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tanford araswat !

EE410 - 2011

Process and Device Simulations

Krishna Saraswat Stanford University

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tanford araswat !

EE410 - 2011

CMOS Process Flow Arsenic

Mask 4 - Active PMOS shield. Arsenic implant for NMOS source/drain formation and gate doping.

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Phosphosilicate glass deposition. Final "do-all" anneal. Mask 5 - Definition of contact holes

Al/Si metallization by sputtering.Mask 6 - Patterning of Al/Si interconnect.

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BF2

BF2 implant for PMOS source/ drain formation and gate doping.

p-poly

n-poly

field oxide LTO-PSG

thin oxide

n-substrate p-well

n+ source/drain

Al/Si alloy

p+ source/drain photoresist

undoped poly

Silicon nitride

Fig. 1 (cont'd). CIS/CMOS-II process summary. Refer to text for details.

________________________________________________________________________ Appendix B: Pad Assignments 7

Phosphorus ions

Phosphorus blanket substrate implant.

Thin oxide and silicon nitride deposition.

Mask 1- active area definition.

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Field oxidation (LOCOS)

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Mask 2 - P-well definition. boron p-well implant and drive-

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Boron ions

Amorphous-Si gate deposition.

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Silicon nitride

n-substrate p-well

n+ source/drain

Al/Si alloy

p+ source/drain photoresist

field oxide LTO-PSG

thin oxide

p-poly

n-poly

undoped poly

Fig. 1. CIS/CMOS-II process summary. Refer to text for details.

________________________________________________________________________ Appendix B: Pad Assignments 6

Gate oxidation, gate a-Si deposition

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tanford araswat !

EE410 - 2011

1. Analytical calculations based on EE212 a) Field and gate oxide thicknesses b) Ion implant profiles c) Junction depths d) Sheet resistance of junctions and poly-Si gate

-  Mobility is a function of doping density. Use either Irwin’s curves or analytical model given in text books, for example in Pirret’s book.

-  Note that mobility in poly-Si is lower. Assume half that of crystalline Si. -  Dopant diffusivity in poly-Si is several orders of magnitude higher than crystalline Si

e) Threshold voltages 2. TSUPREM or Sentaurus simulations on the same topics as in 1 3. Comparison between analytical calculations and simulations

What needs to be done? Al/Si alloy

P+ poly-Si N+ poly-Si

LOCOS Field oxide

LPCVD SiO2

P+ P+ N+ N+ P-well

Gate oxide N-Si

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tanford araswat !

EE410 - 2011

Cross-Sections to be Simulated"PMOS NMOS

Cross-sectionnumber

Description

1 PMOS source/drain2 PMOS channel3 Parasitic metal-field PMOS channel4 Parasitic p+-poly-field PMOS channel5 NMOS source/drain6 NMOS channel7 Parasitic metal-field NMOS channel8 Parasitic n+-poly-field NMOS channel

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tanford araswat !

EE410 - 2011

What is TSUPREM-4?"

•  Two dimensional process simulation program"•  Simulates "

–  Accurately simulates oxidation, ion-implantation, diffusion "–  Approximately simulates etching, deposition, epitaxy"

•  Output: thickness of layers, dopant distribution, certain electrical properties"

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tanford araswat !

EE410 - 2011

Using TSUPREM-4"

•  Create file containing processing info and output statements with TSUPREM commands using Emacs, VI or any of your favorite text editor. Do not use software like Word or Pages. "

•  To run the simulation at the command prompt, type:"" "tsuprem4 "

"•  Output is in file named .out"

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tanford araswat !

EE410 - 2011

Using TSUPREM-4 in 1-D mode: Gridding"

•  We will use TSUPREM-4 in1-D mode, and grid the structure, e.g., in x-direction"

•  Denser grid in areas where a lot of action occurs and where precision of information is important"

•  i.e. thin layers, areas with steep dopant profiles"

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tanford araswat !

EE410 - 2011

Creating a TSUPREM Input File"• Initialize (required before other steps)"

Silicon, 5µm thick, .01µm grid spacing at top of layer, phosphorus concentration of 1e15"

Initialize material=silicon width=5 dx=.01 – units in µm"+ phosphor=1e15 – Initialize Statement!Initialize in.file=struct2 – Read structure from file struct2!

• Process steps"Diffusion with Transient Enhanced Diffusion effect:"

"Method PD.TRANS – diffusion model""Diffusion Temperature=1000 Time=10 DryO2– units in °C and minutes"

Diffusion with temperature ramp: ""Diffusion Temperature=800 T.Final=900 Time=20 Inert"

Examples of other steps""Deposition Thickness=0.6 Aluminum Dx=.02 Spaces=5""Implant Dose=5e15 Energy=45 BF2 Gaussian – units in cm-3 and keV""Etch oxide all"

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tanford araswat !

