1. 서강대학교 기계공학과 최범규(Choi, Bumkyoo)
Micromachining 기술 I
(Silicon Based)
서강대학교 기계공학과
최범규(Choi, Bumkyoo)

2. Oxidation Fundamental process in all silicon device fabrication
Used for
(i) passivation of the silicon surface (i.e., the formation of a chemically and electronically stable surface)
(ii) masking of diffusion and ion implantation (0.5-1µm)
(iii) dielectric films (50-200Å)
(iv) an interface layer between the substrate and other materials

3. A native oxide : about 20Å thick
Process parameter : temp. time
Si+O2  SiO2 for dry oxygen
Si+2H2O  SiO2+2H2 for water vapor

4. The oxide expands to fill a region approximately
SiO2
Original surface
0.54 tox
0.46 tox
Si wafer
The oxide expands to fill a region approximately
54% above and 46% below the original surface of
the wafer

5. Diffusion
The deposition of a high concentration of the desired impurity on the silicon surface
At high temperature ( C), the impurity atoms move from the surface into the silicon crystal

6. Without vacancy “rapid”
Substitutional diffusion
Interstitial diffusion
Interstitial
impurity atom
Substitutional
impurity atom
vacancy
Need vacancy
“slow”
“good”
Without vacancy “rapid”
Speed
Control
* donor 나 acceptor가 되기 위해서
반드시 substitutional site를 차지 해야 함.
Need activation  annealing

7. Governing equation Fick’s first law
J : the particle flux (particle flow/unit area)
N : the particle concentration
Fick’s second law (from the continuity equation)
(*)
CSD : the impurity concentration is held constant at the
surface of the wafer
LSD : the dose Q remains constant throughout the process

8. Constant-Source Diffusion Limited-Source Diffusion
Sol’n
of Eq(*)
No : impurity concentration
at wafer surface(x=0)
Q0 : total # of impurity atom per unit area (dose) Dt
“erfc”
Impurity
concn. N(x)
Distance from surface x
distribution
“Gaussian” x
No1
No2
No3
Dose (Q)

9. Practical diffusion Two-step diffusion
 Predeposition of impurity source (CSD) : D1t1
 Impurity drive-in (LSD) : D2t2
If D1t1 >>D2t2, the resulting impurity profile is approximated by erfc
D1t1 <<D2t2, the resulting impurity profile is approximated by Gaussian

10. Ion Implantation
Ion implanter : a high-voltage particle accelerator producing a high velocity beam of impurity ions which penetrate the surface of Si wafer
Mass
spectrometer
Ion source
Accelerator
X-axis scanner
Y-axis scanner
Target

11. Ion source : high voltage (25kV) produces a plasma containing the desired impurity as well as other undesired species
Mass spectrometer : an analyzer magnet bends the ion beam through a right angle to select the desired impurity ion
High-voltage accelerator : adds energy to the beam (∼175kV) and accelerates the ions to their final velocity
Scanning system : X-and Y-axis deflection plates are used to scan the beam across the wafer to give a uniform implantation and the desired dose

12. Advantage (cf. Diffusion)
1. Low temperature
2. Process flexibility
mask material (SiO2, PR, Nitride, Metals)
wide range of impurity species
3. Process stability
dose control
pure dopant species (mass spectrometer)
Disadvantage
expensive, bulky facility (1-2 million),
source damage

13. Thin film deposition 1. Evaporation
The desired metals are heated to the point of vaporization and evaporate to form a thin film (high temp., low pressure)
E-beam
heating
target

14. 2. Sputtering (Physical)
Sputtering is achieved by bombarding a target with energetic ions. (보통 Ar+)
Atoms at the surface of the target are knocked loose and transported to the substrate, where deposition occurs (metal and nonmetal alloy)
Ar+
target
source

15. 3. CVD (Chemical Vapor Deposition)
CVD forms thin films on the surface of a substrate by thermal decomposition and/or reaction of gaseous compounds. The desired material is deposited directly from the gas phase onto the surface of the substrate
To exhaust
system
Wafers
Susceptor N2 H2
SiCl4+ H2
HCl
Dopant+ H2

16. 4. Epitaxy
CVD processes can be used to deposit silicon onto the surface of a Si wafer under appropriate conditions, the Si wafer acts as a good crystal, and a single-crystal silicon layer is grown on the surface the wafer

17. Step coverage CVD > Sputtering > Evaporation CVD Evaporation
source
CVD
Evaporation
Sputtering
CVD > Sputtering > Evaporation

18. Bulk-Micromachining Silicon Bulk-micromachining Two key capabilities
Wet (chemical) or dry(plasma) etching of silicon substrate, in combination with masking film (etch-resistant layers) to form micromechanical structures
Two key capabilities
1. Anisotropic etchants of silicon : EDP (ethylen-diamine and pyrocatechol), KOH preferentially etch single crystal silicon along given crystal planes
2. Etch masks and etch-stop techniques are available in conjunction with silicon anisotopic etchants

