💻CMOS - sputtered gate stack (WIP)
Overview
This is in support of the development of a CMOS process that uses a Si-Al2O3-Al gate stack. The NMOS process originally developed with Hacker Fab tooling relies on procuring wafers that already have a Si-SiO2-PolySi prefabricated gate. This prevents the expansion of the NMOS process into a CMOS process. For a CMOS process, it is necessary for the gate stack to be fabricated in house with Hacker Fab tooling. Sputtering of the gate stack from just an Al target is pursued as a potential option to mitigate the risks associated with gate oxide contamination. For context, growing a SiO2 gate oxide has been considered, but abandoned due to a few challenges. The main being contamination control of the gate oxide in an open air tube furnace, contamination that can be introduced between oxide growth and metal deposition for the gate, and the added complexity of dopant pile up + depletion (discussed in the Oxidation Effects Section of "Microchip Fabrication: A Practical Guide to Semiconductor Processing by Peter Van Zant"). Overall this intended process simplifies the tooling required to tape out devices (cuts out evaporator and plasma etcher). Sputtering allows for a low contamination oxide to be deposited, which is immediately covered with a metal gate contact without the chip ever exiting the clean vacuum environment of the sputtering chamber. The choice of Al2O3 as the oxide and Al as the metal is motivated by simplified process flow it allows, and relatively inexpensive target material (Al2O3 is sputtered reactively form an Al target, then Al is sputtered immediately after from the same target, avoiding the need for a 2 target system or a target swapping). Al2O3 has favorable dielectric constant and band alignment, but there are risks related to charge traps forming at the Si-Al2O3 interface. Attempts are made to tune the process to mitigate this interface effect.
Thus far only NMOS devices have been fabbed to verify effectiveness of sputtered gate oxide. Details on n well doping process dev to allow for CMO devices coming soon
More details on general approach and methods and next steps coming soon
The sputtered film characterization that precedes this work is linked below.
Film CharacterizationMOSCap Capacitance and Conductance Profiling
MOSCap Capacitance and Conductance Profiling

MOSCap devices were fabricated from the sputtered Al and Al2O3 for Conductave-Voltage and Capacititacne voltage profiling. They were fabricated from the process flow inlcuded below from prime p-Si 5-10 ohm (100) wafers. Oxide thickness, surafce preparation method, and post oxide deposition anealing were modulated to study the impact on capcitance and conductance testing.The individual MOSCaps were defined by applying a shadowmask in between the oxide Deposition nad the firt Al deposition.
0
Substrate Stack
Si p-type 5-10 ohms (100): 525 µm

1
RCA Clean
SC 1 Constituents: H2O:NH4OH(29%):H2O2(30%) (5:1:1) SC 1 Cleaning Time: 10 mins SC 1 Cleaning Temperature: 80 °C HF Concentration: 6:1 BOE HF Dip Time: 15 secs HF Dip Temperature: 20 °C SC 2 Constituents: H2O:HCl(37%):H2O2(30%) (6:1:1) SC 2 Cleaning Time: 10 mins SC 2 Cleaning Temperature: 80 °C "SC 1 and 2 mixed in 50ml pyrex beakers directly before using. Optional HF last processing step (Experimental Option) Rinsed with DI Water then N2 dried. Immediately placed into sputtering chamber for pump down."

2
Sputter w/ CMU Hacker Fab RF Sputtering Chamber
Material: Al2O3 Film thickness: Experimental Option Deposition Rate: .00025 µm/min Deposition Time: 80 mins Sputter Type: RF Sputtering RF or DC Power: 100 Watts Sputter Gas Composition: 9 SCCM UHP Ar : 9 SCCM UHP O2 Chamber Pressure: 4.7E-3 Torr Throw Distance: 3" Base Pressure Pre Deposition: 1E-7 Torr Target Sputter Clean Ar Flow: 30 SCCM Target Sputter Clean Time: 15 Minutes

3
Anneal (Experimental Option)
Performed in RF sputtering Chamber under partial Vacuum. Pump speed at 250Hz with 50 SCCM of either UHP Ar or UHP O2.

