front end processes - cleaning, lithography, oxidation ion implantation, diffusion, deposition and...
TRANSCRIPT
FRONT END PROCESSES - CLEANING, LITHOGRAPHY, OXIDATIONION IMPLANTATION, DIFFUSION, DEPOSITION AND ETCHING
• Cleaning belongs to front end processes and is an important part of fabrication.• Reference - ITRS Roadmap for Front End Processes (class website).
Chapter 4
Semiconductor ManufacturingClean Rooms, Wafer Cleaning and Gettering
Importance of Importance of unwanted impurities unwanted impurities increases with increases with shrinking geometries of shrinking geometries of devices. devices.
75% of the yield loss is 75% of the yield loss is due to defects caused due to defects caused by particles (1/2 of the by particles (1/2 of the min feature size)min feature size)
Crystal originated (45-Crystal originated (45-150nm) particles (COP) 150nm) particles (COP) ~1,000Å=void with ~1,000Å=void with SiOSiOx x -> affect GOI-> affect GOI
-> anneal in H-> anneal in H22 -> ->
oxide decomposes and oxide decomposes and surface reconstructs! & surface reconstructs! & oxide precipitates from oxide precipitates from deep depth in Si.deep depth in Si. Yield -> 90% at the end -> 99% @ each step
Historical Development and Basic Concepts
Contaminants and their role in devices (various elements, various films)
Na+, Ka+
XOX ~10nm QM≈ 6.5x1011cm-2, VTN=0.1V (equivalent to 6.7*1017 cm-3 or 10 ppm contaminations)
! !
!!Life time killers
Poly-Si, silicides
Particles cause defects
QM
SEMICONDUCTOR MANUFACTURING - CLEAN ROOMS, WAFER CLEANING AND GETTERING
• Modern IC factories employ a three tiered approach to controlling unwanted impurities: 1. clean factories 2. wafer cleaning 3. gettering
Silicon Wafer
SiO2
or other thin films
Photoresist
Au
Cu
Fe
Particles
Interconnect Metal
Na
N, P
• Contaminants may consist of particles, organic films (photoresist), heavy metals or alkali ions.
Year of Production 1998 2000 2002 2004 2007 2010 2013 2016 2018
Technology Node (half pitch) 250 nm 180 nm 130 nm 90 nm 65 nm 45 nm 32 nm 22 nm 18 nm
MPU Printed Gate Length 100 nm 70 nm 53 nm 35 nm 25 nm 18 nm 13 nm 10 nm
DRAM Bits/Chip (Sampling) 256M 512M 1G 4G 16G 32G 64G 128G 128G
MPU Transistors/Chip (x106) 550 1100 2200 4400 8800 14,000
Critical Defect Size 125 nm 90 nm 90 nm 90 nm 90 nm 90 nm 65 nm 45 nm 45 nm
Starting Wafer Particles (cm-2 ) <0.35 <0.18 <0.09 <0.09 <0.05 <0.05
Starting Wafer Total Bulk Fe (cm-3 ) 3x1010 1x1010 1x1010 1x1010 1x1010 1x1010 1x1010 1x1010 1x1010
Metal Atoms on Wafer Surface
After Cleaning (cm-2 )
5x109 1x1010 1x1010 1x1010 1x1010 1x1010 1x1010 1x1010 1x1010
Particles on Wafer Surface After
Cleaning (#/wafer)
75 80 86 195 106 168
2003 ITRS Front End processes - see class website
Up till 2018
Dynamic Random Access Memory
Leakage currents discharge the capacitor (mechanism SRH) refresh the charge storage (time ~ a few msec)
Deep-level traps (Cu, Fe, Au etc.)
Pile up at the surface where the devices are located.
Lifetime must be > ~ 25 µsec
Use gettering to keep Nt <1012 cm-3 (Nt< ppb) -> G≈100µsec
write, read Vth~107cm/sec
~10-15cm-2
Role of Surface Cleaning in Processing
Oxide thickness [Å] Residual contaminants, layers affect
kinetics of processes.
