Semi-Aqueous Solvent Applications for 193nm EAPSM Resist Strip

Julio Reyes∗, Curt Jackson, Laurent Dieu

DuPont Photomasks, Inc. 100 Texas Avenue, Round Rock, TX 78664

William Bowers, Russell Stevens

ATMI, 617 River Oaks Parkway, San Jose, CA 93134

ABSTRACT

Photomask resist strip processes have traditionally used the sulfuric-peroxide-mix, known as SPM, or Piranha.

This paper details a recent investigation into the utilization of solvent-based resist strip solutions applied to photomask

resist stripping. Studies of two commercially available solvents are documented in this report: one formulated for

positive resist stripping [Chem A, which contains a primary amine, glycol and is semi-aqueous], and another rated for

‘hard-to-remove’ positive resist stripping [Chem B, which contains glycol ethers, organic cyclics – all proprietary].

Resist types, such as IP3600, and most Chemically Amplified Resists (CAR) will strip easily with any of the chemicals

mentioned, however, other adverse effects may deter one from using them. The screening process employed in this

study monitors effects of processing on EAPSM phase and transmission, AR layer reflectivity changes and surface ionic

analytical comparisons. Chem A and B will show similarly low phase and transmission shifts at higher temperature and

longer process times, while reflectivity data shows lower level changes associated with the use of Chem A (favorable).

As for surface ionic contamination: on F and Cl contaminated surfaces, Chem A shows favorable results. Overall Chem

A seems to be the appropriate choice for more thorough investigation in a production mask-making environment.

Keywords: SPM, Solvent Strip, Reflectivity, Mask, Ionic Contamination, Phase Shift Mask (PSM), Resist

Stripping, Phase, Transmission, and Haze.

1. INTRODUCTION

Resist stripping process technology has focused on the use of SPM for quite some time1. The effects of SPM

on silicon wafers have been studied in detail. Recent work on photomask processing using SPM and other chemicals2

provides information which explains some of the drawbacks inherent with the use of SPM; namely, the presence of

ammonium sulfates and their influence on ‘haze formation’, or contaminant growth3 on photomasks. Emphasis on the

reduction of ammonium sulfates provides the motivation for photomask suppliers to explore alternative chemicals for

their resist strip and clean processes. For related reasons, it also becomes necessary to characterize the differences

between oxidation (SPM, DIO3, and UV/O3) and dissolution (NMP-based) methods of resist stripping.

The chemicals studied for purposes of this report were not originally intended for the application of resist

removal from Anti-Reflective Chrome (ARC) and Embedded Phase Shifter Materials (EPSM), however, the observed

data indicates they may be viable alternatives, useful for achieving good strip process results. Differences observed in

our work relative to the available literature allow us to logically propose this conclusion – most preparatory literature

suggests that SPM is incapable of good resist removal efficiency post dry etch - contradicting our observation of high-

efficiency resist stripping performance with semi-aqueous solvents.

2. EXPERIMENT

2.1 Sample Preparation

a) Samples for Phase and Transmission Measurements:

A 248nm EAPSM sample mask was pre-measured (Phase/Transmission) in a Lasertec MPM-248 metrology

tool for initial data collection. A typical EAPSM mask has about 16 so-called ‘phase boxes’ where phase and

transmission measurements are made. The mask was subsequently divided into 8 pairs of these boxes for

processing.

b) Samples for Surface Analysis

There were several groups of samples prepared for surface analysis testing:

i) Five coated (IP3600) chrome blanks (HOYA AR8). These were subjecte d to a chrome dry etch process

to simulate the condition of the resist surface post- chrome dry etch.

ii) Five chrome blanks (HOYA AR8) treated with a fluorinated etch (Quartz etch chemistry, or C2F6-

based)) for F surface contamination on the chrome surface. Process time was 30sec.

iii) Five chrome blanks (HOYA AR8) treated with a Chlorinated etch (Chrome etch chemistry, or Cl2-

based) for Cl surface contamination. Process time was 30sec.

c) Samples for Anti-Reflective (AR) layer reflectivity:

The samples for group (a) were used for reflectivity data collection, as well. Reflectivity measurements were

made within 5mm of the phase-box measurement sites.

