CHAPTER 3 EXPERIMENTAL - University of Floridapearton.mse.ufl.edu/theses/DavidShuttleworthCH3.pdf · CHAPTER 4 RESULTS AND DISCUSSIONS 4.1 Experiment 1 – Window-1 Interactions 4.1.1

CHAPTER 3 EXPERIMENTAL

3.1. The Scanning Electron Microscopes

The SEMs used in this study are currently in common use in modern IC fabs. The

first SEM is a model ES20XP scanning E-beam inspection system manufactured by

KLA-Tencor. The beam conditions used for this SEM were acceleration voltage of

800eV, and a probe current of 66 nanoamperes. The second SEM is a SEMVision CX

manufactured by Applied Materials, using an acceleration voltage of 1000eV, and a

probe current of 58 picoamperes. The third SEM is a model JWS 7515 wafer inspection

system manufactured by JEOL, using an acceleration voltage of 2500eV and probe

current of 10pA.

3.2. The Wafer Cleans

The three wafer cleans used were ALEG310, NE111, and DSP+. ALEG310 is an

alkaline (pH of ~11) amine-based organic solvent blend. The details of the processing

are ~10 minute batch clean , at 85°C, followed by a deionized water (DI) rinse with CO2

bubbler which brings the pH to ~4. The NE111 clean is a buffered pH-stable acidic (pH

of ____) clean, containing ammonium fluoride NH4F. NE111 is used in a single wafer

spin processor at room temperature, with duration less than one minute. This is followed

by a DI rinse with CO2 bubbler which brings the pH to ~4. The DSP+ clean is an acidic

clean, pH = 1, containing less than 10% sulfuric acid, less than 10% hydrogen peroxide,

and HF with a concentration on the order of parts per million. DSP+ is used in a single

wafer spin processor at room temperature, with duration less than one minute. This is

followed by a (DI) rinse with CO2 bubbler which brings the pH to ~4.

3.3. The Electrical Test Structures investigated

For the first analysis, five window-1 stitches were investigated. A stitch is a two-

point test structure with 2000 windows at nominal sizing, connected in series such that

any single open contact will cause the entire structure to fail. There was a 2000 window

stitch for each of the following types of windows:

• Window-1 between metal-1 and 1.5Volt N+-Source/Drain in P-Tub • Window-1 between metal-1 and 1.5Volt P+-Source/Drain in N-Tub • Window-1 between metal-1 and 3.3 Volt N+-Source/Drain in P-Tub • Window-1 between metal-1 and 3.3Volt P+-Source/Drain in N-Tub • Window-1 between metal-1 and 1.5Volt N-doped poly.

The layout of the source/drain stitch is seen in Figure 1. There are 2000 windows

laid out at 0.20µm X 0.20µm. The metal pads are 0.24µm in width with a 0.02µm

extension on either side of the window and an extension beyond window of 0.06µm.

Figure 1. Schematic of 2000-Window Source/Drain Stitch

The layout of the gate stitch is similar to the thinox stitch. There are 2000 windows laid

out at 0.20µm X 0.2µm. The metal-1 pads are 0.24µm in width with a 0.02µm extension

on either side of the window and an extension beyond window of 0.06µm. In addition,

two references structures were electrically tested to determine the parametric values for

the nominal window-1. These structures did not undergo SEM inspection, and should

therefore have been processed correctly. The first structure was a single window Kelvin

cross-bridge, which will yield the ohmic value for a single contact (Contact Resistance

RC.) This is shown in Figure 2.

Figure 2. Schematic of Single Window Kelvin-Cross-Bride That Measures Window-1 to

Source/Drain Contact Resistance

The second structure was a van de Pauw structure, shown in Figure 3. This test

structure gives sheet resistance for the levels tested (in this case source/drain, gate, and

metal-1) in units of ohms/square (Ω/¨).

Figure 3: Four-Point Thinox Van de Pauw Layout for Measuring Sheet Resistance

With the values from the Kelvin and van de Pauw which did not undergo SEM

inspection, along with the layout of the stitch structures, it is possible to calculate an

expected resistance value for a fully functional stitch. This is a validation of the

resistance measurements from the control cell: the stitches that did not receive the SEM

inspection. Analysis of the stitch structures show that for every window there is one half

of a thinox or poly square. Therefore, the expected value of a Source/Drain or gate stitch

is:

(# of Windows) X (RC ) + (# of Windows) X (S/D Sheet Resistance)/2 (23)

. Metal sheet resistance for the window-1 stitches can be ignored since it is orders

of magnitude less than the source/drain or gate sheet resistance.

For the second analysis, window 2 interactions were investigated. In this case

both a window-2 stitch, and a window-2 Kelvin structure underwent SEM inspection.

The layouts for the 2000 window stitch and the single window Kelvin structures are

similar to analysis one. Window-2 is sized at 0.24µm, with both metal-1 and metal-2 laid

out at 0.24µm width so that the via is borderless. The stitch layout can be seen in Figure

4, the Kelvin structure is laid-out similar to the thinox Kelvin of Figure 2.

Figure 4: Window-2 Stitch Layout

3.4. The Procedure - Experiment One:Window-1

Two wafers (wafers 01 and 02) were processed through metal-1, capped and

pulled from the line for electrical testing and analysis. The wafer layout consists of a

total of 76 flash fields laid out in 10 rows by 11 columns. Figure 5 depicts the wafer with

row labels. Row 1 and 10 did not undergo SEM inspection at any time. Rows 2 and 3

underwent ES20 inspection. Rows 4 and 5 underwent JEOL inspection. Rows 6 and 7

underwent SEMVISION inspection. Rows 8 and 9 underwent inspection by all three

SEMs at precisely the same spots on the stitches so that the inspections overlapped.

