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Direct Analysis of Photoresist by ICP-MSFeaturing the Agilent Technologies 7500s ICP-MS

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Presentation Outline

● How is photoresist used?

● Analytical challenges

● Instrumentation developments

● Analytical approach◆ Tuning◆ Calibration

● Typical analytical figures of merit◆ Detection Limits (DLs)◆ Spike recoveries at the 1 ppb level◆ Stablitity

● Conclusions

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What is Photoresist (PR)?

● Thick resin mixtures

● Two basic types:◆ Positive resist: soluble upon

exposure to radiation – offer higher resolution than negative resists

◆ Negative resist: insoluble upon exposure to radiation

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Process Steps ● Diffusion – a layer of material such as an oxide layer is grown or

deposited on to the wafer surface● Next a positive photoresist layer is applied and cured on the oxide

layer● A glass mask is positioned over the wafer. Ultraviolet light shines

through the clear portions of the mask and exposes the template onto the photo sensitive resist

● The exposed resist becomes soluble to the developer TMAH (2.38%)in the presence of UV light

● The “mask pattern” is then etched on to the wafer using either a wet or dry etching process

● The undeveloped/hardened photoresist is then removed and the process repeated using a different mask pattern

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Trace Metal Requirements for Photoresist (PR)● PR manufacturer and user requirements

◆ control impurities that affect characteristics of semiconductor devices◆ limits on impurities are being reduced

● Typical acceptable levels of metallic impurities in the photoresist are in the range 10 – 30 ppb per element

● Typical elements of interest: Na, Mg, K, Ca, Cr, Mn, Fe, Ni, Cu, Zn

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Impurity Levels in Photoresist:Manufacturers’ Data

20-22<10K

47--30Fe

<20-<1<10Mn

88-11-Ca

<20-1<10Mg

247291110Na

Concentration (ppb)Element

30282830Resin (wt. %)

S(2)S(1)FTManufacturer

PR samples are typically analysed at 1:10 dilution in an appropriate solvent giving 2-3% resin in the sample as analyzed

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Analytical Challenges (1)

Some sample preparation is required prior to analysis of photoresist

● In the past acid digestion was widely used but it is time-consuming and leads to

➝ loss of volatiles - eg B, As➝ contamination from apparatus, acid and other reagents➝ potentially hazardous reactions

● More typically photoresist is diluted using an appropriate solvent e.g. 1:10 in N-methyl pyrolidone (NMP), propylene glycol mono-methyl acetate (PGMEA), ethyl lactate

● Detection limits in the photoresist are limited by impurity level in the solvents

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Metal Impurities in Organic Solvents

0.1300.0510.057NormalV (51)

1.1470.6831.631CoolFe (56)0.0170.0810.534CoolMn (55)2.0350.0860.857CoolCr (52)

2.0100.8871.021NormalTi (47)0.11021.5800.761CoolCa (40)0.0392.8700.459CoolK (39)0.1000.0730.490CoolAl (27)0.0200.2130.885CoolMg (24)0.051-8.201CoolNa (23)1.29617.690-NormalB (10)0.0270.0800.099NormalBe (9)0.0090.0440.013CoolLi (7)

PGMEAEthyl LactateNMPModeElementImpurity (ppb)

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Metal Impurities in Organic Solvents (2)

0.0330.0440.016NormalTa (181)

0.0130.0520.016CoolPb (208)0.0290.0130.027NormalW (182)

0.0210.0340.037NormalBa (138)0.0530.0530.863NormalCd (111)0.0190.0220.109NormalAg (!07)0.0540.0260.062NormalMo (95)0.3480.5586.918CoolZn (68)0.0130.3061.855CoolCu (63)0.0590.0800.455CoolNi (60)0.0220.0430.031CoolCo (59)0.0640.0721.024CoolNi (58)

PGMEAEthyl LactateNMPModeElementImpurity (ppb)

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Analytical Challenges (2)

● Since photoresist is 30% resin, clogging of the nebulizer, torch, interface and drain can be a problem

● Heavy sample matrix could suppress analyte signals

● Carbon based spectral interferences on Mg (C2) and Cr (ArC)

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The Agilent Approach to Photoresist Analysis● Clogging at nebulizer, torch, interface and drain

◆ Specially designed sample introduction system➙ Low sample uptake rate ➙ PR spray chamber drain fitting➝ Quartz nebulizer and torch injector with tapered tip

● Improved instrument design for analysis of high matrix samples◆ Solid state ICP RF Generator – replacement power tubes not needed ◆ Improved plasma stability ◆ Robust, 27.12 MHz high temperature plasma

● Carbon-based spectral interferences◆ ShieldTorch cool plasma analysis at high power◆ 950W power can handle heavy matrices such as PR

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Tapered-tip Torch

A Quartz narrow-bore injector (1.5 mm ID) with a tapered tip is recommended for PR analysis in place of standard 2.5mm ID torch

A tapered tip avoids a point for deposition

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Reducing Carbon-based Interferences● Enhanced ShieldTorch System

