# Assessment of carbon layer growth induced by resists outgassing in multi e-beams lithography

J.C. Marusic, M.L. Pourteau, S. Cêtre, L. Pain, A. Mebiene Engohang, S.

David, S. Labau, J. Boussey

► To cite this version:

J.C. Marusic, M.L. Pourteau, S. Cêtre, L. Pain, A. Mebiene Engohang, et al., Assessment of carbon layer growth induced by resists outgassing in multi e-beams lithography. Conference on Photomask Technology, Sep 2014, Monterey, United States. 10.1117/12.2066152. hal-01798360

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# Assessment of carbon layer growth induced by resists outgassing in multi e-beams lithography

JC. Marusic, ML. Pourteau, S. Cêtre, L. Pain

CEA, LETI, Silicon Technology Division, F-38054 Grenoble, France

jean-christophe.marusic@cea.fr

AP. Mebiene-Engohang

STMicroelectronics, F-38920, Crolles, France

S. David, S. Labau, J. Boussey

LTM-CNRS-UJF/CEA, LETI, F-38054 Grenoble, France

## ABSTRACT

The development of multiple e-beam lithography equipment is seen as an alternative for next generation lithography. However, similarly to EUV lithography, this technology faces important challenges in controlling the contamination of the optics due to deposition of carbon layer induced by the outgassed chemical species from resist under electron bombardment. An experimental setup was designed and built at LETI to study the outgassed species and observe the carbon layer. In this setup, resist coated wafers 100 mm size are exposed under a 5 kV e-beam gun. During exposure, by-products from outgassed species are monitored with a Residual Gas Analyzer (RGA). The identification of outgassed chemical species is done with an *ex-situ* TD-GC-MS analysis (ThermoDesorption-Gaz Chromatography-Mass Spectrometry). In a second part of this investigation, we observed the contamination carbon layer growth induced by the outgassing. Thereby, we fabricated a device which consists of a silicon membrane with micro-machined apertures. During e-beam exposure, this device simulates the multiple parallel beams of the optic system of a maskless lithography tool. The deposited contamination layer on device is then observed and thickness measured under SEM. In this paper, we present the results of outgassing and contamination on 3 chemically amplified resists showing that contamination is not directly dependent of the overall outgassing rate but on first order of the outgassing from Photo Acid Generator (PAG). It also reports on the performance in reducing outgassing and contamination of applying a top-coat layer on top of the resist and shows that reduction is more important for contamination than for outgassing.

**Keywords:** chemically amplified resists; outgassing; optic contamination; top-coat; TD-GC-MS; RGA; SEM

## 1. INTRODUCTION

Similarly to EUV lithography, in e-beam lithography the contamination of optical systems by resist outgassing is a well-known phenomenon¹. It is due to the interaction between electrons radiation and the organic molecules outgassed from resist that are adsorbed onto optics element surface². The nature of this contamination layer has been shown to be composed of more than 90% of carbon³. In time, this layer of carbon eventually degrades the performances of the optics⁴. In a single electron beam equipment, this contamination is less critical due to its intrinsic low throughput. However, with direct-write lithography using massively parallel e-beams as an alternative next generation lithography for IC or mask manufacturing, contamination phenomenon is amplified and can no longer be neglected. It is thus important to understand the influential parameters and the kinetics of contamination layer growth. It was shown in previous investigation⁵ that the contamination layer growth depends on the e-beam exposure dose, the hydrocarbon pressure inside the exposure chamber and the optics projection system's temperature. Beside these results, other teams have studied how photoresist formulation can impact the outgassing and contamination⁶⁻¹⁰. They have shown that in Chemically Amplified Resists (CAR), Photo Acid Generator (PAG) and Leaving Groups (LG) play an important role in outgassing. We presented, in a previous paper¹¹, how the CAR formulation influences the outgassing rate under e-beam exposure and especially the contribution of PAG and LG. This paper also presented results on the use of a top-coat layer above the resist to reduce outgassing. In collaboration with Mapper Lithography, the purpose of the current experimental
investigations presented in this paper is to determine in which conditions the contamination layer induced by resist outgassing may obstruct the mimic holes and modify the e-beams path characteristics. We will present the contamination growth rates measured for different resist formulations and compare them with outgassing results. We will detail the contribution of PAG and LG outgassing rate and how they correlate with contamination growth. Finally, we will present how a top-coat layer above resist effectively reduces outgassing and contamination rates.

## 2. EXPERIMENTAL SETUP

### 2.1 Description of the test setup

In order to study resist outgassing and induced contamination layer growth under e-beam exposure, we designed at LETI an experimental setup which consists of an ultra-high vacuum chamber capable of reaching the low range $10^{-8}$ mbars base pressure, equipped with a 5 keV e-gun from Kimball Physics and a 100 mm size wafer holder. The chamber is also coupled to a load lock in order to introduce wafers without breaking the vacuum. As shown in Figure 1, the test setup can be used in two configurations: one for outgassing and the other one for contamination measurements.

