ECSE-6300 IC Fabrication Laboratory Lecture 2 Thermal Oxidation. Introduction: Oxide

Transcription

1 ECSE-6300 IC Fabrication Laboratory Lecture 2 Thermal Oxidation Prof. Bldg. CII, Rooms 6229 Rensselaer Polytechnic Institute Troy, NY Tel. (518) s: luj@rpi.edu 2-1 Introduction: Oxide Major uses of SiO 2 : Formation Techniques of SiO 2 : Focus of this lecture: Thermal Oxidation 2-2

2 Lecture Outline Thermal Oxidation Deal-Grove Oxidation Model Orientation Dependence Dopant Dependence Oxide Charges Dopant Segregation Oxidation-Induced Stacking Faults Ref.: S.M. Sze, VLSI Technology, 2 nd Ed. McGrraw-Hill, S. Wolf and R.N. Tauber, Silicon Processing for the VLSI Era, Vol. 1 Processing Technology, 2 nd Ed. Lattice Press, Note: The lecture slides were prepared based on the original materials written by Profs. T.P. Chow and J.-Q. Lu 2-3 Deal-Grove Oxidation Model Assumptions: oxidation occurs at the interface between oxide and substrate; oxidant (e.g., O 2, or H 2 O) is not dissociated, and diffuses from the bulk of the ambient gas to the surface. diffuses through the existing oxide layer to the oxidesubstrate interface. reacts with the substrate. Steady State: : Number of oxident molecules incorporated into a unit volume of oxide. 2-4

3 Flux F 1 Deal-Grove Oxidation Model (continued) Using Henry s Law in solid, And ideal gas law,, 1 where. In the oxide, assuming Fick s Law, p G p S 2 Flux at the oxide/si interface, 3 k S : SiO 2 reaction rate C O : equilibrium concentration in the oxide at the outer surface; C * : equilibrium bulk concentration in the oxide 2-5 Deal-Grove Oxidation Model (continued) Limiting cases: Very small diffusivity, C i 0, C o C* Diffusion controlled case D is large, C i C o Reaction controlled case 2-6

4 Let Deal-Grove Oxidation Model, flux: (continued) Solution for : At 2-7 Deal-Grove Oxidation Model (continued) / / Long oxidation times ( t >> ) / (Parabolic Law) Short oxidation times ( ( t + ) << A 2 /4B ) (Linear Law) Deal and Grove, JAP,

5 Linear and Parabolic Rate Constants C* (cm -3 ) in SiO 1000 o C O 2 5.2x10 16 H 2 O 3.0x10 19 For O 2 : N 1 =2.2x10 22 cm -3 D = D 0 exp(-e A /kt) 2-9 Orientation Dependence Linear oxidation rate is larger for (111) than for (100) silicon, particularly at lower temperatures (900ºC) Proposed model attributes this to higher surface bond density Alternate model difference in oxide density? 2-10

6 Dopant Dependence Boron-doped B segregates into oxide, weakens bond structure, increases D 2-11 Dopant Dependence (continued) Phosphorus-doped P accumulates at the oxide/si interface, increases k S 2-12

7 Thin Oxides Low T, P; and rapid thermal oxidation (RTO) Nitric oxide (NO) gas anneal enhances oxide 1.2 nm gate oxide with Intel 90nm tech Example of Thin Gate Oxide Intel, 90 nm tech node in ~2003 => 45 nm Hi-k Metal-Gate in ~

8 High Pressure Oxidation C* p G 2-15 Thermal Oxide Structure SiO 2 SiO 2 Surface d d 0 Original Si Surface (SiO 2 /Si Interface) Quartz Fused Silica SiO 2 dielectrics d O O 2.3 Å d Si O 1.6 Å 2-16

9 Oxide Charges Fixed oxide charges (Q f ) Non stoichiometric SiO x Reduced with N 2 annealing at oxidation temperature Interface Charges (Q it ) From surface asymmetric bonds Reduced with 400ºC H 2 or D 2 annealing Other Charges (Q m, Q ot ) Reduced with clean processing 2-17 Dopant Segregation Boron depletes and phosphorus accumulates at the oxide/si interface Segregation is much worse when oxidation takes place at lower temperatures Boron Boron with H 2 Phosphorus Gallium 2-18