4
Sputter w/ CMU Hacker Fab RF Sputtering Chamber
Material: Al Film thickness: 150 nm Deposition Rate: .0025 µm/min Deposition Time: 60 mins Sputter Type: RF Sputtering RF or DC Power: 100 Watts Sputter Gas Composition: 15 SCCM UHP Ar Chamber Pressure: 3E-3 Torr Throw Distance: 3" Base Pressure Pre Deposition: 1E-7 Torr Target Sputter Clean Ar Flow: 30 SCCM Target Sputter Clean Time: 15 Minutes SWR During Deposition: 1.5

6
Sputter w/ CMU Hacker Fab RF Sputtering Chamber
Material: Al Film thickness: 150 nm Deposition Rate: .0025 µm/min Deposition Time: 60 mins Sputter Type: RF Sputtering RF or DC Power: 100 Watts Sputter Gas Composition: 15 SCCM UHP Ar Chamber Pressure: 3E-3 Torr Throw Distance: 3" Base Pressure Pre Deposition: 1E-7 Torr Target Sputter Clean Ar Flow: 30 SCCM Target Sputter Clean Time: 15 Minutes SWR During Deposition: 1.5

Summary/Comparison




Standard RCA clean
45
7.1
4.7
-0.20
-1.15
0.95
7.2
4.9
-0.20
-1.10
0.90
2.2E-05
2.3E-05
565.95
Standard RCA clean
15
5.3
2.4
-0.80
-1.20
0.40
5.5
2.5
-0.70
-1.00
0.30
8.3E-05
8.3E-05
495.45
HF-Last RCA clean
45
7.4
4.7
-1.90
-2.20
0.30
7.4
4.8
-1.85
-2.15
0.30
1.9E-05
2.0E-05
260.67
HF-Last RCA clean
20
6.6
3.2
-1.05
-1.85
0.80
6.7
3.4
-0.95
-1.70
0.75
7.5E-05
8.3E-05
289.87
HF-Last RCA clean, 450C O2 anneal
20
4.7
2.8
-4.35
-4.70
0.35
5.3
3.1
-4.15
-4.45
0.30
8.1E-05
9.0E-05
910.06
MOSCap Leakage Current

MOSFETs and TLM Test Structures



0
Substrate Stack
p-Si 5-10 ohm (100): 525 µm
1
Acetone + IPA Clean (N2 dry)
Cleaning Agent: Acetone then IPA Squirt with Acetone, then IPA, then dry the surface with the N2 gun.
2
Bake
Bake Temperature: 100 °C Bake Time: 60 secs
3
Spin-Coat
Material Type: Adhesion Promoter Material: HMDS Spin Speed: 4000 rpm Spin Time: 30 secs
4
Bake
Bake Temperature: 100 °C Bake Time: 60 secs
5
Spin Resist
Resist: AZ P4210 Resist Type: Positive Spin Speed: 4000 rpm Spin Time: 30 secs
6
Bake
Bake Temperature: 100 °C Bake Time: 90 secs
7
Hacker Fab Maskless Litho Stepper
Exposure time: 8 secs
8
Develop
Developer: AZ 400K : DI Water (1:3) Develop Time: 30 secs
9
Wet-Etch
Actually performed with SF6 RIE, but can be replaced by a Nitric + HF wet etch solution since this step simply creates alignment marks in the Si.
10
Wet Strip Resist
Stripping Agent(s): Acetone then IPA Blow dry with N2 gun after.
11
RCA Clean
SC 1 Constituents: H2O:NH4OH(29%):H2O2(30%) (5:1:1) SC 1 Cleaning Time: 10 mins SC 1 Cleaning Temperature: 80 °C HF Concentration: 6:1 BOE HF Dip Time: 15 secs HF Dip Temperature: 20 °C SC 2 Constituents: H2O:HCl(37%):H2O2(30%) (6:1:1) SC 2 Cleaning Time: 10 mins SC 2 Cleaning Temperature: 80 °C
12
Spin-Coat
Material Type: Non-Resist Material: 700B (spin on glass) Spin Speed: 4000 rpm Spin Time: 20 secs
13
Bake
Bake Temperature: 400 °C Bake Time: 10 mins
14
Spin-Coat
Material Type: Adhesion Promoter Material: HMDS Spin Speed: 4000 rpm Spin Time: 30 secs
15
Bake
Bake Temperature: 100 °C Bake Time: 60 secs
16
Spin Resist
Resist: AZ P4210 Resist Type: Positive Spin Speed: 4000 rpm Spin Time: 30 secs
17
Bake
Bake Temperature: 100 °C Bake Time: 90 secs
18
Hacker Fab Maskless Litho Stepper
Exposure time: 8 secs
19
Develop
Developer: AZ 400K : DI Water (1:3) Develop Time: 30 secs
20
Wet-Etch
Etch Time: 20 secs Etching Agent(s): 6:1 BOE
21
Wet Strip Resist
Stripping Agent(s): Acetone then IPA Blow dry with N2 gun after.
22
Spin-On Dopant
Spin-On Dopant Name: P504 (Phosphorus source) Spin Speed: 4000 rpm Spin Time: 20 secs
23
Bake
Bake Temperature: 200 °C Bake Time: 10 mins
24
Dopant Diffusion
Diffusion Time: 30 mins Diffusion Temperature: 1100 °C Environmental: false

