How Mass Metrology can be used for TSV monitor and SPC

1. Characterisation of
Through Silicon Via (TSV) processes
utilising Mass Metrology
Liam Cunnane, Adrian Kiermasz PhD, Gary Ditmer
Metryx Ltd., Bristol UK
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2. Outline
Mass Metrology
Principles
Methodology
Through-Silicon Via (TSV) Process Sequence
Deep Silicon Etch & Polymer clean
Oxide Liner
Barrier & Seed Deposition
Copper ECP & CMP
Summary
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3. Principle of Mass Metrology
Normal Distribution
0.4
Normal Distribution
0.4 0.3
density
0.3 0.2
ETCH
density
0.2 0.1
0.1 0
ALD
43 45 47 49 51 53 55 57 59 61 63
0 x
43 45 47 49 51 53 55 57 59 61 63 Normal Distribution
x 0.4
0.3
Normal Distribution
density
0.2 0.4
ΔMass
0.1 0.3
ETCH
density
0 0.2
43 45 47 49 51 53 55 57 59 61 63
x 0.1
PECVD PVD 0.4
Normal Distribution
0
43 45 47 49 51 53
x
55 57 59 61 63
0.3
Normal Distribution
density
0.4 0.2
0.3 0.1
density
0.2 0
CMP
43 45 47 49 51 53 55 57 59 61 63
0.1 x
0
43 45 47 49 51 53 55 57 59 61 63
x
Process Step
All Microelectronic devices are manufactured through a series of
process steps which add or remove material.
Accurate measurement of mass change allows production monitoring or
supports development in determining of a layer’s physical parameters.
Mass metrology provides the advantages of On-product measurement,
an atomic-level sensitivity, and total flexibility in application.
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4. Mass ≠ Weight
Weight Measurement Mass Measurement
Load-cell utilising complex
force measurement
Real-time corrections
for internal and
external forces
influencing
measurement
Unstable, irreproducible, not designed for
Fully automatic wafer handling and host
semiconductor measurement use communication compliant
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5. Standard Mass error (80µg 1σ)
Expressed as thickness
on a 300mm wafer
Detection capability:
~1Å for a dense material such as Ta (TaN)
<5Å for silicon, silicon oxide / nitride.
Advanced structures, with increased complexity, actually improves
the sensitivity of mass metrology.
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6. Simple Measurement
Pre-Measurement STI Oxide Fill deposition
M1 140000
135000
Mass change (µg)
130000
+ 5%
125000
ΔM = | M1-M2 |
120000
115000 - 5%
110000
0 50 100 150 200 250 300
M2 Number
Post-Measurement
Mass Metrology is a non-invasive method for In-line monitoring
‘On-Product-Wafers’ Measurement, Backside Contact only
Mass change is a direct representation of process performance
Mass excursions outside the normal distribution represent process problems
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7. TSV - Process Steps
TSV Etch
TSV Cleaning
Oxide Liner
Barrier/Seed
Cu Plate
CMP
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8. Through Si Via (TSV) Study
40
Two Via types were studied. RA
-TSV A: near straight walled TSV
TSV A
TSV B
B
via with a AR of 5:1 DA
30
Aspect Ratio
-TSV B: highly tapered via
with a AR of ~35:1
Mass change characteristics 20 Mass (mg)
of each via type investigated 0.0-30.0
30.0-60.0
60.0-90.0
10 90.0-120.0
120.0-150.0
150.0-180.0 TSV
180.0-210.0 A
210.0-240.0
0 240.0-270.0
0
0.5
1
1.5
2
2.5
3
Exposed Area %
Via Diameter ~ 5 um
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9. Etch Depth v Cycle time
150
Process requirements of each TSV A
via type are unique. y = -0.0022x2 + 1.2293x + 0.0903
R2 = 0.9992
TSV B
y = -0.0024x2 + 1.1901x + 4.7453
Via etch process change and R2 = 0.995
Depth % of POR
response is compared by 100
Depth TSV A
normalizing to the related Depth TSV B
Process of Record (PoR).
RA
Rate of change in etch rate is
TSV A
50
TSV B
DA
very similar, albeit slightly
more pronounced on TSV B.
0
0 50 100 150
Etch Time % (Cycles)
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10. Mass Loss v Cycle time
Mass vs etch cycle time, more 150
TSV A
clearly shows the difference in y = 0.9672x + 2.2678
behavior between TSV A & B. R2 = 0.9999
Mass Loss % of PoR
TSV B
y = -0.0038x2 + 1.3509x + 3.0548
TSV A exhibits a linear loss in 100 R2 = 0.9893
mass vs cycle time suggesting Mass TSV A
the transport rate of species Mass TSV B
and by products in and out of
the Via remains constant.
RA
50
TSV A
TSV B
DA
However, in the case of TSV B,
the mass loss is rapidly slowing
as the feature becomes deeper.