Surface effects are very important (MORE) in scaled down devices
Level 1 Contamination Reduction: Clean Factories
• Air quality is measured by the “class” of the facility.
(Photo courtesy of Stanford Nanofabrication Facility.)
Factory environment is cleaned by: • Hepa filters and recirculation for the air, • “Bunny suits” for workers. • Filtration of chemicals and gases. • Manufacturing protocols.
Level 1 Contamination Reduction: Clean Factories
Class 1-100,000 mean number of particles, greater than 0.5 m, in a foot of air
Particles ---> people , machines, supplies
suits Material filters Chemicals, water (use DI)
Small particles remain in air (long) coagulate large ones precipitate quickly and deposit on surfaces by (small) Brownian motion and gravitational sedimentation (larger).
Use local clean rooms
from
Ex. Class 100 -> 5 particles/cm, >0.1 µm in 1hr.
Level 2 Contamination Reduction: Wafer Cleaning
Front End Process
Back End
Oxygen plasma
Organic strippers
(do not attack metals)
5 H20 + H2O2 + NH4OH SC1Oxidizes organic films
Oxidizes Si and complexes metals
6H2O : H2O2 : HCl SC2
Small content reduces Si etch (0.05%)
Removes alkali ions & cations Al3+, Fe3+, Mg3+ (insoluble in NH4OH - SC1)
H2S04 +H2O2
Oxygen plasma
Ultrasonic and now megasonic cleaning for particulates removal (20-50 kHz)
Good clean for high T steps Low T - less critical
DI water is necessary: H2O<-> H++OH- [H+]=[OH-]=6x10-13cm-3
Diffusivity of:H+≈9.3x10-5cm2s-1 -> µH+=qD/kT=3.59cm2V-1s-1
of :OH-≈5.3x10-5cm2s-1 -> µOH-=qD/kT=2.04cm2V-1s-1
€
ρ =1
q([H+ ] H+ + [OH−] OH− )=18.5MΩcm
Level 2 Contamination Reduction: Wafer Cleaning
• RCA clean is “standard process” used to remove organics, heavy metals and alkali ions.
• Ultrasonic agitation is used to dislodge particles.
120 - 150˚C
10 min
Strips organics
especially photoresist
DI H2
O Rinse Room T
80 - 90˚C
10 min
Strips organics,
metals and particles
DI H
2
O Rinse Room T
80 - 90˚C
10 min
Strips alkali ions
and metals
Room T
1 min
Strips chemical
oxide
DI H2
O Rinse Room T
H2
SO4
/H2
O2
1:1 to 4:1
HF/H2
O
1:10 to 1:50
NH4
OH/H2
O2
/H2
O
1:1:5 to 0.05:1:5
SC-1
HCl/H2
O2
/H2
O
1:1:6
SC-2
with all contaminants -> H passivation (or F!)
NH4OH small -> reduce surface roughness
Not removed by SC1
HF dip added to remove oxide
Level 3 Contamination Reduction: Gettering• Gettering is used to remove metal ions and alkali ions from device active regions.