25
Wet-Etch
Etch Time: 10 mins Etching Agent(s): 6:1 BOE
26
RCA Clean
SC 1 Constituents: H2O:NH4OH(29%):H2O2(30%) (5:1:1) SC 1 Cleaning Time: 10 mins SC 1 Cleaning Temperature: 80 °C HF Concentration: 6:1 BOE HF Dip Time: 15 secs HF Dip Temperature: 20 °C SC 2 Constituents: H2O:HCl(37%):H2O2(30%) (6:1:1) SC 2 Cleaning Time: 80 mins SC 2 Cleaning Temperature: 75 °C
27
Sputter w/ CMU Hacker Fab RF Sputtering Chamber
Material: Al2O3 Film thickness: 20 nm Deposition Rate: .25 Å/s Deposition Time: 80 mins Sputter Type: RF Sputtering RF or DC Power: 100 Watts Sputter Gas Composition: 9 SCCM UHP Ar : 9 SCCM UHP O2 Chamber Pressure: 4.7E-3 Torr Throw Distance: 3" Base Pressure Pre Deposition: 1E-7 Torr Target Sputter Clean Ar Flow: 30 SCCM Target Sputter Clean Time: 15 Minutes SWR During Deposition: 1.5
28
Sputter w/ CMU Hacker Fab RF Sputtering Chamber
Material: Al Film thickness: 150 nm Deposition Rate: 2.5 Å/s Deposition Time: 60 mins Sputter Type: RF Sputtering RF or DC Power: 100 Watts Sputter Gas Composition: 15 SCCM UHP Ar Chamber Pressure: 3E-3 Torr Throw Distance: 3" Base Pressure Pre Deposition: 1E-7 Torr Target Sputter Clean Ar Flow: 30 SCCM Target Sputter Clean Time: 15 Minutes SWR During Deposition: 1.5
29
Spin Resist
Resist: AZ P4210 Resist Type: Positive Spin Speed: 4000 rpm Spin Time: 30 secs
30
Bake
Bake Temperature: 100 °C Bake Time: 90 secs
31
Hacker Fab Maskless Litho Stepper
Exposure time: 8 secs
32
Develop
Developer: AZ 400K : DI Water (1:3) Develop Time: 30 secs
33
Wet-Etch
Etch Time: 8 mins Etching Agent(s): Type A Al Etch (PAN) Etch Temperature: 40 °C
34
Wet Strip Resist
Stripping Agent(s): Acetone then IPA Blow dry with N2 gun after.
35
Spin-Coat
Material Type: Non-Resist Material: 700B (spin on glass) Spin Speed: 4000 rpm Spin Time: 20 secs
36
Bake
Bake Temperature: 400 °C Bake Time: 10 mins
37
Spin-Coat
Material Type: Adhesion Promoter Material: HMDS Spin Speed: 4000 rpm Spin Time: 30 secs
38
Bake
Bake Temperature: 100 °C Bake Time: 60 secs
39
Spin Resist
Resist: AZ P4210 Resist Type: Positive Spin Speed: 4000 rpm Spin Time: 30 secs
40
Bake
Bake Temperature: 100 °C Bake Time: 90 secs
41
Hacker Fab Maskless Litho Stepper
Exposure time: 8 secs