0
0 50 100 150
Etch Time % (cycles)
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11. Production Monitoring of Si Etch
-59000
Chambers 1 & 2 are well matched
-61000
Reducing Si mass loss as the
wafers are processed is due to
polymer loading in the chamber
Mass Loss (ug)
-63000
If a sudden shift of similar
magnitude occurs in both -65000
chambers, it is known that this
is related to incoming material
-67000
Photolithography
Chm 1 Chm 2 USL Target LSL
Hard-mask issue
-69000
1 26 51 76 101 126 151 176
Measurement Number
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12. Polymer Removal – Etch Chamber Sort
Mass measurement of the polymer
removed in the wet strip process, -300
provides a clear quantitative
measurement -800
Mass Loss (ug)
Chamber B is exhibits a high level -1300
of variability in mass loss during
the wet clean
-1800
Variability is related to the degree
of polymer formed during the etch -2300
Chamber A Chamber B
-2800
Stable
Unstable
Etch Chm
Data randomized across wet clean stations
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13. Polymer Removal – Wet Station Sort
When post etch wafers are
randomized between Wet -600
More Aggressive Less Aggressive
Stations #1 & #2, the same
Mass Removed (ug)
mass should be removed
-800
Mass removed in Station #1 is
greater than Station #2, indicating
a more aggressive chemistry -1000
Mass offers a simple and direct
method to monitor via wet cleaning -1200
and wet chemistry stability
-1400
System 1
System 2
System
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14. Oxide Liner – (HAR, Surface Area & Mass)
30000
Sidewall 40 nm TEOS Liner / 3 % Exposed Area
Top Surface
25000
20000
Mass (ug)
15000
10000
5000
0
1:11 5:12 10:1
3 20:1
4 50:1
5
Aspect Ratio (AR)
Mass of Oxide Liner over topography
increases rapidly with exposed area and
Aspect Ratio, as compared with the
same nominal surface film thickness
ITRS roadmap calls for AR values
as high as 20:1 in the future.
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15. Liner / Barrier Sidewall Coverage
11500
Liner / Barrier deposition,
including sidewall coverage Barrier
monitored non-destructively
Liner Mass (ug)
Liner
Similar to the Oxide Liner,
10500
sidewall contribution of mass
added increases on higher
Aspect Ratio features
9500
8500
1
2
3
4
5
6
7
8
Lot No.
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16. Developing Effective Cu-Fill with Mass
Monitor Mass increase v. Top
Total Mass (mg)
surface thickness Fill Superior
Selective bottom-up, void free fill,
results in the mass increasing Voiding
rapidly as compared to top
surface thickness Fill Marginal
If Voids are present then the Surface thickness ‘x’
mass will increase more slowly Thickness ‘xa’ Process A
compared to increase in top
surface thickness increase
Thickness ‘ya’
Thickness ‘xb’ Process B
Thickness ‘yb’
Process C
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17. Metal Fill & CMP
20.00
Mass removed during DEP
CMP reflects the mass 15.00 CMP
of the Barrier & Cu Fill
10.00
This correlation of mass
Mass Deviation (mg)
removed to mass 5.00
deposited affirms the
CMP process stability 0.00
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
-5.00
-10.00
-15.00
-20.00
Data No
Data is plotted in mg deviation from target
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18. CMP Stability
8.00
DEP Mass (Barrier + ECP) Added
Mass removed during CMP is Cu
well correlated to the mass Barrier
added by Barrier + ECP Liner
3.00
Therefore the CMP is observed
to be well controlled. -8.00 -3.00 2.00 7.00 12.00 17.00
Where there is an undesirable -2.00
CMP under- or over-polish,
CMP Mass (Barrier + ECP)
scatter will be seen in the data
-7.00
Removed
This ‘compensation’ results in Barrier
Liner
a mass stability of the wafers
relative to post contact etch
which is excellent -12.00
y = -1.0001x + 2E-13
2
R = 0.9997
-17.00
Data is plotted in mg deviation from Target
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19. Mass Stability
Mass variation is 15
shown relative to the
mean for the group
10
Box-Whisker plots
indicate the Mass 5
Mass (mg)
stability relative to
the previous step 0
The CMP process -5
accommodates Cu
variance in the Fill
-10
TSV Barrier Barrier
process by adjusting
Liner Liner
the CMP polish
-15
Final Mass Stability Final
shown is relative to TSV Liner
Barrier
Cu ECP CMP
Stability
Etch
the post contact etch
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20. Mass added Post TSV CMP
Mass measured post Etch and 161000
post CMP reflects the mass
filling the TSV.
160000
Mass Added (ug)
Process excursions related to
Via Etch, Via Fill and CMP are 159000
all effectively monitored by
the stability in this mass delta. 158000
Barrier
157000 TSV
Liner
156000
0 20 40 60 80 100
Measurement Number
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21. TSV Metrology – Summary
Through-Silicon Via (TSV) Etch in Silicon
Measure etched silicon volume
Confirms etch depth and profile meet process definition
Oxide Liner
Confirm step coverage of liner
Barrier/Seed
Accurate measurement of multi-stack layer
Determine effective sidewall coverage
Copper – ECD Fill and CMP
Cleaning of Copper Oxide from Seed prior to ECP
Optimization of Bottom Fill and Void prevention
Monitor, affirm process stability
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