various silicates and hence, must be
  purified
• The process of purifying silicon
• Heating to produce 95 98 pure polycrystalline
  silicon
• Using Czochralski (CZ) process to grow single
  crystal silicon

1 rev/s
Liquid silicon
Illustration of CZ process
25
Crystal Growing
26
Wafer Slicing

• This step includes
• Slice the ingot into slices using a diamond saw
• Polish the surface, and
• Sort

27
Film Deposits

• This step is used to add a special layer on the
  surface of the silicon for masking
• Many types of films are used for insulating /
  conducting, including polysilicon, silicon
  nitride, silicon dioxide, tungsten, and titanium.
• Films may be deposited using various method,
  including
• Evaporation
• Sputtering
• Chemical Vapor Deposition (CVD)

28
Film Deposits

• The process of CVD
• Continuous, atmospheric-pressure CVD
• Low-pressure CVC

29
Photolithography

• Photolithography is a process by which an image
  is optically transferred from one surface to
  another, most commonly by the projection of light
  through a mask onto a photosensitive material.
• Photoresist is a material that changes molecular
  structure when exposed to radiation (e.g.
  ultraviolet light). It typically consists of a
  polymer resin, a radiation sensitizer, and a
  carrier solvent.

30
Photolithography

• Adding a photoresist layer on the wafer
• A photomask is typically manifested as a glass
  plate with a thin metal layer, that is
  selectively patterned to define opaque and
  transparent regions.

31
Photolithography

• A positive photoresist is weakened by radiation
  exposure, so the remaining pattern after being
  subject to a developer solution looks just like
  the opaque regions of the mask

A negative photoresist is strengthened by
radiation exposure, so the remaining pattern
after being subject to a developer solution
appears as the inverse of the opaque regions of
the mask.
32
Etching Mask for a Mask
Reusable mask
Photoresist coating
Functional mask

• The mask must withstand the chemical environment.
• A typical mask/substrate combination is oxide on
  silicon.
• Resilient masks are typically grown or deposited
  in whole films, and must therefore be patterned
  through a photosensitive mask

33
Dry Etching Mechanisms

• Physical
• Removal based on impact momentum transfer
• Poor material selectivity
• Good directional control
• High excitation energy
• Lower pressure, lt100 mTorr
• Chemical
• Highest removal rate
• Good material selectivity
• Generally isotropic
• Higher pressure, gt100 mTorr
• Physical/Chemical
• Good directional control
• Intermediate pressure, 100 mTorr

34
Isotropic Wet Etching

• Etch occurs in all crystallographic directions at
  the same rate.
• Most common formulation is mixture of
  hydrofluoric, nitric and acetic acids (HNA HF
  HNO3 CH3COOH).
• Etch rate may be very fast, many microns per
  minute.
• Masks are undercut.
• High aspect ratio difficult because of diffusion
  limits.
• Stirring enhances isotropy.
• Isotropic wet etching is applicable to many
  materials besides silicon

35
Anisotropic Wet Etching

• Etch occurs at different rates depending on
  exposed crystal
• Usually in alkaline solutions (KOH, TMAH).
• Heating typically required for rate control (e.g.
  gt 80 oC).
• Etch rate typically 1 µm/min, limited by
  reactions rather than diffusion.
• Maintains mask boundaries without undercut.
• Angles determined by crystal structure (e.g.
  54.7º).
• Possible to get perfect orthogonal shapes
  outlines using 1-0-0 wafers.

36
Etching a Comparison

• ANISOTROPIC
• Predictable profile
• Better depth control
• No mask undercutting
• Possibility of close feature arrangement

• ISOTROPIC
• Wide variety of materials
• No crystal alignment required
• May be very fast
• Sometimes less demand for mask resilience

Multiple layers are common
37
Etching a Comparison
38
Ion implantation

• Ion implantation is used to alter the electrical
  characters of the silicon in specific regions.
• The process

39
Ion implantation

• The main disadvantage is that it can only process
  a wafer at a time
• Diffusion

40
Bonding and Packaging

• Wires (?25 ?m) are bonded to package leads
• The bond wires are attached using
  thermocompression, ultrasonic, or thermosonic
  techniques

41
Bonding and Packaging

• Packaging is done by surface mount technology