제7장 Chemical Vapor deposition

Issues

1. Chemical Vapor Deposition이 필요한 이유

2. Reaction Mechanism 분석

3. Applications

Key words

1. LPCVD

2. 반응성 가스

3. Metal Organics

4. Epi-taxial Growth

5. out-diffusion

6. Auto-Doping

7. Stagnant layer

Theory

Technology

1. 박막을 형성하는 방법 중 가장 step coverage 가 좋은 방식이므로 aspect ratio 가 높은 고집적 소자의 제조에 유리하다.

2. epi taxial growth가 가능하여 기판의 결정성을 그대로 유지할 수 있다.

3. 미세 소자의 형성에 결정적인 self aligned process가 가능하므로 대단위 집적을 가능하게 한 핵심공정 중 하나이다.

4. 일반적으로 epi 층의 두께는 0.5 mum이며 50 mum 이하, uniformity는 [eqn: +-] 20% 정도이다.

5. 실제 공정에서는 Auto doping, outdiffusion 등의 문제 때문에 정확한 junction depth의 control이 불가능하며 다른 박막 형성 공정에 비해 CVD 박막은 반응가스의 양과 기판 온도에 매우 민감하므로 꼭 필요한 경우를 제외하고는 될 수 있는 대로 적용하지 않는다.

7-1 Applications

Triple-  diffused : 1. No epitaxy

2. High collector R

3. High [eqn: C_BC]

4. Triple compensation of material

Double-diffused Epitaxial : 1. Small collector R

2. Optimized [eqn: C_BC]

3. Independent choice of collector,

substrate and base sheet resistance

따라서 DDE structure가 현재 bipolar TR 제조에 지배적으로 쓰이고 있다.

7-2 CVD systems

7-2-1 APCVD (Atmospheric Pressure CVD)

위 그림에서 보인 system 들은 모두 대기압에서 작동하는 APCVD system 이다. 따라서 wafer 들을 가까이 배치할 수 없다 ( 상세한 이유는 곧 설명될 것이지만 우선 gas phase로부터 reactant들의 이동이 scattering에 의해 쉽지 않기 때문이다) 따라서 APCVD의 경우 생산성이 낮은 것이 문제이다.

7-2-2 LPCVD ( Low Pressure Chemical Vapor Deposition)

1 Torr 정도의 낮은 기압에서 CVD 공정을 진행할 수 있는 데 이 경우 wafer들을 가까이 위치하여 많은 wafer를 한번에 lqoading 할 수 있게 되어 생산성이 높아진다. 그 이유는

[eqn: D ~PROPTO ~1 over P] 따라서 확산계수는 1 Torr에서는 약 1000 배정도 높아지고 이에 따라 gas phase 로부터 기판 표면까지로의 reactant 이동이 더 이상 rate limiting step 이 되지 못한다.

이로써

1. wafer loads much higher

2. Uniformity improved

3. Gas consumption much lower ( generally little or no carrier gas is needed.)

7-3 Single Crystal Epitaxy Growth Kinetics

[eqn: F_1 = h_G~ ( N_G - N_S )]

where [eqn: h_G] = gas phase mass transfer coefficient (cm / sec)

[eqn: F_2 ~=~ k_S ~N_S]

where [eqn: k_S] = chemical surface reaction rate constant ( cm/sec)

at steady state, [eqn: F = F_1 = F_2]

[eqn: therefore~~ h_G ~( N_G - N_S`` )~= ~k_S~ N_S]

or

[eqn: N_S = N_G over {1 + k_S over h_G }]

The growth rate is now given by

[eqn: V= F over n] --------------------------5

where V = growth rate (cm/sec)

n = number of Si atoms incorporated per unit volume in the film = [eqn: 5 times 10^22 /cm^3]

[eqn: therefore V = {k_S ~h_G }over {k_S + h_G}N_G over n]

Let [eqn: y ~==] mole fraction of Si species in gas phase

= [eqn: N_G over N_T]       =      molecules of [eqn: SiH_4]

mol of [eqn: SiH_4] + mol of [eqn: H_2]

[eqn: therefore] [eqn: V = {k_S h_G} over {k_S + h_G} N_T over n y]

[eqn: therefore] 1. [eqn: V ~propto~ y] the concentration of Si species in the gas phase

2. [eqn: V] is determined by the smaller of [eqn: k_S] and [eqn: h_G]

i.e., [eqn: V ~approx~ N_T over n k_S~ y]  surface reaction control ( [eqn: k_S ~<<~ h_G] )

[eqn: V~ approx ~N_T over n h_G~ y]  mass transfer control ( [eqn: h_G ~<<~ k_S] )