Figure 5. Wafer Layout Identifying SEM Inspection Row

As stated above the beam conditions for the three SEMs were 800Vacc and 66nA

Ip for the ES20, 1000Vacc and 58pA Ip for the SEMVISION, and 2500Vacc 10pA Ip for

the JEOL. The field of view for the ES20 was 10µm by 10µm, for a total scanned area of

100µm2. The field of view for the SEMVISION was 3µm by 3µm, for a total scanned

area of 9µm2. The field of view for the JEOL was 3.25µm by 2.5µm for a total scanned

area of 8.125µm2. For the SEMVISION and JEOL, the dwell time on each site was 17 ±

1 second. For the ES20, the dwell time on each site was 328 millisecond. Multiplying

the probe current by the scan time and dividing by the scan area yields the total electron

dose per square micron. As such, the calculated doses are for the ES20

___electrons/µm2, for the SEMVISION 6.84E+8 electrons/µm2, and for the JEOL

1.31E+8 electrons/µm2. These details are summarized in Table 1.

Table 1: SEM Scanning Conditions for the Five Stitches in Experiment One

SEM Vacc Probe Current

Field of View

Scan Area Scan Time Total Electron Dose electrons/µm^2

ES20XP 800eV 66nA 10µm X 10µm

100 µm^2 328 msec

SEMVision CX 1000eV 58pA 3µm X 3µm 9 µm^2 17 sec ~6.84E+08

Jeol JWS7515 2500eV 10pA 3.25µm X 2.5µm

8.125 µm^2 17 sec ~1.31E+08

The battery of SEM scans were performed at four points in the processing. These scan

points were:

Scan #1. On the oxide of dielectric-1, after window-1 pattern and plasma etch and clean, prior to window-1 IMP Ti/TiN liner deposit

Scan #2. After window-1 IMP Ti/TiN liner deposition prior to CVD tungsten deposition

Scan #3. After window-1 tungsten CMP and clean prior to Metal-1 stack deposition Scan #4. After metal-1 patterning and etch

The four areas of the stitches scanned by the SEMs are indicated in Figure 6.

Figure 6. SEM Scan Locations after (1) Window Etch, (2) Window Liner Deposition, (3) Tungsten CMP, (4) Metal-1 Etch, With Accompanying SEM Image

It should be noted here that metal-1 was intentionally misaligned in both the X and Y

direction by ~80µm. This amount of misalign is the upper limit of the specification for

overlay tolerance, but it is within spec. After the two wafers completed the fourth run of

SEM inspections, they were split. One wafer received an ALEG310 clean, and then

dielectric caps were deposited. The other wafer did not receive the ALEG310 clean, but

went directly to caps deposition after SEM scan #4. Either process falls within the

normal flow of a product wafer.

3.5. The Procedure: Experiment 2: Window-2

The same basic stitch and Kelvin layouts used in experiment 1 are used again in

experiment 2. In this case, the window-2 module was investigated. Four wafers were

processed, according to the normal process flow through metal-2 deposition. Metal-2

was then misaligned by ~80µm in both the X and Y direction, exposing the underlying

tungsten plugs. The window-2/metal-2 surface was then scanned by the SEMs. The

ES20 was not available for this experiment so only the SEMVision and JEOL were

utilized in the experiment. The flash-fields scanned remained as detailed in Figure 5 with

the exception of the ES20, but in this run both the 2000 window stitch and the single

window Kelvin cross-bridge structures were scanned by the SEMs. The scan areas

remained the same as in experiment 1, 3µm by 3µm for SEMVISION, and 3.25µm by

2.5µm for JEOL. The beam conditions for the SEMs also remained identical to

experiment 1, but scan time was increased to 25 ±1 seconds. These SEM conditions for

experiment 2 are detailed in Table 2.

Table 2: SEM Scanning Conditions for the Window-2 Stitch and Window-2 Kelvin in Experiment Two

SEM Vacc Probe Current

Field of View

Scan Area Scan Time Total Electron Dose electrons/µm^2

SEMVision CX 1000eV 58pA 3µm X 3µm 9 µm^2 25 sec 1.01E+09

Jeol JWS7515 2500V 10pA 3.25µm X 2.5µm

8.125 µm^2 25 sec 1.92E+08

After the SEM scanning, the wafers were split. One wafer each went through an

ALEG310 clean, an NE111 clean, a DSP clean, and one wafer did not receive any clean.

The details of the cleans split by wafer are given in Table 3

Table 3: Specification of Cleans Split by Wafer

Wafer Clean

03 Skip

04 ALEG310

05 NE111

07 DSP+

06 Control

08 Control

09 Control

The four wafers, in addition to three control wafers were then capped and pulled from the

line for electrical testing and physical analysis. The three control wafers (wafers 06, 08,

and 09) were processed through metal-2 according to the normal process flow of the

production routing. The data for these three wafers were used as the control data for both

experiment 1 and experiment 2.

CHAPTER 4 RESULTS AND DISCUSSIONS

4.1 Experiment 1 – Window-1 Interactions

4.1.1 Electrical Testing Results

The electrical resistance of the 2000 window-1 stitches for each of the five

window-1 types is presented in the following five figures and tables

N+_

CH

AIN

_1V

_Rt (

kOhm

s)

100

200

300

400

500

.Con

trol

.Non

e

ES

20

JEO

L

SE

MV

isio

n

SE

MV

isio

n+E

S20

+JE

OL

SEM

.NoneES20JEOLSEMVision

.01 .05 .10 .25 .50 .75 .90 .95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

Figure 7: Oneway Analysis of Win-1 to 1.5V NSD Stitch Resistance By SEM Inspection,

With Quartile Plot

Table 4: Total Resistance Mean, Standard Deviation, and Quartiles of The Win-1 to 1.5V NSD Stitch Resistance By SEM Inspection (Units of Kilohms)

Level Number Mean Std Dev Min 10% 25% Median 75% 90% Max.Control 218 190.0 123.5 168.7 171.8 174.7 179.5 187.8 194.7 2000.0.None 12 199.5 4.6 191.3 192.2 195.7 199.2 204.2 205.5 205.7ES20 32 189.8 11.0 172.5 175.5 181.0 189.5 196.4 208.2 213.3JEOL 36 178.2 7.9 166.8 168.6 170.6 177.2 184.1 190.9 194.5