<figure><img src="image_2.png" /><figcaption>Figure 1. Schematics of the outgassing/contamination test setup installed at Leti.</figcaption></figure>

During outgassing measurement as shown in Figure 1a, the beam is scanned over the wafer and beam deflection parameters are adjusted to expose a resist area of about 7 x 7 cm² at dose-to-size that corresponds to approximately 2.5 times the dose-to-clear of each resist. The mass spectrum of outgassed by-products is monitored in real time by a Residual Gas Analyzer (RGA) equipped with a Quadrupole Mass Spectrometer (QMS). To perform contamination study, we fabricated devices we call “mimic” which consist of a 100 µm thick silicon membrane with a micro-machined 10 x 10 array of 16 µm diameter holes (Figure 2). They mimic the electron optics component closest to the wafer in Mapper’s multi e-beams exposure tool. The mimic is fixed within the e-beam path close to the resist-coated wafer as shown in
Figure 1b. The wafer is held on a motorized X-Y table in order to scan the wafer and expose the resist at dose-to-size. Such a setup enable to expose wafers with several parallel e-beams and grow contaminating carbon layer onto holes sides as it occurs in a real multi e-beams lithography tool. It allows to perform destructive characterization of the contamination on the mimic without sacrificing complex and expensive electron optics components.

<figure><img src="image_3.png" /><figcaption>Figure 2. SEM top view of the holes array.</figcaption></figure>

## 2.2 Sample preparation

We selected 3 resists with different formulation and one organic top-coat layer. All 3 resists are Positive Chemically Amplified Resist (PCAR). Two of them have an Acrylate polymer matrix and the other one, our Process of Reference (POR) resist, has a mixture of Poly Hydroxystyrene (PHS) / Acrylate polymer matrix. Table 1 summarizes resist formulation as characterized in a previous paper¹¹. The resist process conditions were chosen in compliance with manufacturer recommendations. Resists were spin-coated on 100 mm wafers to a thickness range of 30-40 nm. The top-coat was spin-coated above the resist to 2 different thicknesses of 10 and 30 nm.

<table><caption>Table 1. Characteristics of materials tested.</caption><thead><tr><th>Samples name</th><th>Type</th><th>Polymer matrix type</th><th>Main outgassed by-products (origin)</th></tr></thead><tbody><tr><td>Resist A</td><td>PCAR</td><td>Acrylate (PAG is bonded to polymer matrix)</td><td>benzene (PAG), biphenyl sulphide (PAG), adamantylidene (LG)</td></tr><tr><td>Resist B</td><td>PCAR</td><td>Acrylate</td><td>benzene (PAG), biphenyl sulphide (PAG), adamantylidene (LG)</td></tr><tr><td>Resist POR</td><td>PCAR</td><td>Poly Hydroxystyrene/Acrylate</td><td>benzene (PAG), biphenyl sulphide (PAG), adamantylidene (LG)</td></tr><tr><td>TC</td><td>Top-coat layer</td><td>Acrylate</td><td>not measured</td></tr></tbody></table>

## 2.3 Outgassing test methodology

We introduced resist-coated wafer in the test chamber and let the pressure stabilize in low range of $10^{-8}$ mbar. We then exposed the wafer with the e-beam on an area of 7 x 7 cm². Beam current was fixed at 15 µA and beam raster parameters set to ensure the resist exposure at dose-to-size. Outgassing was monitored during wafer exposure with the RGA equipped with a Quadrupole Mass Spectrometer (QMS) from MKS. The test is repeated 3 times for each resist for repeatability check.

In order to identify chemical species released during exposure, we use the TD-GC-MS technique. After exposure, a 2 x 2 cm² piece of the wafer to be analyzed is cut and introduced into a desorption chamber where it is heated from 30°C up to 110°C with temperature ramp of 5°C/min and maintained at this temperature for 5min. The sub-compounds desorbed
from the sample are then transported by N₂ gas flow towards a Tenax® tube where they get trapped. In a second step, the collected compounds were thermally desorbed from the Tenax® tube and analyzed by GC-MS for components identification.

## 2.4 Contamination test methodology

The mimic of the electron optics is introduced in the chamber and positioned in the beam path at a distance from wafer that can vary from 200 to 800 µm. The side of the mimic opposite to wafer is isolated from the rest of the chamber by a piece of brass with an orifice of 2 mm of diameter. This arrangement, shown in Figure 3, allows controlling the outgassed components pressure at both sides of the mimic holes, P1 and P2 by means of the orifice conductance and the mimic-to-wafer gap distance. After the chamber is pumped down to base pressure, a first wafer is introduced and is scanned over the e-beam at a fixed wafer-to-mimic distance to expose an area of 100 x 25 mm². After the scan is completed, the wafer is removed; a new coated wafer is introduced in the chamber for a new scan after the chamber has returned to base pressure. Typically a total of 15 wafers need to be exposed in order to have a layer of contamination with sufficient thickness for measurement and characterization. The mimic is then taken out of the chamber to be analyzed.