H
1.008
1
3 4
11 12
19 20
Li
6.941
Be
9.012
Na
22.99
Mg
24.31
K
39.10
Ca
40.08
Rb
85.47
Cs
132.9
Fr
223
Sr
87.62
Ba
137.3
Ra
226
37 38
55 56
87 88
Sc
44.96
Ti
47.88
V
50.94
Cr
51.99
Mn
54.94
Fe
55.85
Co
58.93
Ni
58.69
Cu
63.55
Zn
65.39
21 22 23 24 25 26 27 28 29 30
Y
88.91
Zr
91.22
Nb
92.91
Mo
95.94
Tc
98
Ru
101.1
Rh
102.9
Pd
106.4
Ag
107.9
Cd
112.4
39 40 41 42 43 44 45 46 47 48
La
138.9
Hf
178.5
Ta
180.8
W
183.9
Re
186.2
Os
190.2
Ir
192.2
Pt
195.1
Au
197.0
Hg
200.6
57 72 73 74 75 76 77 78 79 80
Ac
227.0
Unq
261
Unp
262
Unh
263
Uns
262
89 104 105 106 107
B
10.81
Al
26.98
Ga
69.72
In
114.8
Tl
204.4
C
12.01
Si
28.09
Ge
72.59
Sn
118.7
Pb
207.2
N
14.01
P
30.97
As
74.92
Sb
121.8
Bi
209.0
O
16.00
S
32.06
Se
78.96
Te
127.6
Po
209
F
19.00
Cl
35.45
Br
79.90
I
126.9
At
210
He
4.003
Ne
20.18
Ar
39.95
Kr
83.80
Xe
131.3
Rn
222
5 6 7 8 9 10
2
13 14 15 16 17 18
31 32 33 34 35 36
49 50 51 52 53 54
81 82 83 84 85 86
Period
1
2
3
4
5
6
7
I
A
II
A
III
B
IV
B
V
A
I
B
II
B
III
A
IV
A
V
B
VI
B
VII
B
VIII
VI
A
VII
A
Noble
Gases
Shallow Donors
Shallow Acceptors
Elemental
Semiconductors
Deep Level Impurites in Silicon
Alkali Ions
• For the alkali ions, gettering generally uses dielectric layers on the topside (PSG or barrier Si3N4 layers).• For metal ions, gettering generally uses traps on the wafer backside or in the wafer bulk.• Backside = extrinsic gettering. Bulk = intrinsic gettering.
Gettering Concepts: contaminants freed diffuse become trapped
Fast Diffusion of Various Impurities
Metal contaminants will be trapped by dislocations and SF (decorate) and far away from ICs
PSG (for alkali ions Na+, K+ and metals) affects E fields (dipoles in PSG) and absorbs water leading to Al corrosion (negative effects)
or Si3N4
Closer to devices than to a backside layer -> high efficiency
metals
Devices in near
surface region
Denuded Zone
or Epi Layer
Intrinsic
Gettering
Region
Backside
Gettering
Region
10 - 20 µm
500+
µ m
PSG Layer
.
10-20
10-18
10-16
10-14
10-12
10-10
10-8
10-6
0.65 0.7 0.75 0.8 0.85 0.9 0.95 11000/T (Kelvin)
SiAs
B, P
AuS
CrPt
FeAu
I
Cu1200 1100 1000 900 800
T (˚C)
InterstitialDiffusers
Dopants
TiDiffusivity (cm2 sec-1)
I Diffusivity
10-4
• Heavy metal gettering relies on: • Metals diffusing very rapidly in silicon. • Metals segregating to “trap” sites.
.
500
700
900
1100
Temperature ̊ C
Time
Outdiffusion
Nucleation
Precipitation
• “Trap” sites can be created by SiO2 precipitates (intrinsic gettering), or by backside damage (extrinsic gettering).
• In intrinsic gettering, CZ silicon is used and SiO2 precipitates are formed in the wafer bulk through temperature cycling at the start of the process.
StackingFault
V I
OI Diffusion
[OI]
SiO2
OI
OI
OIOI
OI SiO2
SiO2 precipitates (white dots) in bulkof wafer.