SEMVision 36 178.1 7.9 168.6 169.2 172.3 176.4 183.4 191.3 197.0SEMVision+ES20+JEOL 30 15734.5 84841.4 185.9 187.8 196.2 225.1 274.5 370.6 464940.0

Quantiles

Wafer-1 in Red Wafer-2 in Black

N+_

CH

AIN

_3V

_Rt (

kOhm

s)

100

1000

.Con

trol

.Non

e

ES

20

JEO

L

SE

MV

isio

n

SE

MV

isio

n+E

S20

+JE

OL

SEM

.01 .05 .10 .25 .50 .75 .90 .95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

Oneway Analysis of N+_CHAIN_3V_Rt (kOhms) By SEM

Figure 8: Oneway Analysis of Win-1 to 3.3V NSD Stitch Resistance By SEM Inspection,

With Quartile Plot

Table 5: Total Resistance Mean, Standard Deviation, and Quartiles of The Win-1 to 3.3V NSD Stitch Resistance By SEM Inspection (Units of Kilohms)

Level Number Mean Std Dev Min 10% 25% Median 75% 90% Max.Control 218 9.18E+07 1.35E+09 168.1 171.0 174.1 179.6 188.5 196.6 2.00E+10.None 12 201.9 4.9 195.9 196.0 198.0 201.0 204.6 211.1 213.6ES20 32 189.6 11.7 171.3 174.2 180.3 189.0 196.9 207.5 216.8JEOL 36 178.0 8.4 166.5 167.9 170.4 176.4 185.3 191.0 195.2

SEMVision 36 178.1 8.9 167.4 168.4 171.8 175.4 184.1 193.9 199.5SEMVision+ES20+JEOL

30 1.67E+05 6.48E+05 186.2 188.68 202.28 228.4 326.3 3635 3.00E+06

Quantiles


P+_

CH

AIN

_1V

_Rt (

kOhm

s)

1000

10000

.Con

trol

.Non

e

ES

20

JEO

L

SE

MV

isio

n

SE

MV

isio

n+E

S20

+JE

OL

SEM

.01 .05 .10 .25 .50 .75 .90 .95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

Quantiles

Oneway Analysis of P+_CHAIN_1V_Rt (kOhms) By SEM

Figure 9: Oneway Analysis of Win-1 to 1.5V PSD Stitch Resistance By SEM Inspection,

With Quartile Plot

Table 6: Total Resistance Mean, Standard Deviation, and Quartiles of The Win-1 to 1.5V PSD Stitch Resistance By SEM Inspection (Units of Kilohms)

Level Number Mean Std Dev Min 10% 25% Median 75% 90% Max.Control 218 1.38E+06 2.03E+07 437.5 459.8 476.3 500.8 562.7 634.0 3.00E+08.None 12 606 57.8 520.6 526.1 556.7 607.9 664.2 683.1 686.8ES20 32 571 39.8 493.8 520.8 533.6 571.4 607.8 628.1 641.0JEOL 36 546 54.3 475.2 480.0 500.7 537.9 589.7 622.6 683.3

SEMVision 36 572 80.0 478.2 487.9 500.6 555.0 630.2 663.6 856.0SEMVision+E

S20+JEOL 30 3442 7808.2 536.4 565.4 641.1 901.4 1484.5 9572.1 34254

Quantiles


P+_

CH

AIN

_3V

_Rt (

kOhm

s)

1000

10000

.Con

trol

.Non

e

ES

20

JEO

L

SE

MV

isio

n

SE

MV

isio

n+E

S20

+JE

OL

SEM

.01 .05 .10 .25 .50 .75 .90 .95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

Oneway Analysis of P+_CHAIN_3V_Rt (kOhms) By SEM

Figure 10: Oneway Analysis of Win-1 to 3.3V PSD Stitch Resistance By SEM

Inspection, With Quartile Plot

Table 7: Total Resistance Mean, Standard Deviation, and Quartiles of The Win-1 to 3.3V PSD Stitch Resistance By SEM Inspection (Units of Kilohms)

Level Number Mean Std Dev Min 10% 25% Median 75% 90% Max.Control 219 539 108 449.1 466.7 484.2 509.4 561.2 636.0 1572.None 12 980 1260 532.2 538.3 562.1 642.3 661.2 3689 4979ES20 32 582 40 501.5 531.7 545.6 582.0 614.7 638.0 653.7JEOL 36 568 103 479.5 493.4 508.6 551.2 593.9 649.0 1090

SEMVision 36 630 326 483.8 496.7 515.5 575.2 640.7 663.7 2498SEMVision+E

S20+JEOL 30 271795 1141549 560.2 601.0 643.4 961.9 3276 70857 6000000

Quantiles


Pol

y_C

HA

IN_R

t (kO

hms)

35

40

45

50

55

.Con

trol

.Non

e

ES

20

JEO

L

SE

MV

isio

n

SE

MV

isio

n+E

S20

+JE

OL

SEM

.Control

.None

ES20JEOLSEMVision

.01 .05 .10 .25 .50 .75 .90 .95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

Oneway Analysis of Poly_CHAIN_Rt (kOhms) By SEM

Figure 11: Oneway Analysis of Win-1 to Poly-Silicon Gate Stack Stitch Resistance By

SEM Inspection, With Quartile Plot

Table 8: Total Resistance Mean, Standard Deviation, and Quartiles of The Win-1 Poly-Silicon Gate Stack Stitch Resistance By SEM Inspection (Units of Kilohms)

Level Number Mean Std Dev Min 10% 25% Median 75% 90% Max.Control 219 41.93 2.02 36.69 38.91 40.25 42.30 43.72 44.36 45.19.None 12 43.38 3.55 39.17 39.25 40.63 42.40 46.84 48.96 49.07ES20 32 42.48 2.38 37.90 38.46 40.61 42.96 44.31 45.64 45.97JEOL 36 43.11 1.67 40.06 40.68 41.96 43.34 44.37 45.21 46.45

SEMVision 36 43.23 2.32 38.27 39.78 41.05 43.85 45.03 45.91 47.18SEMVision+E

S20+JEOL 30 1245 6561.5 41.53 43.34 45.80 47.33 48.61 50.44 35986

Quantiles

It is evident from quartile plots for the four thinox stitches, that the sites that

received SEM inspection by all three SEMs have considerable disturbances in resistance.


