How is a Wafer Fabricated / Wafer Fabrication Procedure?

How is a Wafer Fabricated / Wafer Fabrication Process Steps?
Written By: Kia Javadi

Semiconductor fabrication can be a very time-consuming and complicated procedure. Beginning with a blank wafer (silicon wafer) the steps required to apply a mask and fabricate devices involve microscopes, photolithography equipment, etching stations, developer, etc. Extensive testing must also be done along the way to ensure proper electrical characteristics are obtained and to confirm that the process had been completed appropriately. Much of the work today is computer driven and done with speed and yield considerations in mind. This page is designed to serve as a simple overview over the various processes and design techniques undertaken to go from a blank wafer to a completed semiconductor device (with appropriate contacts). The devices illustrated are a P-N Diode, Schottky Diode, and NMOS.

Design Steps - Image Representation (See Below For Text)...

Design Steps - Text Representation (See Above For Images)...

Step	Name	Description/Requirements	Img#
1	Wafer Reception	Wafers obtained sliced from boule - 4" Diameter <100> Si	1
2	Boron Implantation	P type implant doping of silicon (5x10^12/cm^2)	2

3	Piranha Cleaning	Wafer cleaned with 10:1 H2SO4 Piranha solution to remove organics
4	Drive-in Annealing	120min placement in Oxidation Furnace for annealing	3
5	Field-Oxidation Deposition	Wet Oxide Process - 5000A thickness target	4

6	Thickness Measurement	Nanospec test from each quadrant to check for uniform thickness
7	Demos	Demos and Introduction to Lab Safety
8	Dehydration Bake - 5min	Dry off excess moisture from wafer surface/bottom which can stick to devices
9	HMDS Coating - 10min	Improves adhesion of photoresist
10	Apply Photoresist/Prebake	Applies a layer of photoresist to which we will expose the mask.	5
11	Active Mask	Apply active mask to photoresist prior to development
12	Development	Develop for 60 seconds in Developer solution	6

13	Post-Bake	For 120min @ 120C to complete the development process
14	Plasma Etching	Dry etching process is used to etch photoresist w/ minimal undercutting
15	Wet Etching	6:1 BOE solution - Wet etching process
16	Inspection	Using the nanospec, check to ensure that oxide/silicon is properly etched
17	Strip photoresist	Using 4:1 Piranha solution to remove organics	7
18	RCA1 + RCA2 Cleaning	Strip the organic and inorganic matrials from the wafer
19	Dry Oxide Growth - Gate	In oxidation furnace - 30min - Target thickness 600A	8

20	LPCVD Poly Deposition	Used to deposit uniform layer of poly on the wafer - Target thickness 2000A	9
21	Oxide/Poly Thickness Nanospec	Check above steps by TA for correct measurements.
22	Dehydration Bake - 5min	Dry off excess moisture from wafer surface/bottom which can stick to devices
23	HMDS Coating - 10min	Improves adhesion of photoresist
24	Apply Photoresist/Prebake	Applies a layer of photoresist to which we will expose the mask.
25	Poly Mask	Apply poly mask to photoresist prior to development
26	Development	Develop for 60 seconds in Developer solution
27	Post-Bake	For 120min @ 120C to complete the development process
28	Post-Bake	For 120min @ 120C to complete the development process
29	Plasma Etching	Dry etching process is used to etch photoresist w/ minimal undercutting
30	Polysilicon Etch	DRIE Plasma etch - defines the gate of the NMOS	10

31	Nanospec Inspection	Check for Poly Thickness/Gate Oxide thickness under 20A
32	Backside Poly Etch	The process used above deposits poly on both sides of the wafer, so the backside must also be etched - Approximately 5 minutes	11
33	Wet Oxide Etch	Etch oxide for approximately 5.5min in 6:1 BOE Solution	12

34	4-Point Probe Inspection	Check for resistance values of monitor bars.
35	Strip photoresist	Using 4:1 Piranha solution to remove organics
36	Ion Implantation Procedure	Since we are using a self aligned Source/Gate/Drain, Ion implantation is necessary. Phosphorous implant done on the frontside/Boron implant done on the backside	13
37	RCA Cleaning Procedure	Strip organic and metallic elements off the wafer
38	Activation Anneal	Activation is required to activate the implanted atoms and repair any damage caused by the annealing process	14

39	LTO Oxide Deposition	LPCVD Furnace deposition - Target Thickness 5000A	15
40	Dehydration Bake - 5min	Dry off excess moisture from wafer surface/bottom which can stick to devices
41	HMDS Coating - 10min	Improves adhesion of photoresist
42	Apply Photoresist/Prebake	Applies a layer of photoresist to which we will expose the mask.
43	Contact Mask	Apply contact mask to photoresist prior to development
44	Development	Develop for 60 seconds in Developer solution
45	Post-Bake	For 120min @ 120C to complete the development process
46	Plasma Etching	Dry etching process is used to etch photoresist w/ minimal undercutting - 2min
47	Wet Etching	6:1 BOE solution - Wet etching process - One wafer for 5min, second for 9min	16

48	Inspection	Using the nanospec, check monitor bar measurements Poly/ S/D / Con/ & Schottky
49	4-Point Probe measurements	Measure monitor bars and check resistance values
50	RCA1 + RCA2 Cleaning	Strip the organic and inorganic matrials from the wafer
51	Aluminum Sputtering	Frontside Target 5000A / Backside Target 2500A	17
52	Dehydration Bake - 5min	Dry off excess moisture from wafer surface/bottom which can stick to devices
53	HMDS Coating - 20min	Improves adhesion of photoresist. This step was doubled for increased adhesion during this process.
54	Apply Photoresist / Prebake	Applies a layer of photoresist to which we will expose the mask.
55	Metal Mask	Apply metal mask to photoresist prior to development
56	Development	Develop until proper color change is seen.
57	Post-Bake	For 120min @ 120C to complete the development process
58	Backside Photoresist / Prebake	Applies a layer of photoresist to the backside. Done to protect the backside Aluminum during the etching.
59	Plasma Etching	Dry etching process is used to etch photoresist w/ minimal undercutting - 2min
60	Metal Etching	Very fast procedure (overetching is not recommended and can kill the device). Etch until red photoresist shows - approximately 7min)
61	Plasma Etching	Dry etching process is used to etch photoresist w/ minimal undercutting - 2min
62	Sintering of Metal	Creates a good ohmic contact between the Aluminum and Silicon	18

63	Measurements	Characterization of all required devices using Probes	19