EE THERMAL OXIDATION - Chapter 6. Basic Concepts

Transcription

1 EE 22 FALL THERMAL OXIDATION - Chapter 6 Basic Concepts SiO 2 and the Si/SiO 2 interface are the principal reasons for silicon s dominance in the IC industry. SiO 2 : Easily selectively etched using lithography. Masks most common impurities (B, P, As, Sb). 6 Excellent insulator (ρ> 0 Ωcm, E g > 9 ev). High breakdown field (0 7 Vcm - ) Excellent junction passivation. Stable bulk electrical properties. Stable and reproducible interface with Si. Oxide Thickness Thermally Grown Oxides Deposited Oxide Layers µ Field Oxides Backend Insulators Between Metal Layers 0.µ Masking Oxides Masking Oxides 00Å Gate Oxides, Pad Oxides Tunneling Oxides 0Å Chemical Oxides from Cleaning, Native Oxides

2 2 Year of st DRAM Shipment Minimum Feature Size 0.25µ 0.8µ 0.3µ 0.0µ 0.07µ 0.05µ DRAM Bits/Chip 256M G 4G 6G 64G 256G Minimum Supply Voltage (volts) Gate Oxide T ox Equivalent (nm) <.5 <.0 Thickness Control ± 4 ± 4 ± 4-6 ± 4-8 ± 4-8 ± 4-8 (% 3 σ) Equivalent Maximum E >5 >5 >5 field (MV cm - ) Gate Oxide Leakage <0.0 <0.0 <0.0 <0.0 <0.0 <0.0 (DRAM) (pa µm -2 ) Tunnel Oxide (nm) Maximum Wiring Levels Dielectric Constant, κ <.5 <.5 for Intermetal Insulator Ambient O 2, H 2 O SiO 2 Diffusion Reaction Silicon Si + O 2 SiO 2 or Si + 2H 2 O SiO 2 + 2H Si substrate.3 Si substrate

3 3 Deposited Polysilicon Location of Si 3 N 4 Mask Volume Expansion SiO 2 Original Si Surface Si Substrate Oxidation involves a volume expansion ( 2.2X). Especially in 2D and 3D structures, stress effects play a dominant role. SiO 2 is amorphous even though it grows on a crystalline substrate.

4 4 Q m SiO Na Q ot Transition Region x - K x x x x x Q f Silicon Q it Four charges are associated with insulators and insulator/semiconductor interfaces. Q f - fixed oxide charge Q it - interface trapped charge Q m - mobile oxide charge Q ot - oxide trapped charge Quartz Tube Wafers Quartz Carrier Resistance Heating O 2 H 2 Oxidation systems are conceptually very simple. In practice today, vertical furnaces, RTO systems and fast ramp furnaces all find use.

5 5 C-V Measurements There are a number of measurement techniques used to characterize SiO 2 and the Si/SiO 2 interface. The most powerful of these is the C-V method which is described in the text in detail. a) + V G C CO CO e- N Silicon Doping = N D - + V G b) - V G C x D CO CD CO e- - + V G c) -- V G C Q G CO Q I Q D e - x DMax CO C DMin Holes - V TH + V G

6 6 Electric field lines pass through the perfect insulator and Si/SiO 2 interface, into the substrate where they control charge carriers. Accumulation, depletion and inversion result. QG = qndxd + QI () C Ideal LF C ox C Min Ideal HF Deep Depletion V TH DC Gate Voltage HF curve - inversion layer carriers cannot be generated fast enough to follow the AC signal so C inv is C ox + C D. LF curve - inversion layer carriers follow the AC signal so C inv is just C ox. Deep depletion - DC voltage is applied fast enough that inversion layer carriers cannot follow it, so C D must expand to balance the charge on the gate. C-V measurements can be used to extract quantitative values for: t ox - oxide thickness N A - the substrate doping profile Q f, Q it, Q m, and Q ot - oxide & interface charges. See text for more details on these measurements.