EE410 - 2011

Creating a TSUPREM input file (2)"•  Output commands"

Print Layers"Electrical – Lists out the thicknesses of the various layers and

their sheet resistance!Print Layers""Comment Vt measurement"+ Sweep bias from -10 to 10V in 0.2V step"+ Interface charge Qss concentration of 5e10 added"Electrical x=0 threshold nmos v=“-10 10 0.2”"+ " " Qss=5e10 (Default is 1e10/cm2)!+ " " Out.file=struct2"$ " " "Another way to do comments""Plot.1d electric "$This command plots the resistivity of the region under the

gate as a function of the voltage!

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tanford araswat !

EE410 - 2011

$ EE410 Diode

$ This creates a simple diode

$ Initialize the siliconInitialize material=Silicon width=5 dx=.01+ phosphor=1e15

$ Boron ImplantImplant Dose=5e15 Energy=45 BF2

$ Anneal using transient modelsmethod pd.transDiffusion Temperature=950 Time=30 Nitrogen

$ Deposit AluminumDeposition Thickness=0.6 Aluminum dx=.02 Spaces=5

$ To plot the output to a postscript fileOption device=postscript plot.out=”+ee410diode.ps”

$ Plot the net dopant concentration$ Print the layersSelect z=log10(boron)Plot.1d bottom=14 top=21 right=2Print.1d out.file=Boron.txtSelect z=dopingPrint Layers

Example of an input file"

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tanford araswat !

EE410 - 2011

Create sub-structures (use loadfile or initialize to load structure and savefile to save structure)"

Hierarchy of SUPREM files

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tanford araswat !

EE410 - 2011

$ CIS/CMOS-II Process Flow: STARTING SUBSTRATE WITH FIELD +O X I D E

$ Filename = create_sub

$ Start with silicon, phosphorus-doped to 5.5 ohm-cm.Initialize material=Silicon Phosphorus=8E14+ width=1.5 dX=0.005

$ Initial phosphorus blanket implant.Implant Phosphorus Energy=100 Dose=3.5E12

$ Grow 500nm field oxide.$ CIS furnaces use argon instead of nitrogen.Diffusion Time= 35 Temperature= 800 T.Rate=+5.714N i t r o g e nDiffusion Time= 10 Temperature=1000 DryO2Diffusion Time=100 Temperature=1000 WetO2Diffusion Time= 10 Temperature=1000 DryO2Diffusion Time= 35 Temperature=1000 T.Rate=-5.714 Nitrogen

$ Output results.Option device=postscript plot.out=”EE410Substrate.ps”Print L a y e r sS e l e c t z = l o g 1 0 ( a c t i v e ( p h o s p o r u s ) )P l o t . 1 d bottom=14 top=21 right=2

S a v e f i l e o u t . f i l e = c r e a t e _ s u b . s t r

Structure 1: Starting Substrate With Field Oxide"

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tanford araswat !

EE410 - 2011

Example: PMOS S/D Profiles

Bulk silicon

Gate oxide

Bulk siliconMetal

TSUPREM Simulations Analytical Calculations

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tanford araswat !

EE410 - 2011

Example: PMOS Channel Profiles TSUPREM Simulations

This profile is not accurate

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tanford araswat !

EE410 - 2011

Example: NMOS Channel Profiles TSUPREM Simulations

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tanford araswat !

EE410 - 2011

Example: NMOS S/D Profiles TSUPREM Simulations

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tanford araswat !

EE410 - 2011

Useful tips"•  Create sub-structures (use loadfile or initialize to

load structure and savefile to save structure) •  Skip steps (freckle etch) and ignore effects (layer

undercutting) that TSUPREM cannot simulate •  Do not simulate furnace cycles below 800°C •  Use gaussian or 2-gaussian implant profiles

instead of pearson profiles for all implants •  See handout on CMOS-LOCOS Manual for more

details on TSUPREM •  Simulation report will be due on 2/7/2011

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tanford araswat !

EE410 - 2011

Writing the report"

•  Donʼt spend your time trying to get your hand calculations to match your SUPREM results"

•  Focus on differences between hand calculations and SUPREM results and explain why they are different"

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tanford araswat !

EE410 - 2011

Getting help"

•  Enter the following line in a file and run the file in TSUPREM. Help info will appear in the output file."help "example: help implant ""Examples in the directory: /usr/class/ee410/examples"