Physical Deposition- Sputtering

Physical Deposition- Sputtering
Breakdown voltage: Paschen law
V
Pd
ln( Pd )  b
P: pressure, d: electrode spacing
d
Threshold energy ~10-30 eV
V32
Ion flux J 
mion d 2
• Less directional coverage
Angular dependence
• No melting point limit
• Inert gas pressure ~0.1 torr
• d~ 10 cm
• Maximum yield E~ 1 keV
• E > 10 keV → implantation
Ref: Campbell: 10.6
5-5
Ref: Campbell: 13.8
CVD Metal Deposition
• Plug can reduce contact area
• Cold wall LPCVD
WF6  3H 2  W  6HF
5-6
SiO2 Etch
• H2O:HF = 6: 1 etches thermal oxide at 1200Å/min
SiO2  6HF  H 2 SiF6  2H 2O
• Selectivity: etch rate of SiO2/Si =100
• Buffered HF: maintain HF and pH
NH 4 F  NH 3  HF
NH 4 F : HF  6 : 1
At room temperature, Water: BHF=20:1
etch rate: thermal oxide:300Å/min, Si3N4: 10Å/min
Aluminum Etch
CH3COOH:H3PO4:HNO3 = 20:77:3
acetic
acid
phosphoric
acid
nitric
acid
5-7
Isotropic Silicon Etch
•
HNO3 oxidizes Si and HF etches the SiO2 hereby formed
Si  HNO3  6HF  H 2 SiF6  HNO2  H 2  H 2O
• HNO3 (70%): HF (49%): CH3COOH = 20: 60: 20 produces an etch rate of 165 µm/min
Region 1: High HF concentrations
• Reaction limited by HNO3 , follow constant
HNO3% lines.
• Rate limited by oxidation, etched wafer
surface have some oxide.
1
2
Rate increase
Ref: Campbell: 11.1
Region 2: High HNO3 concentrations
• Reaction limited by HF, follow
constant HF % lines.
• Rate limited by reduction, etched
wafer surface have more oxide.
5-8
Anisotropic Silicon Etch
• Strong alkaline solutions (pH>12) such as KOH and TMAH (tetramethylammonium
hydroxide N(CH3) 4OH•5H2O) etch Si via
𝑆𝑖 + 4𝑂𝐻− → 𝑆𝑖 𝑂𝐻
KOH (6 M, or 25.4 wt%)
4
+ 4𝑒 −
• SiO2 or Si3N4 are the preferred masking materials; Au can be a mask too.
Etch rate:
Si(100)=13 µm/h
SiO2 will also be etched
Etch rate ratios in different crystal orientations:
Si(100)/Si(111)=300,
Si(110)/Si(111)=600
[100]
[110]
Si(111)
PR72-09
5-9
Anisotropic Silicon Etch
PR69-17
PR69-38
PR69-18
PR69-23
PR69-30
PR74B-01
5-9
Reactive Ion Etch (RIE)
• Reduce wet chemical waste, enhance anisotropic etching
• Capacitively coupled plasmas (DC, RF: 50 kHz, 13.56 MHz), Inductively coupled
plasmas (ICP), Electron cyclotron resonance (ECR: 2.45 GHz microwave)
Barrel reactor: uniformity issue, lack of
temperature control
Downstream reactor
Parallel plate reactor
• Gases: O2 , CF4, CCl4 , CHF3, CCl3F, CCl2F2, SF6
• Volatile products removed from surface.
5-10
Reactive Ion Etch (RIE)
• DC bias generated:
𝐴𝑔𝑟𝑜𝑢𝑛𝑑
𝑉𝐷𝐶 ~
𝐴𝑝𝑜𝑤𝑒𝑟
plasma
𝑛
0V
• RIE-1C
Dark sheath
13.56 MHz, 75-225 mTorr gas pressure
-VDC
Power: increase energy and density of electrons
• Etch and passivation: SF6/O2 etch Si
Si  F   SiF
SF6  e  SF5  F  e
SiF  F   SiF2
SiF3  F   SiF3 g  SiF2  SiF2  SiF4  g   Si
SF5  e  SF4  F  e
O2  e  O  e
O  Si  SiOx
SiOx  F   SiFx  SiOx Fy
• Polymer formation: can affect profile
• Selectivity
• Increase vertical wall profile needs long mean free path or low pressure but
denser plasma: ICP operate a 0.1-1 mTorr
5-11
SF6/O2 RIE
SF6:O2=10:5 sccm
SF6:O2=20:5 sccm
KOH: 32 min, 50 °C
SF6:O2=18:5 sccm
0.16 Torr, 50W, 10 min
0.22 Torr, 50W, 7 min
0.18 Torr, 50W, 2 min
SiO2
SiO2
SiO2
Si
5.4 µm
Si
PR9315-B5_02
PR76I6_06
SF6:O2=20:5 sccm
SF6:O2=10:10 sccm
0.22 Torr, 50W, 7 min
0.15 Torr, 50W, 1 min
PR76N6_40
Black Si formation
SiO2
SiO2
2.7 µm
Si
Si
PR76P8_07
PR93-02
PR93-05
5-12
Kinetic Theory of Gases
Ref: Campbell: 10.2
• Most gas systems can be treated as ideal gas
• Probability of finding a gas molecule with a speed v and v+dv per unit volume is given by
Maxwell distribution
vave
vmax
32
 m  2 mv 2 kT
vrms
Pv dv  4 
dv
 ve
 2kT 
2

