Chapter 4 Clean room, wafer cleaning and gettering

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Chapter 4 Clean room, wafer cleaning and gettering

1. Introduction.2. Clean room.3. Wafer cleaning.4. Gettering.5. Measurement methods.

NE 343: Microfabrication and thin film technologyInstructor: Bo Cui, ECE, University of Waterloo; http://ece.uwaterloo.ca/~bcui/Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin

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Effect of defect and contamination on semiconductor industry

Importance of unwanted impurities increases with shrinking geometries of devices. 75% of the yield loss is due to defects caused by particles (1/2 of the min feature size).

LLS: localized light scatters (use laser to detect and count particles)

GOI: gate oxide integrity, by electrical measurement

Requirement different for DRAM and logic chip, due to greater gate insulator area on DRAM chip.

109/cm2 0.0001% monolayer

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Silicon Wafer

SiO2 or other thin films

PhotoresistAu

Cu

Fe

Particles

Interconnect Metal

Na

N, P

Contaminants may consist of particles, organic films, photoresist, heavy metals or alkali ions.

Modern IC factories employ a three tiered approach to controlling unwanted impurities:

1. clean factories2. wafer cleaning3. gettering

Type of contaminants

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Effects on MOSFET: two examplesMOSFET threshold voltage is given by:

If tox=10nm, then a 0.1V Vth shift can be caused by Na+ or K+ of QM=2.151011 ions /cm2 (<0.1% monolayer or 10ppm in the oxide).0=8.8510-12F/m, ox=3.9

For MOS DRAM, refresh time of several msec requires a generation lifetime of

This requires trap density Nt1012/cm3, or 0.02ppb (1012/(51022)=0.02ppb).( is trap capture cross-section, vth is minority carrier thermal velocity; Vth107cm/sec, 10-15cm-2) Deep-level traps (Cu, Fe, Au etc.) pile up

at the surface where the devices are located. This causes leak current. Need refresh/recharge the MOS capacitor.

DRAM: Dynamic Random Access Memory

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Residual contaminants affect kinetics of processes, here oxidation.

Effects of cleaning on thermal oxidation

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Particle contaminants

Particle sources: air, people, equipment and chemicals.A typical person emits 5-10 million particles per minute.

>0.2m >0.5m

NH4OH 130-240 15-30

H2O2 20-100 5-20

HF 0-1 0

HCl 2-7 1-2

H2SO4 180-1150 10-80

Particle density (number/ml) for ULSI grade chemicals

ULSI: ultra-large-scale integration

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Fe, Cu, Ni, Cr, W, Ti…Na, K, Li…

Metal contamination

Sources: chemicals, ion implantation, reactive ion etching, resist removal, oxidation.

Effects: defects at interface degrade device; leads to leak current of p-n junction, reduces minority carrier life time.

Dry etching

Ion implantation

Photoresist removal

Wet oxidation

9 10 11 12 13Log (concentration/cm2)

Fe Ni Cu

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NE 343 Microfabrication and thin film technologyInstructor: Bo Cui, ECE, University of WaterlooTextbook: Silicon VLSI Technology by Plummer, Deal and Griffin

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Modern IC factories employ a three tiered approach to controlling unwanted impurities:

1. clean factories2. wafer cleaning3. gettering

Clean factory is the first approach against contamination

Clean factory Wafer cleaning Gettering

Factory environment is cleaned by:• HEPA filters and recirculation

for the air.• “Bunny suits” for workers.• Filtration of chemicals and

gases.• Manufacturing protocols.

Clean roomHEPA: High Efficiency Particulate Air• HEPA filters composed of thin porous sheets of

ultrafine glass fibers (<1m diameter). • It is 99.97% efficient at removing particles from air. • Room air forced through the filter at 50cm/sec.• Large particles trapped, small ones stick to the fibers

due to electrostatic forces.• The exit air is typically better than class 1.

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Class of a clean room• Air quality is measured by the “class” of the facility.• Class 1-100,000 mean number of particles, greater than 0.5m, in a cubit foot of air.• A typical office building is about class 100,000.• The particle size that is of most concern is 10nm – 10m. Particles <10nm tend to

coagulate into large ones; those >10m are heavy and precipitate quickly.• Particles deposit on surfaces by Brownian motion (most important for those <0.5m)

and gravitational sedimentation (for larger ones).

Class 0.1 0.3 0.5 5.0

1 35 3 1

10 350 30 10

100 300 100

1000 1000 7

10000 10000 70

100000 100000 700

Particle diameter (m)

by definition

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Particle contamination and yield• Generally, particles on the order of the technology minimum features size or

larger will cause defect.• 75 yield loss in modern VLSI fabrication facilities is due to particle

contamination.• Yield models depend on information about the description of particles.• Particles on the order of 0.1-0.3m are the most troublesome: larger particles

precipitate easily; smaller one coagulate into larger particles.