점 : 실험 data

실선 : 앞 식에 의한 이론 치

[eqn: k_S = k_o e ^{-E_a / kT}]

[eqn: h_G] = constant

[eqn: therefore][eqn: h_G ~approx] 5 - 10 cm/sec

[eqn: k_So approx 1 times 10^7] cm / sec

[eqn: E_a ~approx] 1.9 eV

[eqn: H_2] 를 carrier gas 로 사용했을 경우의 실험치

1. at high T, V independent of T [eqn: therefore ~~h_G] or mass transfer limited [eqn: V_SiH_4 ~> V_{SiH_2 Cl_2} ~> V_{SiCl_3 H} ~> V_SiCl_4]

i.e., [eqn: V~ propto ~1 over sqrt {mol ~wgt}]

2. also note that at low T all [eqn: E_A] 's are almost same. Rate limiting step is arrangement of Si atoms on the surface. ( [eqn: E_A] for Si surface diffusion is [eqn: approx] 1.9 eV )

7-4. Silicon sources

7-5 Wafer to wafer Uniformity (Stagnant film model)

The frictional force of the gas stream on the stationary susceptor is given by

[eqn: F = mu~ {partial mu} over {partial y}]

[eqn: mu] = viscosity of gas

[eqn: {partial mu}over {partial y}] = velocity of gradient perpendicular to susceptor

[eqn: therefore]  velocity next to susceptor must be zero or [eqn: {partial mu}over {partial y}] is infinite and so F is infinite which would force the velocity to zero.

따라서 boundary layer 또는 stagnant region 이 susceptor의 길이를 따라 gas stream 속으로 보다 깊이 이동된다. 여기에 더불어 growth rate는

[eqn: V~ propto~ y] ( partial pressure of Si bearing species)

따라서 susceptor의 길이방향으로 [eqn: SiH_4] 이 소모될수록 y 값은 떨어지게 되고 이에 따라 V 역시 작아지게 된다.

또한 susceptor 길이방향으로 stagnant layer 의 두께가 증가하게 되고 이에 따라 ,ass transport 또는 diffusion limited region ( high temperature)에서는 deposition rate가 감소하게 된다. 따라서 우리는 susceptor 길이방향으로 균일한 증착을 얻기 위해서 reactor의 geometry를 변화시킬 필요가 있다.

susceptor tilt의 효과 :

1. U increases along length [eqn: therefore~ delta] can be held almost constant or decreased to compensate for decreasing partial pressure of Si in gas phase.

2. Experimentally U increase leads to increased deposition rate.

따라서 by choosing the proper tilt angle and the proper gas velocity at the imput, it is possible to obtain uniform deposition along the length of the susceptor.

즉 적절히 tilt 함으로써 U 값을 증가 시킬 수 있고 이에 따라 감소되는 y 값을 보상하기 위해 [eqn: delta] 값이 작아지게 된다.

higher U -> smaller [eqn: delta]

typically :

tilt = few degrees

flow = 50 - 150 liters/min             for typical reactor

U [eqn: approx] 25 - 100 cm /sec

7-6 Doping of epitaxial layers

Doping is achieved by mixing a gaseous source of the desired dipant with the silicon source and a carrier gas ([eqn: H_2] ). the steps involved in the process are similar to those described earlier for Si deposition. Common sources are [eqn: AsH_3] , [eqn: PH_3] , [eqn: B_2][eqn: H_6] and [eqn: Sb(CH_3 )_3]

1) Dopant transport to the deposition region

2) Mass transport through the boundary layer by diffusion

3) Chemical reactions in the gas phase near the surface

4) Adsorption on Si surface

5) Dissociation

[eqn: AsH_3 -> As + 3H]

6) Lattice site incorporation

7)Burying by Si atoms

8) Desorption of by products

[eqn: H + H ``->`` H_2 uparrow]

Recent work has demonstrated that steps 5-7 are the rate limiting steps, not step w as was previously believed.

For low growth rates, it is reasonable to assume thermal equilibrium for all steps and thus, from Henry's Law,

N = H [eqn: P_g]

where N is doping density in the eip layer and P_g is the gas phase dopant partial pressure, near the growing surface. Under equilibrium,

[eqn: P_g ~propto~ P_i]

In practice, a particular reaction is normally experimentally characterized since the constant relating N and [eqn: P_i] may depend on reactor parameters. ( Compare this with predep "solid solubility" )

At higher growth rates, the system is not thermal euqilibrium and the analysis is much more complicated. Experimentally, it is found that

[eqn: N = k P_i over V]

where V is the growth rate

Normal reactor operation is in the [eqn: N~ propto` P_i] region

If the dopant concentration is changed during epi growth, a transient response is associated with the epi doping.