<figure><img src="image_4.png" /><figcaption>Figure 3. Schematic of the setup configuration showing wafer, mimic and orifice.</figcaption></figure>

The mimic is cleaved along the holes to observe the cross-section by using the SEM technique and to measure the thickness of the deposited carbon layer (Figure 4). After a few preliminary runs, we fixed the conditions of current density at 20 A/m² and wafer-to-mimic gap at 200 µm which corresponds to the current density and conductance conditions found inside the Mapper's multi e-beams lithography tool. We then performed contamination run under these conditions for each resist A, B and POR with and without top-coat.
<figure><img src="image_5.png" /><figcaption>Figure 4. SEM views of a mimic hole after a contamination run showing deposited carbon layer thickness (picture extracted from reference 11).</figcaption></figure>

### 3. RESULTS AND DISCUSSION

We first compared the outgassing rate to the contamination growth rate for the 3 resists selected. By means of TD-GC-MS analysis performed, we were able to identify the origin of the molecules fragments detected by RGA during outgassing and therefore assess the outgassing rate contribution from the leaving group (LG), from the Photo Acid Generator (PAG) or from other molecules. The comparison is shown in Figure 5.

<figure><img src="image_6.png" /><figcaption>Figure 5. Outgassing rate and contamination growth rate for 3 resists.</figcaption></figure>

We can see that the POR resist has the highest growth rate among the 3 resist tested. However, the resist with the highest outgassing rate is not the POR resist, but resist B. Hence, a higher outgassing rate does not necessarily lead to higher growth rate. Indeed, the growth rate will most likely be influenced by the relative proportion of the different groups of outgassed species. We can see that the outgassing rate of PAG for POR resist is the highest and the lowest for resist A. This indicates that PAG component outgassing is most likely to be a major contributor to contamination growth rate.

Secondly, we evaluated the impact of applying a top-coat on the resist. We first evaluated the impact of the top-coat thickness on outgassing and growth rate. Two thicknesses were tested: one at 10 nm which is best suited for e-beam
lithography resolution and the other at 30 nm in accordance with manufacturer's recommendations. Figure 6 shows the results for POR resist with a top-coat of 10 nm and 30 nm.

<figure><img src="image_7.png" /><figcaption>Figure 6. Relative outgassing rate and contamination growth rate for POR resist only and POR resist with top-coat layer of 10 nm and 30 nm.</figcaption></figure>

We observe that the top-coat reduces the outgassing rate by 36% for 10 nm and 46% for 30 nm. As we would expect, the thickness improves the top-coat efficiency in reducing outgassing. If we look at the contamination growth, it is a 70% and 89% decrease respectively for 10 nm and 30 nm. The effect on contamination is greater than what we would expect looking at outgassing effect. We also measured and compared the efficiency of a 10 nm thick top-coat layer on outgassing and contamination rates for 2 resists: POR and B resists. We can see in Figure 7 that without top-coat, the resist B outgasses less PAG than POR resist and has a lower contamination rate. For both POR and B resists, the top-coat layer reduces PAG outgassing and contamination rate and both resists with a top-coat layer show similar contamination growth and PAG outgassing. These results suggest that contamination growth reduction with top-coat seems to be related to the efficiency of top-coat in reducing PAG outgassing. As detailed in Table 1, the POR and B resists have outgassed species coming from PAG in common and hence top-coat efficiency to limit outgassing is very similar. We have shown in previous work¹¹ that this ability of top-coat to reduce outgassing depends on the size and polarity trends of outgassed by-products. Indeed, large outgassed species move with more difficulty in the top coat film and, therefore, outgas less. These results indicate ways to design resists formulation to reduce outgassing and contamination from e-beam resists by limiting PAG by-products outgassing. It also suggests that top-coat formulation should also focus on limiting PAG by-products outgassing.
<figure><img src="image_8.png" /><figcaption>Figure 7. Relative outgassing rate and contamination growth rate comparison for 2 resists only and with a 10 nm top-coat layer.</figcaption></figure>

## 4. CONCLUSIONS

The outgassing and the growth rate were measured for 3 PCAR resists with different formulations. The results were compared together and we have shown that the growth rate is not directly related to the overall outgassing rate. The outgassed by-products from PAG are major contributors to the contamination. We assessed the impact of a top-coat layer on top of the resist layer. We observed that the top-coat reduces significantly the outgassing and contamination and its benefit increases with thickness. We also noticed that the reduction for contamination is more important than it is for outgassing. The comparison of outgassing and contamination rate suggests that top-coat efficiency to reduce contamination is related to its ability to limit PAG outgassing. These results show how the use of a top-coat layer and the resist formulation can influence the outgassing and the contamination induced.

The research leading to these results has been performed in the frame of the industrial collaborative consortium IMAGINE.

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