10 Dopant Segregation Orientation dependence Segregation can influence oxide quality Segregation can change device electrical parameters (such as BV, threshold voltage) 2-19 Oxidation-Induced Stacking Faults Agglomerated surplus interstitials in Si Stacking fault growth strongly depends on orientation, conductivity type and defect nuclei (100) > (111) N type > P type Stacking fault formation is worse for high pressure oxidation 2-20

11 Summary Dry O 2 Si + O 2 SiO 2 Wet O 2 Si + 2H 2 O SiO 2 + 2H 2 Deal-Grove Oxidation Model Diffusion- or Reaction-controlled case (Parabolic law) (Linear law) - Orientation Dependence - Dopant Dependence - Thin Oxide - High Pressure Homework #1 Other Topics - Oxide Charges - Dopant Segregation - Oxidation-Induced Stacking Faults 2-21 Introduction to TSUPREM4 Simulation Stanford University PRocess Engineering Module (SUPREM) Unix based Program Version 4 is a two dimensional simulation program Simulates implantation, diffusion, oxidation, etching, deposition, lithography and epitaxy Output: Stresses, boundaries of various layers, impurity distribution 2-22

12 Getting Started with TSUPREM4 Login/Access Computer TSUPREM4 is loaded on the ecse-cad machines Can use any of the ECSE lab workstations (machines in CII-6118 are available M-F 9 to 5) Login to ECSE machines with same procedure as if logging into RCS machines (use your ECSE username & password) Remote access also possible 2-23 Getting Started with TSUPREM4 Writing & Editing the Code Use a text editor like vi, emacs, pico to enter the code of the input file After entering the code, save the file as <filename.inp>, to specify it as input file 2-24

13 Getting Started with TSUPREM4 Simulation & Saving Output To run the program, just type tsuprem4 <filename.inp> Statements contained in the input file are executed and the output is plotted on the screen. TSUPREM 4 stores the output as <filename.out> in the home directory The plots can be grabbed using XV or Snapshots utility 2-25 Commonly used TSUPREM4 Statements INITIALIZE - sets up the mesh for simulation LINE to specify detailed mesh SAVEFILE -writes mesh and solution information into a file DEPOSITION deposits specified material on top surface ETCH removes specified portion of the current structure IMPLANT simulates ion implantation DIFFUSION used for both diffusion and oxidation EPITAXY grow epitaxy layer SELECT - to evaluate a quantity to be printed or plotted PRINT.1D PLOT.1D PLOT.2D LABEL 2-26

14 Typical Input File Structure Basic Statements Title, Comment ($) Initialize Process Sequence Statements Implant Diffusion Deposition Etch Output Specification Statements Plot, Print Savefile Calculation Statements VThreshold 2-27 Example 1 2D $ Automatic grid generation for the initial substrate INITIALIZE <100> WIDTH=10 PHOSPHORUS=2E15 $ Plot the structure SELECT Z=doping TITLE="Initialization" PLOT.2D Y.MAX=10 BOUNDARY COLOR COLOR=3 SILICON $ Wet Oxidation diffusion temp=1000 time=210 steam $ Plot the structure after oxidation SELECT Z=doping TITLE="Wet oxidation" PLOT.2D Y.MAX=10 BOUNDARY COLOR COLOR=3 SILICON COLOR COLOR=4 OXIDE 2-28

15 Example 1 2D Initial Structure After Oxidation 2-29 Example 2 1D $ Automatic grid generation for the initial substrate initialize <100> width=10 phosphorus=2e15 $ Ion Implant Boron and drive-in implant boron dose=1e13 energy=100 diffusion temp=950 time=30 inert diffusion temp=1050 time=120 inert $ Plotting the doping profile select z=log10(boron) plot.1d x.value=10 bottom=13 top=21 right=5 line.typ=5 color=2 select z=log10(phosphorus) plot.1d x.v=10 ^axes ^clear line.typ=2 color=3 2-30

16 Example 2 1D $ Ion Implant Arsenic and drive-in implant arsenic dose=1e15 energy=50 diffusion temp=1000 time=30 inert $ Plotting the doping profile select z=log10(boron) plot.1d x.value=10 bottom=13 top=21 right=5 line.typ=5 color=2 select z=log10(phosphorus) plot.1d x.v=10 ^axes ^clear line.typ=2 color=3 select z=log10(arsenic) plot.1d x.v=10 ^axes ^clear line.typ=2 color= Example 2 1D After Boron diffusion After Arsenic implant/diffusion 2-32