The mean, median and standard deviation of the stitch resistance for the combined cell is

significantly greater than any of the other cells of the experiment as well as the control

cell. While the upper and lower quartiles for all but the combined SEM NSD stitches fall

between approximately 170 and 200 kO for both the 1.5V and 3.3V, the NSD quartiles

for the combined SEM cell fall between ~200 and 325kO. While the upper and lower

quartiles for all but the combined SEM PSD stitches fall between approximately 475 and

650 kO for both the 1.5V and 3.3V, the PSD quartiles for the combined SEM cell fall

between ~640 and 3300 kO. In addition, for the four source/drain stitches approximately

10% of the combined SEM structures have measured resistances in the megohms up to

and including electrical opens. For the other five cells the few opens that occurred were

obvious outliers and the distributions were much tighter. Scanning electron microscope

imaging has caused the electrical failure of window-1 to source/drain contacts.

For the window-1 stitch to gate, there is also a statistically significant increase in

measured resistance, however, the gate stitches did not show the high number of

catastrophic failures that the source/drain stitches showed. Figure 12 replots the window-

1 to poly stitch excluding a single outlier data point from the combined SEM cell.

Pol

y_C

HA

IN_R

t (kO

hms)

36

38

40

42

44

46

48

50

.Con

trol

.Non

e

ES

20

JEO

L

SE

MV

isio

n

SE

MV

isio

n+E

S20

+JE

OL

SEM

Each PairStudent's t 0.05

.Control

.NoneES20JEOLSEMVision

SEMVision+ES20+JEOL

Level

36.69

39.17 37.9 40.06 38.27

41.53

Minimum

38.91

39.248 38.462 40.675 39.777

43.14

10%

40.25

40.6325 40.61

41.9575 41.045

45.77

25%

42.3

42.395 42.96 43.34 43.845

47.27

Median

43.72

46.835 44.3125 44.3725 45.0325

48.485

75%

44.36

48.962 45.637 45.212 45.914

50.39

90%

45.19

49.07 45.97 46.45 47.18

50.63

Maximum

Quantiles

.Control

.NoneES20JEOL

SEMVisionSEMVision+ES20+JEOL

Level

219 12 32 36

36 29

Number

41.9282 43.3758 42.4794 43.1092

43.2256 47.0376

Mean

2.01582 3.55013 2.38251 1.67499

2.32092 2.21079

Std Dev

0.1362 1.0248 0.4212 0.2792

0.3868 0.4105

Std Err Mean

41.660 41.120 41.620 42.542

42.440 46.197

Lower 95%

42.197 45.631 43.338 43.676

44.011 47.879

Upper 95%

Means and Std Deviations

Oneway Analysis of Poly_CHAIN_Rt (kOhms) By SEM

Figure 12: Oneway Analysis of Win-1 to Poly-Silicon Gate Stack Stitch Resistance By SEM Inspection, Excluding a Single Outlier Point From Combined SEM Cell

In this case, the mean of the combined SEM cell increased from ~43kO to ~47kO

as compared to the other experimental cells, which is an increase of only ~9%. In

addition, the standard deviation remained similar between the cells with no increased

spread in the distribution of the resistances for the combined SEM cell. This percentage

increase may seem inconsequential, but if it can be assumed that only one of the four

scan points was responsible for the increase (not necessarily a safe assumption) then that

4kO resistance was caused by a scan area of only a few square microns, encompassing

twenty windows or less. This is, potentially, an average increase of 200O per contact, for

a gate contact resistance that normally measures ~10 O per contact. It is also interesting

that the average measured resistances for every experimental cell is approximately 1-1.5

kO greater than the average measured resistance for the control cell. The most likely

explanation for this is that the experimental wafers (wafers 01 and 02) experienced a

significant amount of time lag between processing steps, whereas the control wafers

(wafers 06, 08, 09) flowed through the process relatively unencumbered. It is a known

phenomenon that significant time delays between certain processing steps in the backend

will lead to increased contact resistances. This should not be a concern, in that on the

experimental wafers, the data from rows 1 and 10 (labeled as the None cell) which did

not receive SEM scans can be considered as a control reference. If this is assumed, then

Figure 12 would indicate that the poly stitches that only received a single SEM inspection

are not significantly different from poly stitches that did not receive any SEM inspection.

The electrical test data also included measurements of contact resistance and

source/drain sheet resistance. These Kelvin and van de Pauw structures did not undergo

SEM inspection. From these measurement values, it is possible to calculate the expected

stitch value for the corresponding window flavor, on a point by point basis. As stated

above, for a 2000 window stitch with one half of a source/drain thinox square per

window, the calculated stitch resistance would be (2000 X W1 Rc) + (1000 X Rsheet).

This is not an exact method since some assumptions have been made, but it is a valid

approximation to the first order. The ratio of the measured stitch resistance to the

calculated theoretical stitch resistance should approach unity if there is no SEM

interaction with the sample that affects the integrity of the tungsten plugs. The following

four figures plot, by SEM split, the ratio of measured stitch resistance to calculated

theoretical stitch resistance, for the four types of window-1 to source/ drain stitch.