Experimentally,

[eqn: N ``=`` k ``P_i ````( 1 +- e^{t/ tau}``)] where [eqn: tau]  [eqn: approx] 41 sec for this system. This time constant is mostly related to surface effects and is approximately independent of reactor.

7-7. Out-diffusion during epitaxy

The general solution to this problem is quite complex because we have a moving boundary.

But generally Gt >> [eqn: sqrt Dt]

Gt = (growth rate cm/sec) (time)

= characteristic length of growth process (cm)

[eqn: sqrt Dt] = characteristic diffusion length (cm)

i.e., the diffusion problem at the substrate - epi interface may generally be solved with an "infinite" epitaxial layer thickness. In this case,

[eqn: N_1 (x,t) ``= ``N_S over 2 left( 1 - erf x over {sqrt {D_s ``t}}right)## N_2 (x,t)``=`` N_e over 2 left( 1 + erf x over {2 sqrt {D_e`` t}}right)]

where [eqn: N_s] = substrate concentration

[eqn: N_e] = epitaxial layer concentration

[eqn: D_s] = diffusion coefficient of substrate impurity

[eqn: D_e] = diffusion coefficient of epi layer impurity

x = distance into epi layer from substrate

Note that x is negative for the [eqn: N_1] profile in the substrate but erf(-x) = - erf(x)

Note that

[eqn: N_1 (x,t) -> N_s] as [eqn: x `````-> - inf]

[eqn: N_2 (x,t) -> N_e] as [eqn: x```` -> + inf] since erf([eqn: inf]) = 1

Note

1. [eqn: N^+] B.L. may move several microns into epitaxial layer

2. [eqn: D_As] and [eqn: D_Sb] [eqn: ~ << ~ D_P]  therefore As or Sb preferred for buried layers

3. Total Dt for outdiffusion is given by

[eqn: Dt ``= Sigma ~( Dt_epi + Dt_iso +~ cdots~ )]

4. [eqn: SiH_4] is slower T process than [eqn: SiCl_4]  [eqn: therefore] less outdiffusion with [eqn: SiH_4]

7-8 Auto-doping during epitaxy

Dopant may be "added" to the gas stream by

1. Evaporation from wafer backside

2. Evaporation from the front side during the HCl etch or during

deposition

3. "Upstream" sources - susceptor other wafers etc

These effects add to the out-diffusion problem described above, to make the epi layer profile non-ideal. For high temperature processes, i.e., [eqn: SiCl_4] epi followed by long iso drive-ins, out-diffusion dominates. For [eqn: SiH_4] , thin epi structures, auto-doping can dominate. Low pressure helps because [eqn: D~ propto~ 1 over P]

As an example, consider an As doped [eqn: n^+] substrate with a [eqn: SiH_4] ( [eqn: 1000 ^o] C ) epi layer grown over it. The "extra" dopant is due to frontside and backside evaporation from the Si substrate into the stagnant layer.

Experimentally, [eqn: N_{Auto-doping }= C ~N_S~ e^{-phi x}]

where [eqn: N_S] = substrate surface concentration

[eqn: phi] = experimentally determined constant

N decreases by 10 X for every 2.7[eqn: mu] in the As example above. The problem is less important with some B.L. dopants ( likely related to vapor pressure)

As [eqn: >>] B [eqn: >>] P [eqn: >>] Sb

Auto doping can be minimized by

1. covering backside of wafer with [eqn: SiO_2]

2. Using Sb for buried layers

3. If As is used ( for lower [eqn: rho_S] ) , a "two step" deposition can be used i.e, a) grow [eqn: approx] 1 [eqn: mu]  b) switch to [eqn: H_2] to "flush" dopant from stagnant layer c) resume growth

Note regions of profile where

a) outdiffusion

b) autodoping due to frontside

c) backside autodoping

Note the differences between As and Sb burried layers

1. As has higher solid solution ( [eqn: 10^21] vs [eqn: 5 times 10^19] / [eqn: cm^3] ) 따라서 lower [eqn: rho_s] can be achieved ( 5 [eqn: Omega] / sq AS  vs 15 Ohm / sq Sb )

2. Sb has a larger misfit factor than As. Higher epi defect density may result with Sb.

7-9 Defects in Epitaxial Layers

7-10 Poly Silicon

If any substrate other than a single crystal substrate is used epitaxial growth is not possible and poly silicon or amorphous silicon will deposit.