∗ julio.reyes@photomask.com; phone (512) 310-6317

2.2 Sample processing

a) Group A samples were processed as indicated in Table 1.

SAMPLE# CHEMICAL TEMP [C] DIP TIME [min]

1 CHEM A 30 15 2 CHEM B 30 15 3 CHEM A 60 15 4 CHEM B 60 30 5 CHEM B 60 30 6 CHEM A 30 30 7 CHEM B 30 15 8 CHEM A 60 30

Table 1. Group A sample process.

The lower times were selected based on initial strip processing from three (3) of the coated samples in group (b)

above. The rinse times for all samples were 5min in DiW at room temperature, followed by an N2 dry.

b) Group B Sample Processing

Based on the longest times and highest temperatures processed in Table 1, the Group B samples were processed

as follows, in Table 2:

SAMPLE# CHEMICAL TEMP [C]

DIP TIME [min]

1 CONTROL 2 SPM PROCESS A 3 SPM PROCESS B

(w/172nm UV)

4 CHEM A 60 30 5 CHEM B 60 30

Table 2. Group B Process Matrix.

c) Group C Sample Processing

Same as Group A samples, since the reflectivity data was taken from the same Group A samples.

2.3 Sample Analysis, Results and Discussions.

a) Group A. Phase and Transmission.

Phase and Transmission data shows some variability; however, the phase data shows more consistent lower

deviations with the higher temperature and time for each chemical (Figure 1). The higher temperatures and

time constantly exposed to the MoSi surface do not seem to cause any major damage to the surface. This is

encouraging in that based on phase and transmission alone, either chemical is acceptable for further testing. See

below for Group B results.

b) Group B. Surface Ionic Analysis.

Anion surface analysis by DiW extraction (70C for 2hrs), and shown in ppb/150ml, shows the effect on Cl, F

and sulfate residues on the chrome surface (Table 3). Note the lower F and Cl levels for Chem A. Note also the

rather poor performance of Chem B on Cl and F removal. However, since both Chem A and B do not contain

sulfates, it is easy to understand the low sulfate levels (in one case it is below the control levels).

Since Chem A is an aqueous-based chemical (approx. 18% water), we can explain the better F removal

performance. It is also reasonable to say the Cl residues may be somewhat soluble in the aqueous solution of

Chem B.

Phase/Transmission Shift Screen

0.400.55

0.03

0.42 0.38

0.71

0.05

-0.12

1.741.65

-0.22

0.85

-0.26

-2.11

0.080.20

0.020.03

-0.02-0.03

0.09

0.04

0.060.05

0.040.05

0.03

-0.03

0.010.00

-0.02

0.01

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

CH

EM

A_3

0_15

CH

EM

A_3

0_15

CH

EM

A_3

0_30

CH

EM

A_3

0_30

CH

EM

A_6

0_15

CH

EM

A_6

0_15

CH

EM

A_6

0_30

CH

EM

A_6

0_30

CH

EM

B_3

0_15

CH

EM

B_3

0_15

CH

EM

B_3

0_15

CH

EM

B_3

0_15

CH

EM

B_6

0_30

CH

EM

B_6

0_30

CH

EM

B_6

0_30

CH

EM

B_6

0_30

Test Type Interaction

Ph

ase

An

gle

Sh

ift [d

eg

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

Tra

ns

shift

[%]

Delta-Phase Delta-Trans

Figure 1. Effect of Chem A and B on Phase and Transmission.