◆ Long life shield plate◆ Self aligning shield mount◆ Cool plasma AutoTune

● The combination of the ShieldTorch System and cool plasma conditions give effective interference removal of carbon based interferences on Mg (C2) and Cr (ArC)

Load coil

Shield plate

Plasma Torch

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Experimental – Standard Analytical Method for PR Analysis

● Define the analytical method

● Fix optional gas flow rate

● Tune cool plasma operating conditions

● Tune normal plasma operating conditions

● Sample preparation – simple dilution

● Calibration using Method of Standard Addition

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7500s Tuning for Photoresist Analysis● General parameters

◆ Fix sampling depth at 17.5mm◆ Add 20% oxygen (0.2 L/min) to the plasma ◆ Free aspiration using 0.3mm capillary tubing

● Tune Cool plasma conditions◆ Optimize 59Co sensitivity (10ppb aqueous solution)◆ Verify reduction of Ar2 dimer at mass 80 ◆ Verify reduction of 12C (carbon does not ionize in cool plasma)

● Tune Normal plasma conditions◆ Optimize sensitivity for Li, Y and Tl with sampling depth and oxygen flow

rate fixed

● Aspirate solution of PGME for approx. 10 minutes for change over to organic sample introduction

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Analysis of Photoresist● Dilute resist samples 1:10 with propylene glycol mono-methyl acetate

(PGMEA)

● Prepare calibration standards (MSA) by spiking the diluted sample with 1, 2, 3 ppb multi-element standard

● Analyze

● A rinse step (using PGMEA) between samples is recommended to prevent signal drift resulting from PR deposition

● Following this protocol trades off sensitivity for simplicity. However, there are no gains to be had for this analysis by optimizing the ICP-MS for best possible performance

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Method of Standard Addition (MSA) Calibration

• MSA calibration on one sample can be applied to all samples of the same matrix type, so analysis time is not compromised

• External calibration (with or without blank subtraction) can give under-reporting (reported value lower than true value), due to subtraction of too high a blank signal or signal changes due to sample transport and nebulization

Measured signal and so reported concentration is sum of 1) background, 2) interference and 3) contamination. Instrument optimisation is used to minimise the contribution of 1) and 2)

Residual Interference

Contamination

Random Background

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7500s Operating Parameters

0.99 sec 0.99 sec Integration time / mass2 deg C2 deg CSpray chamber temp20%20%Optional gas flow rate0.46 L/min0 L/minMake up gas flow rate1.0 L/min1.0 L/minCarrier gas flow rate17.517.5Sampling depth950W1450WRF PowerCool PlasmaNormal Plasma

Concentric Nebulizer (Self aspiration)NebulizerQuartz, 1.5 mm tapered injectorTorchAgilent 7500s ShieldTorch SystemICP-MS

Cool plasma conditions are achieved - even at 950W forward power. This ensures the plasma has enough energy to break down the sample matrix

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Analysis of Photoresist Sample

0.2410.006NDNDND

0.1270.2330.0100.1050.3690.036ND

0.001

A - B

2.0241.9830.057NormalV (51)

1.3771.6180.3CoolFe (56)0.0030.0090.006CoolMn (55)2.0241.9830.057CoolCr (52)

2.0601.7770.2NormalTi (47)0.1620.2880.02CoolCa (40)0.0440.2770.009CoolK (39)0.1030.1130.06CoolAl (27)0.0150.1190.005CoolMg (24)0.0500.4190.01CoolNa (23)0.8730.9090.08NormalB (10)0.0060.0030.002NormalBe (9)0.0000.0010.0004CoolLi (7)

PGMEA (B) (ppb)

*Photoresist (A) (ppb)

Detection Limit (ppb)

ModeElement

ND = not detected

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Analysis of Photoresist Sample (2)

0.0040.0050.0090.004CoolPb (208)0.0070.0090.0160.007NormalW (182)ND0.1050.0050.007NormalTa (181)ND0.0170.0010.004NormalBa (138)ND0.0230.0160.01NormalCd (111)

0.0030.0010.0040.005NormalAg (!07)0.0100.0370.0470.01NormalMo (95)0.2450.4180.6630.08CoolZn (68)0.0020.0690.0710.01CoolCu (63)0.0360.0650.1020.03CoolNi (60)0.0000.0020.0020.0008CoolCo (59)0.0230.0780.1010.06CoolNi (58)

A - BPGMEA (B) (ppb)

*Photoresist (A) (ppb)

Detection Limit (ppb)

ModeElement

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Spike Recoveries

1001.1100.106NormalV (51)

1121.1920.272CoolFe (56)1121.1150.000CoolMn (55)1191.2760.084CoolCr (52)

910.9110.000NormalTi (47)1191.3080.115CoolCa (40)1171.4040.233CoolK (39)1121.1170.000CoolAl (27)1071.1540.088CoolMg (24)1141.4940.352CoolNa (23)910.9350.021NormalB (10)970.9670.002NormalBe (9)940.9400.003CoolLi (7)