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• Cleaning involves removing particles, organics and metals from wafer surfaces.• Particles are largely removed by ultrasonic agitation during cleaning.• Organics (photoresist) are removed in O2 plasma or in H2SO4/H2O2 (Piranha) solutions.

• The “RCA clean” is used to remove metals and any remaining organics.

Modern wafer cleaning

Typical person emit 5-10 million particle per minute.Most modern IC plants use robots for wafer handling.

A cassette of wafers

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RCA clean is “standard process” used to remove organics, heavy metals and alkali ions.Ultrasonic agitation is used to dislodge particles.SC: Standard CleaningRCA: Radio Corporation of America, now makes TV, stereos…

Standard RCA cleaning procedure

DI water: de-ionized water

120 - 150ÞC 10 min

Strips organics especially photoresist

DI H2O Rinse Room T

80 - 90ÞC 10 min

Strips organics, metals and particles

DI H2O Rinse Room T

80 - 90ÞC 10 min

Strips alkali ions and metals

Room T 1 min

Strips chemical oxide

DI H2O Rinse Room T

H2SO4/H2O2 1:1 to 4:1

HF/H2O 1:10 to 1:50

NH4OH/H2O2/H2O 1:1:5 to 0.05:1:5

SC-1

HCl/H2O2/H2O 1:1:6 SC-2

and all contaminants on top of it, but induces H passivated surface (bad)

Less NH4OH will reduce surface roughness

not removed by SC-1

HF dip added to remove oxide

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SC-1: NH4OH(28%):H2O2(30%):H2O=1:1:5 - 1:2:7; 70-80C, 10min, high pH.

• Oxidize organic contamination (form CO2, H2O…)

• Form complex such as Cu(NH3)4+2 with metals (IB, IIB, Au, Ag, Cu, Ni, Zn, Cd, Co, Cr).

• Slowly dissolve native oxide and grow back new oxide, which removes particles on oxide.• But NH4OH etches Si and make the surface rough, thus less NH4OH is used today.

Standard cleaning (SC)

SC-2: HCl(73%):H2O2(30%):H2O=1:1:6 - 1:2:8; 70 - 80C; 10min, low pH.

• Remove alkali ions and cations like Al+3, Fe+3 and Mg+2 that form NH4OH insoluble hydroxides in basic solutions like SC-1.

• These metals precipitate onto wafer surface in the SC-1 solution, while they form soluble complexes in SC-2 solution.

• SC-2 also complete the removal of metallic contaminates such as Au that may not have been completely removed by SC-1 step.

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Si 2H2O SiO2 4H 4e

M Mz ze

(1)

(2)

Principles of metal cleaning

If we have a water solution with a Si wafer and metal atoms and ions, two reactions take place.

The two reactions will proceed in opposite directions, one providing electrons, which will then be consumed by the other (forming an oxidation/reduction couple). In this couple, the stronger reaction will dominate.Generally, (2) is driven to the left and (1) to the right so that SiO2 is formed and M plates out on the wafer.Good cleaning solutions drive (2) to the right since M+ is soluble and will be desorbed from the wafer surface.

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Reaction goes to the left

The strongest oxidants are at the bottom (H2O2 and O3). These reactions go to the left, grabbing electrons and forcing (2) in previous slide to the right.Fundamentally the RCA clean works by using H2O2 as a strong oxidant.

Principles of metal cleaning

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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

cmMOHHq

OHH

5.18)][]([

1

Ultrasonic cleaning and DI water

RCA cleaning with ultrasonic agitation is more effective in removing particles.Ultrasonic cleaning:• Highly effective for removing surface contaminants• Mechanical agitation of cleaning fluid by high-frequency vibrations (between 20 and 45

kHz) to cause cavitation - formation of low pressure vapor bubbles that scrub the surface.• Higher frequencies (>45kHz) form smaller bubbles, thus less effective.• However, megasonic (1MHz) cleaning is also found effective in particle removal.

DI (de-ionized) water is used for wafer cleaning. One monitors DI water by measuring its resistivity, which should be >18Mcm.

Einstein relation: µ=qD/kT, http://en.wikipedia.org/wiki/Einstein_relation_%28kinetic_theory%29

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Other cleaning methods

Dry (vapor phase) cleaning:Energy may come from plasma, ion beam, short-wavelength (UV) radiation or heating.• HF/H2O vapor cleaning

• UV-ozone cleaning (UVOC)• H2/Ar plasma cleaning

• Thermal cleaning

Ohmi cleaning: room temperature, fewer chemicals

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Gettering• For the alkali ions, gettering generally uses dielectric layers on the topside; PSG for

trapping, or Si3N4 layer for blocking them from getting into the device region.