1.5V

NS

D (m

srd/

calc

'd)

1

10

.Con

trol

.Non

e

ES

20

JEO

L

SE

MV

isio

n

SE

MV

isio

n+E

S20

+JE

OL

SEM

.Control

SEMVision+ES20+JEOL

.01 .05 .10 .25 .50 .75 .90 .95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

.Control

.NoneES20JEOLSEMVisionSEMVision+ES20+JEOL

Level

0.8950310.899902 0.863940.8354360.8752090.889767

Minimum

0.9380190.9024420.8863080.8701410.8925070.919474

10%

0.9561990.9153440.9075550.9111220.9082480.993259

25%

0.9829740.9589760.9206420.9327180.9399491.093473

Median

1.005860.9808240.9504520.9527040.9535661.269629

75%

1.031180.9877070.9775370.9735770.973433 1.76216

90%

9.5634290.9900631.0248341.0053851.0107532133.633

Maximum

Quantiles

.Control


Level

218 12 32 36 36 30

Number

1.0228 0.9489 0.9291 0.9280 0.9340 72.2693

Mean

0.582 0.035 0.037 0.037 0.033

389.330

Std Dev

0.039 0.010 0.007 0.006 0.005 71.082

Std Err Mean

0.95 0.93 0.92 0.92 0.92

-73.11

Lower 95%

1.10 0.97 0.94 0.94 0.95

217.65

Upper 95%


Oneway Analysis of 1.5V NSD (msrd stitch Rt/ calc'd stitch Rt) By SEM Split

Figure 13: Ratio of 1.5V NSD Measured Stitch Resistance to Calculated Theoretical Stitch Resistance Value by SEM Split


3.3V

NS

D (m

srd/

calc

'd)

1

10

.Con

trol

.Non

e

ES

20

JEO

L

SE

MV

isio

n

SE

MV

isio

n+E

S20

+JE

OL

SEM

.Control

SEMVision+ES20+JEOL

.01 .05 .10 .25 .50 .75 .90 .95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

.Control


Level0.9054590.8770020.9228660.867225 0.873310.977866

Minimum0.9503950.8845750.9320830.9047520.9020830.997798

10%0.9697040.9235940.9413770.9178530.9188911.027294

25%0.9911970.9626240.9620160.9481310.9469521.186114

Median1.0276660.9850960.977416 0.977270.9670741.600372

75%1.0545191.0198360.9896520.9861570.99080516.54986

90%971817301.0235951.0119961.030394

1.025613638.84

Maximum

Quantiles

.Control


Level 218 12 32 36 36 30

Number 445836

1 1 1 1

807

Mean 6581974

0 0 0 0

3092

Std Dev 445788

0 0 0 0

564

Std Err Mean-432792

1 1 1 1

-347

Lower 95%1324463.80.98294980.96894810.95923650.95698521961.8246

Upper 95%


Oneway Analysis of 3.3V NSD (msrd stitch Rt/ calc'd stitch Rt) By SEM Split

Figure 14: Ratio of 3.3V NSD Measured Stitch Resistance to Calculated Theoretical Stitch Resistance Value by SEM Split


1.5V

PS

D (m

srd/

calc

'd)

1

10

.Con

trol

.Non

e

ES

20

JEO

L

SE

MV

isio

n

SE

MV

isio

n+E

S20

+JE

OL

SEM

.Control

SEMVision+ES20+JEOL

.01 .05 .10 .25 .50 .75 .90 .95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

.Control


Level

0.8371720.8744750.8066950.8736050.813122 0.84812

Minimum

0.9164250.879747 0.83011

0.905562 0.87168

0.930398

10%

0.953542 0.919980.8673890.9343540.936893 1.02401

25%

0.9938080.9708290.921047 1.024141.0131181.276079

Median

1.1369561.0208680.9869341.0883071.1492012.449486

75%

1.2952361.1963751.0676781.2116461.22561511.99895

90%

570342.21.2532561.4215581.2789861.54764140.88565

Maximum

Quantiles

.Control


Level

218 12 32 36 36 30

Number

2617.51 0.99 0.94 1.03 1.04 4.53

Mean

38628.4 0.1 0.1 0.1 0.1 9.6

Std Dev

2616.2 0.0 0.0 0.0 0.0 1.8

Std Err Mean

-2539 1 1 1 1 1

Lower 95%

7774.0 1.0 1.0 1.1 1.1 8.1

Upper 95%


Oneway Analysis of 1.5V PSD (msrd stitch Rt/ calc'd stitch Rt) By SEM Split

Figure 15: Ratio of 1.5V PSD Measured Stitch Resistance to Calculated Theoretical Stitch Resistance Value by SEM Split


3.3V

PS

D (

msr

d/ca

lc'd

)

1

10

.Con

trol

.Non

e

ES

20

JEO

L

SE

MV

isio

n

SE

MV

isio

n+E

S20

+JE

OL

SEM

.None

SEMVision+ES20+JEOL

.01 .05 .10 .25 .50 .75 .90 .95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

.Control


Level

0.7816580.8929530.8008490.8560040.918126 0.86678

Minimum

0.9205690.8946620.8616960.9159510.9404290.998452

10%

0.9693760.9230980.8813520.9858210.967093 1.22921

25%

1.0281091.030149 0.944131.0597221.1137851.479641

Median

1.1636281.2213461.015173 1.213171.2191645.051506

75%

1.2765926.3742911.2604931.3136411.40435681.17646

90%

3.1036538.5329911.3193312.0047825.0060127742.935

Maximum

Quantiles

.Control


Level

218 12 32 36 36 30

Number

1.090 1.675 0.979 1.114 1.223

355.191

Mean

0.23 2.16 0.14 0.21 0.67

1480.49

Std Dev

0.02 0.62 0.02 0.04 0.11

270.30

Std Err Mean

1.1 0.3 0.9 1.0 1.0

-197.6

Lower 95%

1.12 3.05 1.03 1.19 1.45

908.02

Upper 95%


Oneway Analysis of 3.3V PSD (msrd stitch Rt/ calc'd stitch Rt) By SEM Split

Figure 16: Ratio of 3.3V PSD Measured Stitch Resistance to Calculated Theoretical Stitch Resistance Value by SEM Split

Kelvin and van de Pauw window-1-to-gate data was not available for this

analysis. In considering the above four figures it becomes clear that for the stitches

which underwent only a single SEM inspection no disturbance in the electrical integrity


of the windows took place. In general, the mean and median values for the ratio is

centered at unity plus or minus ~10%, and in most cases the 90th percentile is within 10%

of the expected value. In fact, for a majority of these stitches, the theoretical value

overestimates the actual measured resistance, and the ratio is less than one.