42
Develop
Developer: AZ 400K : DI Water (1:3) Develop Time: 30 secs

43
Wet-Etch
Etch Time: 20 secs Etching Agent(s): 6:1 BOE

44
Wet Strip Resist
Stripping Agent(s): Acetone then IPA Blow dry with N2 gun after.

45
Sputter w/ CMU Hacker Fab RF Sputtering Chamber
Material: Al Film thickness: 300 nm Deposition Rate: 2.5 Å/s Deposition Time: 120 mins Sputter Type: RF Sputtering RF or DC Power: 100 Watts Sputter Gas Composition: 15 SCCM UHP Ar Chamber Pressure: 3E-3 Torr Throw Distance: 3" Base Pressure Pre Deposition: 1E-7 Torr Target Sputter Clean Ar Flow: 30 SCCM Target Sputter Clean Time: 15 Minutes SWR During Deposition: 1.5

46
Spin Resist
Resist: AZ P4210 Resist Type: Positive Spin Speed: 4000 rpm Spin Time: 30 secs

47
Bake
Bake Temperature: 100 °C Bake Time: 90 secs

48
Hacker Fab Maskless Litho Stepper
Exposure time: 8 secs

49
Develop
Developer: AZ 400K : DI Water (1:3) Develop Time: 30 secs

50
Wet-Etch
Etch Time: 4 mins Etching Agent(s): Type A Al Etchant (PAN)

51
Wet Strip Resist
Stripping Agent(s): Acetone then IPA Blow dry with N2 gun after.

Steps 12-25 replaced by the below sequence in chip 1 for "maskless" diffusion:
Spin-On and Bake P504 Dopant (400 rpm 20s, 200C 10 mins)
Pattern on top of P504 Dopant
Wet-Etch P504 Dopant with 6:1 BOE (20s)
Dopant Diffusion (1100C 30 mins)
MOSFET masks, images, and results
All curves below recorded on lower row (50 um gate length, 21.4 um channel length)
Chip 1 electrical testing



Chip 2 pre anneal electrical testing



Chip 2 post anneal electrical testing



Chip 1 "maskless" doping
3.2E-3
8.8E-4
3.6
Chip 2 "masked" doping - best
4.4E-4
1.2E-6
369
Chip 2 400C annealed "masked" doping
4.9E-4
2.2E-6
227
TLM Structures (contact resistance and sheet resistance results)
Built on same chip(s) as FETs









Chip 1 "maskless doping"
7.15E-4
56.32
Chip 2 "masked doping"
1.78E-3
113.25
Chip 2 "masked doping" - 400C annealed
3.75E-4
92.27
Appendix
9:9, 3hr, Standard RCA Clean












Small (0.2 mm²)
10KHz
0.20
7.428647
5.073352
0.35
-0.90
1.25
9.863421
4.937214
0.05
-1.35
1.40
0.000012
0.000012
20992.490094
Small (0.2 mm²)
100KHz
0.20
7.114325
4.743494
-0.20
-1.15
0.95
7.186758
4.910686
-0.20
-1.10
0.90
0.000022
0.000023
565.949437
Small (0.2 mm²)
1MHz
0.20
6.768461
1.728415
-0.60
-1.40
0.80
7.379437
6.553137
-0.50
-1.30
0.80
0.000512
0.000514
165.844804
Medium (0.79 mm²)
10KHz
0.79
6.697895
5.073352
0.45
-0.60
1.05
6.800300
4.937214
0.50
-0.55
1.05
0.000009
0.000010
1349.595597
Medium (0.79 mm²)
100KHz
0.79
6.529448
4.743494
-0.10
-0.95
0.85
6.571378
4.910686
0.00
-0.85
0.85
0.000095
0.000099
124.666071
Medium (0.79 mm²)
1MHz
0.79
5.318851
1.728415
-0.45
-1.30
0.85
7.888473
6.553137
-0.40
-1.25
0.85
0.003640
0.003644
90.601948
Large (1.77 mm²)
10KHz
1.77
5.409678
5.073352
0.40
-0.25
0.65
5.535893
4.937214
0.45
-0.10
0.55
0.000023
0.000023
1002.470871
Large (1.77 mm²)
100KHz
1.77
5.092326
4.743494
-0.15
-0.60
0.45
5.241898
4.910686
0.00
-0.45
0.45
0.000241
0.000233
132.262058
Large (1.77 mm²)
1MHz
1.77
2.455902
1.728415
-0.35
-0.40
0.05
6.723377
6.553137
-0.45
-0.50
0.05
0.006887
0.006341
107.769918
9:9, 1hr, Standard RCA Clean