19. Isotropic etching : uniform at all orientations
Anisotropic etching : depend on crystal orientation
Isotropic with agitation
Isotropic w/o agitation
<100>
<110>
54.75°
Anisotropic on (100) surface
Anisotropic on (110) surface

20. Etchant and Etch masks 1) Isotropic etchant
HF : HNO3 : CH3COOH = 7 : 3 : 1 (HNA system)
mask-SiO2, Si3N4
2) Anisotropic etchant
EDP (Ethylene Diamine and Pyrocatechol)
KOH
* Etch Rate
R(100) R(110) R(111) mask
EDP : : SiO2(2Å/min),Si3N Au,Cr,Ag,Cu,Ta
KOH : : Si3N4, SiO2 (14Å/min)
* KOH 사용시 EDP 때보다 Oxide의 두께는 훨씬
두꺼워야 한다

21. Etch rate
1) Agitation increases the supply of reactant material to the surface, thus increasing the etch rate (stirring)
2) Temperature
3) The supply of minority carriers to the surface
4) Creation of electron-hole pairs on the surface (by illumination or currents)
5) Substrate-crystal orientation, type and concentration of doping atoms, lattice defects, and surface structure

22. Etch stop & thickness control
1. Dopant dependent etch stop
The etching process is fundamentally a charge-transfer mechanism and etch rates depend on dopant type and concentration
Highly doped material might be expected to exhibit higher etch rate because of the greater availability of mobile carriers
etch rate 1-3 µm/min at p or n 1018/cm3
essentially zero less than 1017/cm3
Si heavily doped with boron (7x1019cm-3) reduce the etch rate by about 1/20 with KOH & EDP (p+)
2. Electrochemical etch stop
3. p-n junction etch stop
4. Various etch stop method (case by case)

23. Dry etching
Involves the removal of substrate materials by gaseous etchants without wet chemicals or rinsing
Plasma etching
Plasma is a neutral ionized gas carrying a large number of free electrons and positively charged ions
A common source of energy for generating plasma is a RF source
The process involves adding a chemically reactive gas such as CCl2F2 to the plasma
The reactive gas produces reactive neutrals when it is ionized in the plasma
The reactive neutrals bombard the target on both the sidewalls as well as the normal surface, whereas the charged ions bombard only the normal surface of the substrate
Etching of the substrate materials is accomplished by the high energy ions in the plasma bombarding the substrate surface with simultaneous chemical reactions between the reactive neutral ions and the substrate material

24. Dry etching (2) Plasma etching (cont’d)
The high energy reaction causes local evaporation, and thus results in the removal of the substrate material
Conventional dry etching is a very slow process at a rate of about 0.1 mm/min or 100 A/min
Plasma etching can increase the etching rates in the order of 2000A/min by increasing the mean free path of the reacting gas molecules in the depth to be etched
Plasma etching is normally performed in high vacuum
Dry etching is typically faster and cleaner than wet etching
Like wet etching, it also suffers the shortcoming of being limited to producing shallow trenches
For dry etching, the aspect ratio is less than 15
Another problem is the contamination of the substrate surface by residues

25. Deep Reactive Ion Etching (DRIE)
Despite the significant increase in the etching rate and the depth of the etched trench that can be achieved with the use of plasma, the etched walls in the trenches remain at a wide angle (q) to the depth
DRIE is a process that produces the high aspect ratio MEMS with virtually vertical walls ( )
The DRIE process differs from plasma etching in that it produces thin protective films of a few micrometers on the sidewalls during the etching process

26. Deep Reactive Ion Etching (DRIE)
It involves the use of a high density plasma source, which allows alternating processes of plasma (ion) etching of the substrate material and the deposition of etching protective material on the sidewalls
Suitable etching protective materials is SiO2. Polymers are also frequently used for this purpose
Recent developments have substantially improved the performance and Si substrates with A/P over 100 was achieved with

27. Surface - Micromachining
Machining of thin film to make microstructures
cf) Bulk-micromachining : Silicon wafer
Surface-micromachining : Thin film on Si wafer / free
standing from substrate
IC-fabrication : Surface of Si wafer / fixed on substrate
*The ultimate success of surface-micromachined devices are determined by the availablility of thin films of high quality and good stability

28. 1. Thin film deposition 2. Thin film patterning Evaporation Sputtering
CVD
2. Thin film patterning
Subtractive process : Wet chemical etching
Dry etching
cf) Bulk micromachining 에서는 (orientation-dependant) 결정면에 따라 etching 속도가 다른 것을 이용
Surface micromachining 은 material-dependent (selective etching)
Additive process : Wet electroplating
Dry lift off

29. Subtractive : film 올리고 필요 없는 부분 제거
Additive : 필요한 부분만 film 생성 PR
metal PR
Electroplating
Lift-off

30. Etchants

31. Sacrificial Materials
Thin film materials
1. Poly-Si and amorphous Si
2. Single-crystal Si
3. Si nitride (Si3N4)
4. Si dioxide (SiO2)
5. Organic films : Polymide
Sacrificial Materials
1. LTO (Low-Temperature Oxide)
2. PSG (Phosphorsilicate glass)
3. Polymide or photoresist

32. Surface-Micromachining Sample