On the other hand, the window-1 to source/drain stitches that underwent SEM

inspection by all three SEMs had ratios that averaged in the 10’s to 100’s, which is of

course skewed by the high number of megohm resistances. Nevertheless, even the

median values ranged from +10% to +47%, the 75th percentile ranged in value from

+27% to 400%, and the 90th percentile ranged in value from +76% to 8000%. The above

four plots also reveal that the PSD stitches are more adversely affected by SEM

inspection than the NSD stitches. There does not appear to be any difference in

resistance distributions between the 1.5V and 3.3V PSD stitches or NSD stitches

The next four figures display the stitch resistance by the post-metal-1 treatment.

Wafer 01 received no post-metal etch clean prior to capping. Wafer 02 received an

ALEG310 clean prior to capping. The three control wafers, which did not receive any

SEM inspection, did not receive a post-metal-etch clean prior to capping.

N+_

CH

AIN

_1V

_Rt (

kOhm

s)

100

1000

.Control Aleg310 Skip

Cleans

.01 .05 .10 .25 .50 .75 .90 .95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

.ControlAleg310Skip

Level

171.5 185.9 187.5

Minimum

173.26 186.68 189.84

10%

177.35 196.4 195.7

25%

182.2 213.7 236.4

Median

188.95 242.9 291.8

75%

198.14 399.18

186199.4

90%

203.4 580.8

464940

Maximum

Quantiles

.ControlAleg310Skip

Level

45 15 15

Number

184.1 241.2

31227.9

Mean

8 97

119983

Std Dev

1 25

30979

Std Err Mean

182 187

-35216

Lower 95%

187 295

97672

Upper 95%


NSD_CHAIN_1V_Rt (kOhms) By Cleans, (Rows 8 and 9 only)

Figure 17: Resistance of Window-1 to 1.5 NSD stitch by Cleans Split (Data From Rows 8

and 9 only)

N+_

CH

AIN

_3V

_Rt (

kOhm

s)

100

1000


Cleans

.01 .05.10 .25 .50 .75 .90.95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

.ControlAleg310Skip

Level 170.9 186.2 188.5

Minimum 173.16 187.16 189.88

10% 177.5 210.4 201.3

25% 183.3 223.5 233.2

Median 192.55 271.2 830.2

75% 202.86 1804.28 2400000

90% 2000000

3869 3000000

Maximum

Quantiles

.ControlAleg310Skip

Level 45 15 15

Number 44625 484

333665

Mean 298115

938 899604

Std Dev 44440 242

232277

Std Err Mean -44938

-36-164519

Lower 95% 134189 1003

831849

Upper 95%


NSD CHAIN_3V_Rt (kOhms) By Cleans, (Rows 8 and 9 only)

Figure 18: Resistance of Window-1 to 3.3V NSD stitch by Cleans Split (Data From Rows

8 and 9 only)

P+

_CH

AIN

_1V

_Rt (

kOhm

s)

1000

10000


Cleans

.01 .05.10 .25 .50 .75 .90.95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

.ControlAleg310Skip

Level

468.6 536.4 578.2

Minimum

487.64 541.2 580.3

10%

492.7 614.1 908.6

25%

519.8 700.8 1241

Median

618.65 894.1 5487

75%

690.06 1807.4 30538.8

90%

785.9 2840 34254

Maximum

Quantiles

.ControlAleg310Skip

Level

45 15 15

Number

556.54 877.87 6006.23

Mean

84.5 566.7

10577.4

Std Dev

12.6 146.3 2731.1

Std Err Mean

531.14 564.01 148.67

Lower 95%

582 1192 11864

Upper 95%


PSD CHAIN_1V_Rt (kOhms) By Cleans, (Rows 8 and 9 only)

Figure 19: Resistance of Window-1 to 1.5 PSD stitch by Cleans Split (Data From Rows 8

and 9 only)

P+

_CH

AIN

_3V

_Rt (

kOhm

s)

1000

10000


Cleans

.01 .05.10 .25 .50 .75 .90.95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

.ControlAleg310Skip

Level

466.7 560.2 582.8

Minimum

496.18 584.56 599.48

10%

510.65 607.8 796.8

25%

546.4 757.2 1002

Median

612.4 2353 5708

75%

675.72 844796 2421978

90%

1177 2000000 6000000

Maximum

Quantiles

.ControlAleg310Skip

Level

45 15 15

Number

573 139289 404301

Mean

111 515099 1548030

Std Dev

17 132998 399700

Std Err Mean

540-145963-452970

Lower 95%

606.83057424541.081261571.4

Upper 95%


PSD CHAIN_3V_Rt (kOhms) By Cleans, (Rows 8 and 9 only)

Figure 20: Resistance of Window-1 to 3.3V PSD Stitch by Cleans Split (Data From Rows

8 and 9 only)

VIA

1_C

HA

IN_R

t (kO

hms)

35

40

45

50

55


Cleans

.Control

Aleg310

.01 .05.10 .25 .50 .75 .90.95 .99

-3 -2 -1 0 1 2 3

Normal Quantile

.ControlAleg310Skip

Level 38.56 41.53 43.14

Minimum 39.582 42.226 44.754

10% 42.11 45.32 47.38

25% 43.79 46.4 48.57

Median 44.24 47.27 50.39

75% 44.608 47.694

14424.78

90% 45.19 47.7

35986

Maximum

Quantiles

.ControlAleg310Skip

Level 45 15 15

Number 43.01 45.96

2444.05

Mean 1.80 1.80

9279.10

Std Dev 0.3 0.5

2395.9

Std Err Mean 42 45

-2695

Lower 95% 43.5 47.0

7582.6

Upper 95%


WN1-to-POLY CHAIN_Rt (kOhms) By Cleans, (Rows 8 and 9 only)

Figure 21: Resistance of Window-1 Poly Stitch by Cleans Split (Data From Rows 8 and 9

only)

The cleans split did not present a clear signal. There is some indication that

catastrophic fails were reduced by the ALEG bath. In general, the resistance were less

for the ALEG split than the skip clean split – this is especially the case for the 1.5V PSD

stitch (Figure 19) where the ALEG data showed a ~400kO reduction in resistance. The

data does not conclusively determine if the post-metal-etch cleans interacts with an SEM

damaged stitch to alleviate or exacerbate the damage. It certainly, however, provides a

signal that is worth investigating further.