1. Si gate MOS

2. High [eqn: rho_s] resistors

3. Solar cells

4. interconnect lines in MOS and BJT circuits

In general,

[eqn: N_2] or [eqn: H_2] + [eqn: SiH_4 ->] Si + [eqn: 2 H_2 + N_2] or [eqn: H_2]

Poly is usually deposited at 700 - 900 oC. In this range, with [eqn: H_s], reaction is surface reaction rate limited and very sensitive to T ( [eqn: +-]10 % in T [eqn: ~~->~~] 25% in thickness )

따라서 낮은온도에서 [eqn: N_2] carrier gas를 쓰는 것이 바람직하다.

[eqn: N_2 + SiH_4 -> Si + 2H_2 + N_2]

If [eqn: H_2] is used reaction is driven to left and decomposition is more T sensitive. 따라서 use [eqn: N_2] and curves on previous page are shifted to right. -> better control  따라서 [eqn: N_2]를 사용하면 낮은 온도에서도 mass transport limited의 반응을 얻을 수 있다.

1. active dopant in grain

2. segregated dopant at grain boundaries

Grain boundaries play a very important role in the electrical and physical properties of polysilicon.

Carrier trapping at grain boundaries -> potential barrier across grain boundaries

i.e., [eqn: {d^2 phi}over dx^2 = q N over E]

and for uniform doping, a parabolic potential barrier analogous to a PN junction is obtained across the grain boundary

The density of carrier traps is on the order of [eqn: 10^17] - [eqn: 10^18] /[eqn: cm^3]

therefore there is an abrupt change in rho at this [eqn: N_D] (Exact behavior depends on grain size.)

Low pressure deposition is another practical way of getting around the temperature sensitivity inherent in RF heated surface reaction rate limited systems.

Resistance heated [eqn: therefore] [eqn: +- 1] T control possible. [eqn: therefore]surface reaction rate limited operatioqn is possible. -> This implies Si deposition on tube inside, which is OK for resistance heating but -> frequent tube cleaning

recall [eqn: V = {k_S~ h_G~}over {k_S~ + h_G }N_T over n y]

where [eqn: h_G] mass transport  and [eqn: k_S] surface reaction

[eqn: h_G ~~propto~~D~~propto~~1 over P]

[eqn: therefore] decreasing pressure to approx 1 Torr leads to D uparrow by 100 to 1000 times and [eqn: h_G] is vastly increased. ([eqn: h_G] is also [eqn: propto] [eqn: 1 over delta] where [eqn: delta] = boundary layer thickness. [eqn: delta uparrow] 3 - 10[eqn: times] at low P but D [eqn: uparrow] more than compensates for this.)

[eqn: therefore] Mass transport becomes unimportant and wafers can be closely spaced.

[eqn: therefore] [eqn: V~ approx~ k_S ``N_T over n y]  i.e., surface controlled

Typical process conditions

Note that we don not want mass transport limited deposition in these systems so that

1. T can not be too high or [eqn: k_S] [eqn: uparrow] and [eqn: h_G] becomes important

2. P can not be too high or D [eqn: downarrow] and [eqn: h_G] becomes important

3. Wafer spacing con not be too close or consumption of reactants I too high and too much reactant depletion occurs.

To compensate for reactant depletion, it is common to introduce a temperature differential imput -> output of approx [eqn: 20 ^o~][eqn: ^o~]C   Since [eqn: k_S~ propto~ e ^{E_A / kT}], this [eqn: uparrow] [eqn: k_S] at exhaust end to compensate for depletion of reactants.

Core Findings

1. poly Si 형성이 가능하므로 Self-aligned Process에 의해 source 및 drain 과 gate edge와의 overlap을 없앨 수 있어 미세소자 제조가 가능해 졌다.

2. [eqn: SiO_2] 의 경우 thermal oxidation과 달리 기판 실리콘의 소모없이 산화막을 형성할 수 있으며 또한 산화막 형성시 부피 팽창같은 문제가 없다.

3. 최근 초고집적소자에서 interconnection으로 쓰이는 알루미늄 금속의 CVD 가 중요한 연구과제인데 이는 집적도가 높아질수록 aspect ratio가 커지고 이에 따라 step coverage 가 좋은 박막 형성 기법이 보다 중요해 지기 때문이다.

4. Epi-taxial growth는 bipolar transistor의 제작에 가장 중요한 공정중 하나이며 이에 따른 dopant의 거동에 관해서는 거의 정확한 예측이 가능하다.

5. Epi-taxial growth는 주로 높은 온도에서 행해지기 때문에 Out-diffusion과 Auto doping 효과를 결코 무시할 수 없다.