Small (0.2 mm²)
10KHz
0.20
6.141816
0.904621
-0.30
-1.00
0.70
13.045352
0.173698
-0.75
-0.80
0.05
0.000052
0.000056
9025.245375
Small (0.2 mm²)
100KHz
0.20
5.301111
2.379235
-0.80
-1.20
0.40
5.513058
2.470873
-0.70
-1.00
0.30
0.000083
0.000083
495.445897
Small (0.2 mm²)
1MHz
0.20
3.828395
0.381960
-1.10
-1.55
0.45
5.758140
3.908771
-1.00
-1.35
0.35
0.002048
0.002042
166.487007
Medium (0.79 mm²)
10KHz
0.79
3.996518
0.904621
-0.25
-0.60
0.35
5.358427
0.173698
-0.40
-0.70
0.30
0.000075
0.000072
3649.048078
Medium (0.79 mm²)
100KHz
0.79
3.610729
2.379235
-0.55
-0.95
0.40
3.720343
2.470873
-0.50
-0.80
0.30
0.000190
0.000199
159.210655
Medium (0.79 mm²)
1MHz
0.79
1.724243
0.381960
-0.85
-1.25
0.40
4.774321
3.908771
-1.00
-1.30
0.30
0.006874
0.006886
95.271610
Large (1.77 mm²)
10KHz
1.77
1.616946
0.904621
-1.45
-0.60
0.85
1.759600
0.173698
-1.40
-0.60
0.80
0.000002
0.000061
1696.143692
Large (1.77 mm²)
100KHz
1.77
2.742960
2.379235
-0.35
-0.75
0.40
2.847340
2.470873
-0.35
-0.70
0.35
0.000355
0.000355
104.022923
Large (1.77 mm²)
1MHz
1.77
0.803455
0.381960
-0.40
-0.75
0.35
4.144864
3.908771
-0.55
-0.95
0.40
0.010591
0.010604
77.807216
9:9, 3 hr, HF last












Small (0.2 mm²)
10KHz
0.20
7.685834
5.625961
-1.45
-1.90
0.45
7.766772
5.877118
-1.40
-1.85
0.45
0.000002
0.000002
3053.672852
Small (0.2 mm²)
100KHz
0.20
7.396978
4.707130
-1.90
-2.20
0.30
7.414154
4.847823
-1.85
-2.15
0.30
0.000019
0.000020
260.665467
Small (0.2 mm²)
1MHz
0.20
6.890245
2.242337
-2.25
-2.55
0.30
7.407294
4.727527
-2.20
-2.45
0.25
0.000470
0.000471
148.915132
Medium (0.79 mm²)
10KHz
0.79
7.129632
5.625961
-0.90
-1.30
0.40
7.175604
5.877118
-0.85
-1.25
0.40
0.000008
0.000009
1143.266648
Medium (0.79 mm²)
100KHz
0.79
6.360197
4.707130
-1.25
-1.60
0.35
6.415378
4.847823
-1.20
-1.55
0.35
0.000085
0.000090
147.965410
Medium (0.79 mm²)
1MHz
0.79
5.041438
2.242337
-1.55
-1.95
0.40
6.663681
4.727527
-1.50
-1.85
0.35
0.002853
0.002849
87.454312
Large (1.77 mm²)
10KHz
1.77
5.925761
5.625961
-1.75
-2.05
0.30
6.146619
5.877118
-1.60
-1.85
0.25
0.000026
0.000026
1447.212115
Large (1.77 mm²)
100KHz
1.77
5.077424
4.707130
-2.15
-2.35
0.20
5.200458
4.847823
-2.05
-2.25
0.20
0.000191
0.000195
136.210088
Large (1.77 mm²)
1MHz
1.77
2.878274
2.242337
-2.25
-2.50
0.25
5.116181
4.727527
-2.30
-2.55
0.25
0.005637
0.005627
79.692297
9:9, 80 min, HF last












