-
1/21/11
1
1
tanford araswat !
EE410 - 2011
Process and Device Simulations
Krishna Saraswat Stanford University
2
tanford araswat !
EE410 - 2011
CMOS Process Flow Arsenic
Mask 4 - Active PMOS shield. Arsenic implant for NMOS source/drain formation and gate doping.
7
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.
8
9
10
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.
1
2
3
Field oxidation (LOCOS)
4
Mask 2 - P-well definition. boron p-well implant and drive-
5
Boron ions
Amorphous-Si gate deposition.
6
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
-
1/21/11
2
3
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
4
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
2
-
1/21/11
3
5
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"
6
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"
-
1/21/11
4
7
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"
8
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"
-
1/21/11
5
9
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!
10
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"
-
1/21/11
6
11
tanford araswat !
EE410 - 2011
Create sub-structures (use loadfile or initialize to load structure and savefile to save structure)"
Hierarchy of SUPREM files
12
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"
-
1/21/11
7
13
tanford araswat !
EE410 - 2011
Example: PMOS S/D Profiles
Bulk silicon
Gate oxide
Bulk siliconMetal
TSUPREM Simulations Analytical Calculations
14
tanford araswat !
EE410 - 2011
Example: PMOS Channel Profiles TSUPREM Simulations
This profile is not accurate
-
1/21/11
8
15
tanford araswat !
EE410 - 2011
Example: NMOS Channel Profiles TSUPREM Simulations
16
tanford araswat !
EE410 - 2011
Example: NMOS S/D Profiles TSUPREM Simulations
-
1/21/11
9
17
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
18
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"
-
1/21/11
10
19
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"