v   Pv vdv 
0
vrms  v 2 
vmax 
8kT
m
3kT
m
2kT
m
• Number of particles hitting a surface per unit area per unit time J  14 nv
Ideal gas law
p  nkT
At 300K and 760 torr, N2
J
p
2mkT
v  500 m / s
J  31023 cm 2 s 1
6-1
Ref: Campbell: 10.2
Conductance of Gas Flows
Viscous flow regime: dimensions of the container > l (mean free path) d: diameter of molecule
Molecular flow regime: dimensions of the container < l
l~
1
kT

2nd 2
2 Pd 2
5
At 760 torr, 300 K, l ~ 3 10 cm, so it is in viscous flow, small tubing is Ok.
But P < 10-2 torr, l  2.3 cm, it is in molecular flow regime, large inlets are necessary.
Conductance of a long round tube of diameter D and length L in the molecular flow
regime at 300 K is C  121D3 L m3 sec
Conductance of an orifice of area A in the molecular flow regime at 300 K is
C  116 A
m
3
sec

Conductance of a series of tubing and inlets
1 1
1
1
 

 ...
C C1 C2 C3
The total conductance is less than that of the smallest component.
6-2
Vacuum Pumps
Ref: Campbell: 10.3
Rough
Rough vacuum: atm to 110-3 torr
medium
Mechanical pump: Oil change, gas ballast
Oil backstreaming
Molecular sieve trap
High vacuum: 110-3 to 110-8 torr
Turbomolecular pump:
Needs a backing pump
Don’t vent through backing pump
Vent when the rotor is spinning
Air cool or water cool
Ultrahigh vacuum: <110-8 torr
Ion pump:
Needs forepump, needs bakeout, no moving parts
Titanium sublimation pump:
No moving parts, non-continuous pumping
6-3
Vacuum Gauges
Ref: Campbell: 10.4
Thermocouple : 10-110-3 torr
Convectron gauge: 103 to 110-4 torr
Ionization gauge: 10-3-110-10 torr
Capacitance manometer
25,000 to 0.1 torr
6-4
Vacuum Seals
KF/QF/NW (Klein Flansche) flanges:
atm to 110-8 torr , 0-150 °C
Compression fitting; Swagelok/Yor-lok
Parker A-lok
Back ferrule
Front ferrule
NPT
ConFlat (CF) flanges: atm to 110-13 torr, -196 - 450°C
Cajon -VCR
gasket
F-nut
gland
6-5
Vacuum Compatible Materials
Residual gas in a stainless steel UHV chamber –Residual Gas Analyzer
JAP 42, 1208 (1971)
Common materials for UHV
OFHC copper
304, 316 stainless steel
6061, 5052, 4043 Aluminum
Kovar/Invar: Fe-Ni alloy
Fused silica (< 1580 °C)
Al2O3 Alumina (< 1700 °C)
Macor (< 800 °C)
Low vacuum elastomers
• Low vapor pressure; no Pb, Zn, Cd, Se, S (SS303)
Teflon, viton (< 150 ° C)
• Low gas permeability: especially for H2
Buna-N (<80 ° C)
Al< Mo<Ag<Cu<Pt<Fe<Ni<Pd
• Low outgassing: 18-8 SS has Cr2O3 barrier
6-6