Anions DL* Control

SPM Process A

SPM Process B Chem A Chem B

Chloride (Cl-) 0.05 61 8.1 18 5.9 28

Fluoride (F-) 0.05 470 140 150 93 410 Sulfate-I (SO4

2-) 0.05 2.0 42 24 1.1 1.5

Sulfate-II (SO42-) 0.05 1.6 37 13 5.9 4.0

Table 3 . Anion surface analysis

c) Group C. AR Reflectivity.

Figure 2 shows the effect of the Chem A and Chem B process treatments on the chrome AR surface. An N&K

1512 RT was used to collect the data. Reflectivity data for 248nm and 193nm wavelengths were collected for

this report. Hoya NTAR5 and NTAR7 AR types were not studied in this screening. Noting the effect of these

processing conditions on other films would be of great interest. At first glance, there appears to be a slightly

higher reflectivity level for Chem B compared to Chem A. The data shows an increase with more aggressive

processing (higher time and temp). However, the reflectivity seems to drop for the 60C-30min processes. This

may be due to the level of erosion of the AR layer, which can cause the optical effect on reflectivity.

AR Reflectivity vs Process Conditions

0.00

20.00

40.00

60.00

80.00

100.00

120.00

CHEM A

_30_

15

CHEM A

_30_

15

CHEM A

_30_

30

CHEM A

_30_

30

CHEM A

_60_

15

CHEM A

_60_

15

CHEM A

_60_

30

CHEM A

_60_

30

CHEM B

_30_

15

CHEM B

_30_

15

CHEM B

_30_

15

CHEM B

_30_

15

CHEM B

_60_

30

CHEM B

_60_

30

CHEM B

_60_

30

CHEM B

_60_

30

Proces Conditions

Ref

lect

ivity

-Nor

mal

ized

to M

ax-M

in

248nm 193nm

Figure 2. AR Reflectivity normalized as Max-Min=100%

Another important observation noted is the slightly higher levels of reflectivity for Chem B at the 248nm

wavelength, while no higher reflectivity is observed at the 193nm level. Overall, AR reflectivity for both

chemicals at both wavelengths seems to be relatively stable for the 30deg-temperature process condition. This

is an indication that the chemicals are more reactive at the higher temperature, and of course, higher time.

3. CONCLUSIONS

This simple screening test has shown that there is justification for exploring solvent chemicals, particularly

Chem A, which is semi-aqueous (18% Water), for resist strip applications in photomask manufacturing. The

solubility properties for F and Cl residues for Chem A give it an advantage over Chem B. The phase and

transmission can hold up fairly well through a range of process conditions (although much less aggressive

conditions will accomplish the desired photomask resist strip). Reflectivity data shows no major AR

degradation, but some thinning at the 60C/30min-process condition occurs. Further work should be done to

clearly understand the effects of the amines on different mask surface materials (i.e.: MoSi, Quartz, and

Chrome). Particle removal performance will, of course, also be of great interest when an in-house evaluation is

performed.

4. ACKNOWLEDGEMENTS

The authors would like to thank Christian Chovino, Eric Johnstone, K.T. Park and Gary Doyle, all from DuPont

Photomasks, Inc., for their contributions to sample measurement and data analysis contributing to this article.

REFERENCES

1. L. H. Kaplan and B. K. Bergin, “Residue from Wet Processing of Positive Resists”, J. Electrochemical

Society, Vol. 127, No.2, p. 986, 1980

2. Hitoshi Handa, Masumi Takahashi, Hisatgusu Shirai, “Study of Reticle Cleaning Process for 130nm

Lithography and Beyond”, Photomask and Next Generation Lithography Mask Technology VIII,

Proceedings of SPIE Vol. 4409 (2001), p.430

3. Battacharrya, K., Volk, W., Brown, D., Ayala, J. and Grenon, B.J.,”Investigation of Reticle Defect Formation at

DUV Lithography”, 22nd Annual BACUS Symposium on Photomask Technology, Proceedings

of SPIE, Vol. 4889, pp.478-87, October 2002.