PSG: phosphosilicate glass, is a P2O5/SiO2 glass that is normally deposited by CVD, usually contains 5% by weight phosphorus.PSG traps alkali ions (Na+, K+) and form stable compounds.At higher than room temperature, alkali ions can diffuse into PSG from device region and trapped there.Problems with PSG: it affects electric fields since dipoles exist in PSG, and it absorb water, leading to Al corrosion.

• For metal ions, gettering generally uses traps on the wafer backside or in the wafer bulk. Here gettering works because the metals (Au…) do not “fit” in the silicon lattice easily because of their very different atomic size, thus they prefer to stay at defect sites.

• Therefore, the idea of gettering is to create such defect sites outside of active device region.

• Backside = external gettering: roughing/damaging the backside of the wafer, or depositing a poly-silicon layer, to provide a low energy “sink” for impurities.

• Bulk = intrinsic (or internal) gettering: using internal defects to trap impurities, thus moving them away from the active region of the wafer.

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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

IIA

III BIV

B

V A

I B IIB

III A IV A

VB

VIB

VIIB

VIII

VIA VIIA

Noble Gases

Shal

low

Don

ors

Shal

low

Acc

epto

rs

Ele

men

tal

Sem

icon

duct

ors

Deep Level Impurites in Silicon

Alkali Ions

Deep level impurities in silicon:large diffusivity, easily trapped by mechanical defects or chemical traps.

Gettering

Figure 4-6 Periodic table indicating the elements that are of most concern in gettering.

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Fast diffusion of various impurities

Heavy metal gettering relies on metal’s very high diffusivity (when in interstitial sites) in silicon, and its preference to segregate to “trap” sites.

Diff

usiv

ity (c

m2 /s

ec)

Those metal diffuses fast because they do so as interstitials.Whereas dopants are substitutional and diffuse by interacting with point defects.

I: interstitialS: substitutional

They can diffuse from front-side to backside of the wafer (>0.5mm distance)

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PSG layerDevices in near surface regionDenuded zone or epitaxy layer

Intrinsic gettering region

Backside gettering region

500+

m

1

0-20

m

Gettering mechanism

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Gettering consists of:1. Making metal atoms mobile.2. Migration of these atoms to trapping sites.3. Trapping of atoms.

Step 1 generally happens by kicking out the substitutional (s) atom into an interstitial (i) site. One possible reaction is: (I = interstitial Si)

Step 2 usually happens easily once the metal is interstitial since most metals diffuse rapidly in this form.

Step 3 happens because heavy metals segregate preferentially to damaged regions (dislocation or stacking fault) or to N+ regions, or pair with effective getters like P (AuP).

Step 1 can be facilitated by introducing large amount of Si interstitials, by such as high density phosphorus diffusion, ion implantation damage or SiO2 precipitation.

AuS I Aui

Gettering mechanism

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Intrinsic gettering

120 sec53.2exp13.0

cmkT

D

Oxygen diffusivity:

D0 >> Ddopants but D0<< Dmetals

1100

900

700

500

Out-diffusion of O

Nucleation of SiO2

Precipitation(growth of SiO2)

denuded zone = oxygen free; thickness several tens of µm

50-100nm in size

Slow ramp

1-3 nm min size of nuclei, concentrations ≈ 1011cm-3

Tem

pera

ture

o C

Time

In intrinsic gettering, the metal atoms segregate to dislocations (formed because of volume mismatch of SiO2 and host Si lattice) around SiO2 precipitates.15 to 20 ppm oxygen wafers are required:

<10 ppm - precipitate density is too sparse to be an effective getterer. >20 ppm - wafers tend to warp during the high temperature process.

Note: devices that use the entire wafer as the active region (solar cells, thyristors, power diodes, etc...) can not use this technique, but can use extrinsic gettering.

Today, most wafer manufactures perform this intrinsic gettering task that is better controlled.

Precipitates (size) grow @ high TDensity of nucleation sites grow @ low TTherefore, low T to increase density, and high T to grow its size.

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StackingFault

V I

OI Diffusion

[OI]

SiO2

OI

OI

OIOI

OI SiO2

Intrinsic Gettering: SiO2 precipitates

no SiO2

SiO2 precipitates (50-100nm)SiO2 precipitates (white dots) in bulk Si

No SiO2 on top surface(denuded zone)

Figure 4-13

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Un-patterned wafers (blank)• Count particles in microscope• Laser scanning systems that give 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• Test structure (not in high volume manufacturing)

Particle contamination detection

Test structures design to detect defects

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Monitoring the wafer cleaning efficiencyConcentrations of impurities determined by surface analysis.

Excite Identify (unique atomic signature) Count concentrations

Primary beam electron good lateral resolution (e can be focused, but not x-ray)Detected beam electron 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

emitted

Chapter 4 Clean room, wafer cleaning and gettering

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