Intrinsic Gettering Oxygen ~ 1018 cm-3; 15-20 ppm
Oi>20ppm -> too much precipitation-> strength decreases and warpage increases
Oi<10ppm -> no precipitation-> no gettering
denuded zone = oxygen free; thickness several tens of µm 50-100 µm in size
€
D0 = 0.13exp−2.53
kT
⎛
⎝ ⎜
⎞
⎠ ⎟cm2 sec−1
Slow ramp
1-3 nm min size of nuclei, concentrations ≈ 1011cm-3
>> Ddopants but D0<< Dmetals
Intrinsic Gettering Due to Oxide Precipitates
Precipitates (size) grow @ high T
Density of nucleation sites grow @ low T
The largest & the most dense defects -> the most efficient gettering
Measurement Methods
Clean factories = particle control. Detect concentrations < 10/wafer of particles smaller than 0.1 µm
Unpatterened wafers (blank)• Count particles in microscope• Laser scanning systems -> maps of particles down to ≈ 0.2 µm
Patterned wafers• Optical system compares a die with a “known good reference” die (adjacent die, chip design - its appearance) • Image processing identifies defects (SEM)• Test structure (not in high volume manufacturing)
Test Structures
Trapped charge
QT VTH change
Dielectric breakdown due to particles, metals etc.
Water – measure water resistivity Deionized Water ρ=18.5 MΩ
H2O H+ + OH-
€
ρ =1
q μH + ⋅N H + + μ
OH − ⋅NOH +( )
Models relate type of defects (typical for processes) with yields
Monitoring the Wafer Cleaning Efficiency
Concentrations of impurities determined by surface analysis
Primary beam – e - good lateral resolution
Detected beam – e – good depth resolution and surface sensitivity
X-ray poor depth resolution and poor surface sensitivity
ions (SIMS) excellent
ions (RBS) good depth resolution, reasonable sensitivity (0.1 atomic%)
works with SEM He+ 1-3 MeV
O+ or Cs+ sputtering and mass analyses
•Excite•Identify (unique atomic signature)•Count concentrations
emitted
Electrons in Analytical Methods
Inelastic collisions with target electrons, which
are then emitted from the solid
Elastic collision of incoming electrons with atoms (reflected back) ~ the same energy as for the incoming electrons
~ 5 eV
(as in SEM)
Analytical Techniques
kicked out a core electron
This scheme is for lighter elements (Z=33 as is crossover b/w Auger and X-Ray
Several keV
X-Ray Electron SpectroscopyElectron Microprobe
If X-Ray is at the input:
• el. Emitted= X-ray Photoelectron Spectroscopy (XPS)
• X-ray emitted= X-ray Fluorescence (XRF)
XPS usually more dominant for lighter elements, XRF for heavier
Auger El. Spectroscopy
The core electron energy levels
Several keV
1
3
2
1
2
3
Monitoring of Gettering Through Device Properties and Dielectric
p – n leakage, refresh time DRAM junction and dielectric breakdown, of n-p-n
Material properties : G(>>R) in the bulk and on the surface
€
n(t) = Δn(0)exp(−t /τ R )
Photoconductive Decay Measurements
∆n=gopG
•Carriers are generated due to light •Decrease resistivity•Recombine
emission<-> capture
==+
€
t =10(NA /ni)τ G
Carrier Generation Lifetime
Deep Depletion - Return to Inversion via Carrier Generation (measure G) and surface recombination (s)
DLinversion
Zerbst technique:
€
ddt
(CoxC
)2 → G
ddt
(CoxC
)2 → s if plotted vs. (Cmin/C)-1
s=f(NST, )
=Capture cross section
€
VD (t) = VD (0) − kT ln(erfct
τ R
)
€
R =kT /q
dVD /dt
Lifetime Measurements: Open Circuit Voltage Decay
Diode switched from ON VD when carriers recombine
Measurements include surface and bulk recombination
t=0
≈0.7V
off
Use also DLTS: identifies traps (Et) and concentrationsThermal or photoexcitation processes in voltage modulable space-charge region (Schottky Diode, p-n junction, MOS Capacitor)Measured: capacitance, currents or conductance
for t>4
Excess Carrier Concentrations Decays: minority carriers
x=tL/td
Experiment to calculate the diffusion constant Dp, (n) for minority carriers (pn) -> µp, (n)
oscilloscope screen
€
D=kTq
Pulse
vd-> µ
diffusion
€
Dp=(x )2
16td
Drift: vd=L/td
µp=vd/E
Drift&diffusion
Mobility of minority carriers
Mobility of minority carriers