% Recovered1 ppb SpikeConc (ppb)ModeElement

External calibration + internal standard In (cool) & Rh (normal)*Photoresist sample was diluted 1:10 with PGMEA

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Spike Recoveries (2)

% Recovered1 ppb SpikeConc (ppb)ModeElement

1031.0280.000NormalTa (181)

1001.0030.000CoolPb (208)1031.0310.000NormalW (182)

980.9810.000NormalBa (138)1021.0180.000NormalCd (111)990.9880.000NormalAg (!07)980.9900.011NormalMo (95)1071.2480.173CoolZn (68)1111.1210.009CoolCu (63)1151.1830.034CoolNi (60)1141.1350.000CoolCo (59)1161.1900.026CoolNi (58)

External calibration + internal standard In (cool) & Rh (normal)*Photoresist sample was diluted 1:10 with PGMEA

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3 % Photoresist Analysis: Cool Plasma1.5 min Uptake, 2.5 min Analysis, 5 min rinse with PGME

0

0.5

1

1.5

2

2.5

3

3.5

0 20 40 60 80 100 120

Time (min)

Con

cent

atio

n (p

pb)

7 Li (3.0%)24 Mg (0.9%)25 Mg (1.0%)27 Al (0.7%)39 K (0.9%)40 Ca (2.3%)52 Cr (1.0%)53 Cr (0.9%)55 Mn (0.6%)56 Fe (3.0%)58 Ni (1.3%)59 Co (1.0%)60 Ni (1.3%)63 Cu (1.3%)65 Cu (1.9%)68 Zn (6.7%)208 Pb (1.7%)

Excellent long-term stability, despite very complex matrix

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3 % Photoresist Analysis:Normal Plasma1.5 min Uptake, 2.5 min Analysis, 5 min rinse with PGME

0

0.5

1

1.5

2

2.5

0 20 40 60 80 100 120

Time (min)

Con

cent

ratio

n (p

pb)

10 B (2.3%)

47 Ti (1.6%)

51 V (0.9%)

66 Zn (1.4%)

95 Mo (0.8%)

111 Cd (1.0%)

181 Ta (0.7%)

182 W (0.7%)

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7500s Operating Parameters:Optimized for Normal and Cool Plasma

0.99 sec 0.99 sec Integration time / mass2 deg C2 deg CSpray chamber temp20%20%Optional gas flow rate0.37 L/min0 L/minMake up gas flow rate1.0 L/min1.0 L/minCarrier gas flow rate17.514.5Sampling depth850W1450WRF PowerCool PlasmaNormal Plasma

Concentric Nebulizer (Self aspiration)NebulizerQuartz, 1.5 mm tapered injectorTorchAgilent 7500s ShieldTorch SystemICP-MS

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Analysis of Photoresist Sample

0.221NDND

0.119ND

0.1200.235ND

0.0930.3790.048ND

0.006

A - B

0.2290.3470.057NormalV (51)

1.4491.6700.3CoolFe (56)0.0800.0660.006CoolMn (55)2.1542.0490.057CoolCr (52)

2.0681.8090.2NormalTi (47)0.2500.3700.02CoolCa (40)0.1020.3370.009CoolK (39)0.2000.1860.06CoolAl (27)0.0860.1790.005CoolMg (24)0.1080.4870.01CoolNa (23)0.8740.9220.08NormalB (10)0.0250.0180.002NormalBe (9)0.0600.0660.0004CoolLi (7)

PGMEA (B) (ppb)

*Photoresist (A) (ppb)

Detection Limit (ppb)

ModeElement

ND = Not Detected

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Analysis of Photoresist Sample (2)

0.0150.0770.0920.004CoolPb (208)ND0.0500.0360.007NormalW (182)ND0.1180.0440.007NormalTa (181)ND0.0430.0330.004NormalBa (138)ND0.0620.0570.01NormalCd (111)ND0.0330.0280.005NormalAg (!07)

0.0000.0600.0600.01NormalMo (95)0.2330.5160.7490.08CoolZn (68)0.0020.1300.1320.01CoolCu (63)0.0250.1380.1630.03CoolNi (60)ND0.0810.0750.0008CoolCo (59)

0.0020.1510.1530.06CoolNi (58)

A - BPGMEA (B) (ppb)

*Photoresist (A) (ppb)

Detection Limit (ppb)

ModeElement

•No observed different in DL’s when operating under optimized conditions

•DL’s are fundamentally limited by the metals impurities in the solvent blank

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Photoresist Analysis - Summary

● Reliable photoresist analysis requires several key design considerations:◆ Specially designed sample introduction system including a tapered torch◆ Highly efficient plasma rf generator◆ Flexible gas control including the ability for oxygen addition◆ ShieldTorch System for high power, cool plasma operation

● Higher power (950W) cool plasma effectively breaks down the heavy resist matrix (sample analysed as 2-3% resins) and eliminates C-based interferences on Mg and Cr, as well as Ar-based interferences on K, Ca and Fe

● Instrument optimization is quick and easy even when operating in multiple plasma modes

● The Agilent 7500s has the capability to reproducibly measure therequired analytes at the levels required by the industry