7 7 SiO 2 Growth Kinetics Models A. Deal Grove Model µm 500 µm x O CG CS CO CIC I Gas Oxide Silicon F F 2 F 3 The basic model for oxidation was developed in 965 by Deal and Grove. Si + O 2 SiO 2 (2) Si + 2H O SiO + 2H (3) Three first order flux equations describe the three series parts of the process. ( ) (4) F = h C C G G S F D N D C O C 2 = x = x O I (5) F 3 = k C I (6) S

8 8 Under steady state conditions, F = F 2 = F 3, so C I = * * C C k k x k x h D D S S O S O (7) C O = * k x C S + D k k x + + h D O S S O C * (8) Note that the simplifications are made by neglecting F which is a very good approximation. Combining (6) and (7), we have dx dt F ksc = = N k k x + + h D * S S O (9) Integrating this equation (see text), results in the linear parabolic model. x 2 2 O i O i x B x x + B/ A = t (0) where B DC = 2 * N (parabolic rate constant) ()

9 9 B A = C N + k h S * * Ck N S (linear rate constant) (2) (0) can also be written with oxide thickness as a function of time. A t xo = + + τ 2 2 A / 4B (3) where τ= x 2 i + Ax B i (4) The rate constants B and B/A have physical meaning (oxidant diffusion and interface reaction rate respectively). ( ) B= C exp E / kt (5) B A = C 2exp ( E 2 / kt ) (6) Ambient B B/A Dry O 2 C = 7.72 x 0 2 µ 2 hr - E =.23 ev Wet O 2 C = 2.4 x 0 2 µ 2 hr - E = 0.7 ev H 2 O C = 3.86 x 0 2 µ 2 hr - E = 0.78 ev C 2 = 6.23 x 0 6 µ hr - E = 2.0 ev C 2 = 8.95 x 0 7 µ hr - E = 2.05 ev C 2 =.63 x 0 8 µ hr - E = 2.05 ev

10 T ( C) B/A H 2 O B µm 2 hr - B/A µm hr B Dry O 2 B/A Dry O 2 B H 2 O /T (Kelvin) Oxide Thickness - microns C 00 C 000 C C 800 C Time - hours Calculated dry O 2 oxidation rates using Deal Grove.

11 2 Oxide Thickness - microns C 000 C 900 C 800 C C Time - hours Calculated H 2 O oxidation rates using Deal Grove. B. Thin Oxide Growth Kinetics A major problem with the Deal Grove model was recognized when it was first proposed - it does not correctly model thin O 2 growth kinetics. Experimentally O 2 oxides grow much faster for 200 Å than Deal Grove predicts. MANY suggestions have been made in the literature about why. None have been widely accepted.. Reisman et. al. Model b xo = a t+ ti or xo a t ( ) = + xi a b b (7)

12 2 Simple power law fits the data over the whole range of oxide thicknesses. a and b are experimentally extracted parameters. Physically - interface reaction controlled, volume expansion and viscous flow of SiO 2 control growth. 2. Han and Helms Model dx dt O = B + 2x + A 2x O B2 + A O 2 (8) Second parallel reaction added - fits the data over the whole range of oxide thicknesses. Three parameters (one of the A values is 0). Physically - second process may be outdiffusion of O V and reaction at the gas/sio 2 interface. 3. Massoud et. al. Model dx dt O = B x A C O + exp (9) 2 + xl O Second term added to Deal Grove model which gives a higher dx/dt during initial growth. L 70 Å so the second term disappears for thicker oxides. Because it is simply implemented along with the Deal Grove model, this model has been used in process simulators. Experimental data agrees with the Reisman, Han and Massoud models. (800 C dry O 2 model comparison below.)

13 Oxide Thickness - microns Deal Grove Model (τ = 8 hrs) Reisman et. al. Model Han & Helms Model Deal Grove Model (τ = 0) Time - hours C. 2D SiO 2 Growth Kinetics 950 C oxidation (left), 00 C oxidation right (Marcus and Sheng).

14 4 These effects were investigated in detail experimentally by Kao et. al. about 0 years ago. a) Etched Si Ring Si Substrate Side Views Top Views b) c) Polysilicon SiO 2 Si d) Typical experimental results (from Kao et.al.)