To restate, it is evident that in- line scanning electron microscope imaging of

source/ drain contacts has modified the electrical characteristics of the window-1 contact

resistance, and caused the electrical failure of a significant number of those windows. In

addition, in- line scanning electron microscope imaging of gate contacts has modified the

electrical characteristics of the window-1 contact resistance, although it did not cause

catastrophic electrical failure those stitches. It remains to be seen however, which of the

four SEM scan points caused the source/drain stitch failure and what physical mechanism

was responsible for it.

4.1.2 Failure Mode Analysis

Voltage Contrast SEM imaging was attempted in order to isolate some of the

failing windows. This method has been used with great success in locating failing

windows between metal levels as well as windows to gate. The method used is as

follows. One of the two bond pads from a failing stitch is laser-blasted. This shorts the

metal-1 pad to the substrate, grounding one side of the stitch. The capping layer is then

removed using a CHF3 plasma etch, exposing the metal-1 pads. The die is then imaged in

the SEM using a low-voltage high-current beam. The grounded side of the stitch will

image bright up to the point of fail. Beyond the point of fail, the floating side of the

stitch will image dark (see Error! Reference source not found..) The identified failing

via is then typically cross-sectioned by focused ion beam (FIB), for SEM or TEM

imaging. Unfortunately, no voltage contrast imaging was detectable from the

source/drain stitches. It is speculated that the charge build up that should occur on the

floating side of the stitch is dissipated by the junction leakage of the N+ to Ptub or P+ to

Ntub junctions underneath the window-1 contacts.

Multiple random FIB cross-sections were then taken from failing stitches. The

location of the cuts were the four scan areas as detailed in Figure 6. A representative

STEM for each of the four scan areas on a failing PSD stitch can be seen in Figure 22

Figure 22: Scanning TEM Cross-Sectional Micrographs of the Four SEM Scan Areas From Electrically Open Failing Stitches

The random cross-sections did not identify either the failure locations or the failure

mechanism. In fact, cross-sections indicate well formed and filled windows, with no

apparent foreign interfaces, between silicon and liner, between liner and tungsten, or

between tungsten and Ti/TiN barrier.

Multiple failing stitches were then deprocessed by CHF3 plasma etch of the caps.

The metal-1 aluminum was then etched away in an HCl bath, and the Ti/TiN barrier was

polished off via CMP. Top-down SEM images of these failed stit ches were taken.

Figure 23: Top-Down SEM of the Four Scanned Areas On a Failing 3.3V PSD Stitch

After the Removal of Metal-1 – Tungsten Coring Evident In Scan Area #1

Figure 24: Top-Down SEM of the Four Scanned Areas On a Failing 1.5V PSD Stitch


Figure 25: Top-Down SEM of the Four Scanned Areas On a Failing 3.3V NSD Stitch


Figure 26: Top-Down SEM of the Four Scanned Areas On a Failing 3.3V NSD Stitch After the Removal of Metal-1 – Tungsten Coring Evident In Scan Area #1

Top-down SEM imaging of the failing source/drain stitches gives an indication

that the source of the electrical variation may be in SEM scanning of the oxide surface

(scan #1) prior to the window Ti/TiN liner. High magnification images of these windows

clearly indicate incomplete tungsten fill.

Figure 27: High Magnification SEM Images of Incompletely Filled Window-1 in Scan

Area #1.

In some instances, the area that received SEM scans after liner deposition (scan #2) also

showed some degree of incomplete fill. Scan areas #3 and #4 did not show any coring,

rather the windows appeared to be correctly filled. Because the precise location of the

failing window was not pinpointed, the incompletely filled windows must be considered

circumstantial, but they are at the very least an indication

As stated in the introduction, the physical failure mechanism will most likely fall

into one of three categories

1. The SEM deposits a thin hydrocarbon interfacial film that disrupts the electrical integrity of the thin films

2. SEM interaction disrupts the normal deposition of the thin films or the normal structural integrity of the thin films

3. The SEM injects a stored charge on the sample surface that upon subsequent processing, drives electrochemical/galvanic corrosion of one of the metal thin films.

The fact that source/drain stitches showed catastrophic fails and a wide

distribution of resistance whereas the poly stitches showed neither catastrophic fails nor a

wide distribution in resistances eliminates the first premise. A purely physical deposition

of a carbon ‘burn’ mark would not preferentially choose a patterned oxide over thinox

versus a patterned oxide over gate stack. Likewise a hydrocarbon interface between

Ti/TiN liner and tungsten or between tungsten and metal barrier, would not be expected

to preferentially deposit over a thinox stitch as compared to a poly stitch.

As to the second premise, thin film perturbations, several mechanism are

conceivable that could account for the observations. Source/ drain contact resistance

relies on the silicidation of the deposited Ti/TiN window liner to form an ohmic contact.

For the window to gate contact, the tungsten plug is landing on a deposited metal silicide

in the gate stack so the Ti-silicidation is not a requirement for good ohmic contact. A

hydrocarbon interface between silicon and liner could retard or prevent the silicidation

reaction of the source/drain reaction. This mechanism would not explain the evident

coring of the plugs in the Scan #1 and Scan #2 areas, since the tungsten nucleation and

deposition would not be disturbed. However, since the tungsten coring is circumstantial

the aforementioned mechanism can not be eliminated. Alternatively, a charge field

injected into the oxide by the SEM prior to Ti/TiN liner deposition could impact the

coherence of that liner. The impinging titanium is ionized, and therefore would be

subject to electrical fields associated with the surface. The sidewall thickness of the

window is on the order of only tens of angstroms, so even minor alterations of the liner

deposition could lead to discontinuous coverage over the sidewall. An incoherent liner

would impact the nucleation of tungsten, which would explain the cored plugs.

Additionally, hydrocarbon deposition on the surface of the liner could also disrupt the

nucleation of tungsten, which could explain the cored tungsten. The disruption of

tungsten nucleation, would not however, be expected to occur preferentially over thinox

as compared to poly.