Small (0.2 mm²)
10KHz
0.20
7.226328
4.517307
-0.30
-1.45
1.15
11.470554
4.553309
-1.85
-1.70
0.15
0.000034
0.000027
3265.030912
Small (0.2 mm²)
100KHz
0.20
6.614153
3.185460
-1.05
-1.85
0.80
6.721767
3.417425
-0.95
-1.70
0.75
0.000075
0.000083
289.871676
Small (0.2 mm²)
1MHz
0.20
4.991604
0.455764
-1.55
-2.45
0.90
7.040173
5.204334
-1.35
-2.05
0.70
0.001855
0.001850
162.998687
Medium (0.79 mm²)
10KHz
0.79
6.424663
4.517307
-0.45
-1.65
1.20
6.597815
4.553309
-0.40
-1.55
1.15
0.000027
0.000027
1121.137288
Medium (0.79 mm²)
100KHz
0.79
5.074745
3.185460
-1.15
-1.90
0.75
5.259125
3.417425
-1.10
-1.75
0.65
0.000245
0.000268
163.635326
Medium (0.79 mm²)
1MHz
0.79
2.018435
0.455764
-1.50
-2.35
0.85
6.804791
5.204334
-1.50
-2.25
0.75
0.006929
0.006923
105.118284
Large (1.77 mm²)
10KHz
1.77
4.906795
4.517307
-0.90
-1.85
0.95
5.354940
4.553309
-0.85
-1.70
0.85
0.000085
0.000083
1191.643675
Large (1.77 mm²)
100KHz
1.77
3.642597
3.185460
-1.60
-2.20
0.60
3.859411
3.417425
-1.50
-2.00
0.50
0.000497
0.000502
128.614905
Large (1.77 mm²)
1MHz
1.77
0.996573
0.455764
-1.75
-2.35
0.60
5.489297
5.204334
-1.90
-2.45
0.55
0.010393
0.010387
82.599948
9:9, 80 min, HF last, 450C 30 min O2 anneal












Small (0.2 mm²)
10KHz
0.20
5.725439
3.625706
-3.35
-4.30
0.95
10.969794
3.796601
-3.20
-4.55
1.35
0.000007
0.000026
4665.743838
Small (0.2 mm²)
100KHz
0.20
4.742700
2.765370
-4.35
-4.70
0.35
5.292607
3.124594
-4.15
-4.45
0.30
0.000081
0.000090
910.056283
Small (0.2 mm²)
1MHz
0.20
2.901555
0.793532
-4.75
-4.95
0.20
4.029582
2.940564
-4.65
-4.85
0.20
0.000823
0.000622
273.774244
Medium (0.79 mm²)
10KHz
0.79
4.972657
3.625706
-3.15
-4.30
1.15
5.803228
3.796601
-2.95
-3.70
0.75
0.000029
0.000053
2254.233872
Medium (0.79 mm²)
100KHz
0.79
3.760431
2.765370
-4.50
-4.75
0.25
4.186907
3.124594
-4.30
-4.55
0.25
0.000289
0.000286
309.539446
Medium (0.79 mm²)
1MHz
0.79
1.665962
0.793532
-4.75
-4.95
0.20
3.416144
2.940564
-4.80
-5.00
0.20
0.002619
0.001648
137.554599
Large (1.77 mm²)
10KHz
1.77
3.918623
3.625706
-3.10
-4.35
1.25
4.620144
3.796601
-3.05
-3.90
0.85
0.000062
0.000073
1385.010681
Large (1.77 mm²)
100KHz
1.77
3.022550
2.765370
-4.30
-4.65
0.35
3.404616
3.124594
-4.10
-4.50
0.40
0.000523
0.000518
210.306410
Large (1.77 mm²)
1MHz
1.77
1.054850
0.793532
-4.35
-4.65
0.30
3.077261
2.940564
-4.65
-4.90
0.25
0.005672
0.003891
97.212422
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