15 C C Normalized Oxide Thickness C 000 C 900 C 800 C 900 C 000 C 0.3 Convex Radii 0.2 Concave Radii 0. µm 0.2 µm 0.25 µm /r µm - Several physical mechanisms are important in explaining these results: Crystal orientation 2D oxidant diffusion - simple to implement in a 2D numerical simulator Stress due to volume expansion To model the stress effects, Kao et. al. suggested modifying the Deal Grove parameters. nvr tvt ks( stress) = ksexp σ exp σ kt kt (20)

16 6 P VD D( stress) = Dexp ( )( ) kt * * C ( stress) C exp P VS kt = ( )( ) (2) (22) where σ n and σ t are the normal and tangential stresses at the interface. VR, VT and VS are reaction volumes and are fitting parameters. Finally, the flow properties of the SiO 2 are described by a stress dependent viscosity σsvc/ 2kT η( stress) = η( T) sinh σ VC/ 2kT ( ) S (23) where σ S is the shear stress in the oxide and VC is again a fitting parameter. These models have been implemented in modern process simulators and allow them to predict shapes and stress levels for VLSI structures.

17 SiO 2 Si 3 N 4 0 Microns Silicon Microns Microns Left - no stress dependent parameters. Right - including stress dependence. (ATHENA.) D. Point Defect Based Models The oxidation models we have considered to this point are macroscopic models (diffusion coefficients, chemical reactions etc.). There is also an atomistic picture of oxidation that has emerged in recent years. Most of these ideas are driven by the volume expansion occurring during oxidation and the need for free volume.

18 * Class Handout # O 2 Diffusion * V H 2 O I * I Oxide Silicon In Chapter 3 we described internal oxidation in the following way: ( + 2γ) Si Si + 2OI + 2βV SiO2 + 2γI + stress (24) Surface oxidation can be thought of in the same way. The connection between oxidation and other processes can then be modeled as shown below. O2 Surface Recombination G R Bulk Recombination I Inert Diffusion Buried Dopant Marker Layer OED

19 9 Microns Si 3 N 4 SiO 2 Inert Diffusion OED Microns Example - ATHENA simulation of OED. E. Silicide Oxidation Models Other materials are often oxidized in silicon structures (poly, silicides). Models have been developed, based on the Deal Grove model for Si oxidation. O 2 H 2 O Si or M SiO 2 /MO x Silicide Poly SiO 2 Silicon

20 20 In silicides, the physical processes include oxidant diffusion through the oxide and reaction at the oxide/silicide interface. Thus the basic linear parabolic model usually works. x 2 2 O i O i x B x x + B/ A = t (0) In most cases, SiO 2 forms rather than Mo x (see text). If SiO 2 forms, then B is the same as for Si oxidation. B/A values for silicide oxidation are generally much larger than the Si values (0-20X). Apparently the bond breaking process at the SiO 2 /silicide interface is much easier than at the SiO 2 /Si interface. Therefore the overall kinetics may be just parabolic. F. Complete Process Simulation of Oxidation Many of the models described above (and others that are in Chapter 5), have been implemented in programs like SUPREM. In an integrated simulator, these models must work in harmony with each other.

21 2 Si 3 N 4 0 Microns Silicon Si 3 N 4 Microns SiO 2 Silicon Microns Simulation of a recessed LOCOS isolation structure using SSUPREM IV. The initial structure (top left) is formed by depositing an SiO 2 /Si 3 N 4 structure followed by etching of this stack on the left side. The silicon is then etched to form a recessed oxide and the structure is oxidized for 90 min at 000 C in H 2 O. The time evolution of the bird s head shape during the oxidation is shown in the simulations.

22 22 Microns Microns Microns Simulation of an advanced isolation structure (the SWAMI process originally developed by Hewlett- Packard), using SSUPREM IV. The structure prior to oxidation is on the top left. This structure is formed by depositing an oxide followed by a thick Si 3 N 4 layer, both of which are etched away on the right side. A silicon etch on the right side is then followed by a second oxide and nitride deposition. These layers are then etched away on the far right side, leaving the thin SiO 2 /Si 3 N 4 stack covering the sidewall of the silicon. A 450 min H 2 O oxidation at 000 C is then performed which results in the structure on the top right. An experimental structure fabricated with a similar process flow is shown on the bottom right. The stress levels in the growing SiO 2 are shown 00 min into the oxidation on the bottom left.