Finally, the third premise appears quite feasible. As discussed in the literature

review, stored charge driven electrochemical corrosion has been reported for both the

tungsten plugs and the titanium metal barrier. In fact the observations made by Lee et

alError! Bookmark not defined. agree with the electrical findings of this study: plugs

electrically connected to the source/ drains failed whereas plugs terminating at gate did

not. The argument against this premise is that the findings of these reports generally

found completely voided windows, whereas the deprocessed stitches in this study showed

only slight perturbations if any at all. Even so, the third premise seems most likely in

light of the Lee finding.

4.2 Experiment 2 – Window-2 Interactions

The electrical resistance of the 2000 window-2 stitches and the single window

metal-2 to metal-1 are presented in the following two figures. In this experiment, wafers

were fully processed through metal-2, with a metal-2 misalignment. The SEM scans

were then only performed on the metal/exposed tungsten plugs, which corresponded to

SEM Scan #4 in experiment 1. One wafer each was run through an ALEG310 clean, an

NE111 clean, and a DSP+ clean, while one wafer skipped the cleans. The three control

cells did not receive any SEM inspection or cleans. The resultant data is displayed as a

variability chart with the SEM split within the metal cleans split.

82~V

IA2_

CH

AIN

_Rt (

kOhm

s)

9

10

11

12

13

14

15

16

17

18

.Con

trol

.Non

e

JEO

L

SE

MV

isio

n

SE

MV

isio

n+JE

OL

.Non

e

JEO

L

SE

MV

isio

n

SE

MV

isio

n+JE

OL

.Non

e

JEO

L

SE

MV

isio

n

SE

MV

isio

n+JE

OL

.Non

e

JEO

L

SE

MV

isio

n

SE

MV

isio

n+JE

OL

.Control ALEG310 DSP+ NE111 Skip

SEM within Cleans Figure 28: Variability Chart for Window-2 Stitch Resistance by SEM Split Within Cleans

Split

142~

RC

_MT

2/M

T1_

Rc

4.5

5.0

5.5

6.0

6.5

7.0

7.5

.Con

trol

.Non

e

JEO

L

SE

MV

isio

n

SE

MV

isio

n+JE

OL

.Non

e

JEO

L

SE

MV

isio

n

SE

MV

isio

n+JE

OL

.Non

e

JEO

L

SE

MV

isio

n

SE

MV

isio

n+JE

OL

.Non

e

JEO

L

SE

MV

isio

n

SE

MV

isio

n+JE

OL

.Control ALEG310 DSP+ NE111 Skip

SEM within Cleans Figure 29: Variability Chart for Window-2 Kelvin Contact Resistance by SEM Split

Within Cleans Split

Although the distribution is very tight and there is a large degree of overlap

between the datasets, the data for the 2000 window stitches does indicate that the

combined SEM split has the highest mean resistance within every clean cell. This trend

does not occur in the single window Kelvin measurements. This data however, does not

implicate the SEM inspection. The distribution for the control cell is greater than any of

the individual distributions. In addition, if the stitch resistances for only the control

wafers is plotted by row it is evident that what appears to an increased resistance of the

combined SEM cell is merely a processed induced phenomenon, exacerbated by the

small sample size of the data set from single wafers. Figure 30 shows that the average of

the stitch resistances increases with increasing row number in a standard processed wafer.

82~V

IA2_

CH

AIN

_Rt (

kohm

s)

9

10

11

12

13

14

15

16

17

18

1 2 3 4 5 6 7 8 9 10

.Control

.Control

Figure 30: Metal-2 to Metal-1 2000 Window Stitch Resistance By Wafer Row for the

Three Control Wafers Only

Recall that for Figure 28 and Figure 29, the control cell data is taken from all rows of the

control wafers, the None cell data is taken from rows 1,2,3,and 10, the JEOL cell data is

taken from rows 4 and 5, the SEMVision cell data is taken from rows 6 and 7, and the

combined SEM cell data is taken from rows 8 and 9.

While the various cleans did shift the average resistances of the stitches and

Kelvins, there was no SEM scan interaction with these cleans. Evidently then, SEM

scans by the two SEMs did not induce a shift electrical characteristics in the window-2

stitches or window-2 Kelvin regardless of the post-metal-etch processing scheme

employed.

CHAPTER 5 SUMMARY AND CONCLUSIONS

The interactions of SEM inspection in an IC processing environment were

investigated. A new failure mechanism by scanning electron microscope induced

electrical failure of a window-1 contact was reported. It was found that in the event of

multiple SEM inspections of a source/ drain contact at various points in the wafer

processing, the electrical resistance of the inspected structures increased dramatically - up

to and including electrical opens. Furthermore, there was some indication that a post-

metal-etch clean served to decrease the overall resistance of the SEM scanned stitches,

however the resistance did not return the level of the control data. The experimental

procedure used obeyed the normal production processing guidelines of a 0.16µm process

technology. This indicates a potential source of defectivity in high volume integrated

circuit manufacturing, as the use of SEM defect detection and defect analysis steadily

increases. In addition, it was found that multiple SEM inspections of a gate contact did

not cause catastrophic electrical fail but did cause an increase in the resistance of those

gate contacts, possibly by as much as 200O per contact.

Failure analysis by cross-sectional STEM and top-down SEM of the high

resistance source/drain stitches did not conclusively determine the failure mode. Some

indication of the source of the disturbance was indicated, however, by the presence of

partially cored out tungsten plugs in the areas that received SEM scans on patterned oxide

and on window liner. Some speculations on potential physical mechanisms for the

failures were given.

Finally, SEM scanning of exposed window-2 tungsten plugs underneath

intentionally misaligned metal-2, was found to not cause a change in the electrical

characteristics of those structures investigated. This conclusion holds true for wafers that

received post-metal-etch cleans after SEM scans that were acidic, alkaline and neutral, as

well as structures that received no clean at all.

LIST OF REFERENCES

Documents

CHAPTER 3 EXPERIMENTAL - University of Floridapearton.mse.ufl.edu/theses/DavidShuttleworthCH3.pdf · CHAPTER 4 RESULTS AND DISCUSSIONS 4.1 Experiment 1 – Window-1 Interactions 4.1.1