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