Chapter 11
Exploration of Chemically Amplified Resist Mechanisms and Performance at Small Linewidths James W. Taylor, Paul M . Dentinger, Steven J. Rhyner, and Geoffrey W. Reynolds Department of Chemistry, Center for X-ray Lithography, University of Wisconsin at Madison, Madison, WI 53589
When exposed to X-rays, chemically-amplified resists show very high resolution at the required sensitivity, but fundamental quantitative questions about the reaction and performance remain. What and how much chemical change is necessary to sufficiently decrease the dissolution rate? How many cycles does the acid undergo during the post-exposure bake? What is the sidewall roughness? For Shipley SAL 605 negative-tone resist, quantitative measurements show that under lithographic conditions: 5 x10-06 moles/cm of acid are produced, the acid cycles about 26 times, 2% of the phenols are protected, a cross-linking reaction is not necessary for the dissolution rate to be sufficiently changed, and the sidewall roughness is on the order of 5.2 nm. The mechanistic implications of these quantitative observations on resist performance will be discussed. 3
Semiconductor manufacturing applications, using 1.0 nm X-rays as the exposure source, require resist sensitivities on the order of 50-100 m J / c m in order to meet the desired throughput. Conventional resists, where an interaction between the exposing radiation and resist directly defines sensitivity, have not been able to meet this need. Attention has turned to chemically-amplified resists where the exposure creates a species which catalyzes multiple chemical events during the post-exposure bake(PEB). There are a variety of resist systems that can demonstrate chemical amplification, but Shipley S A L 605 - a negative-tone resist showing sensitivities on the order of 100 m J / c m - is the system chosen for this quantitative study. The mechanism for this resist has been described as the creation of an acid from the interaction of the exposing radiation with a photoacid generator(PAG) within the film matrix, a rate-limiting reaction between the acid and hexamethoxymethylmelamine(HMMM), and a cross-linking reaction between the cationic intermediate on the H M M M and the novolac matrix (7). The presumed mechanism is shown in Figure 1 where one molecule of H M M M is shown coupling with one hydroxy on a novolac. O-alkylation has been shown to occur quantitatively over C-alkylation (2) as shown in the Figure. Evidence of the coupling reaction is found at 988 c n r in the IR, (3) and corresponds to the formation of an ether bond between the iminium ion and the oxygen from the novolac polymer. The H M M M has 2
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© 1 9 9 8 American Chemical Society
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CH3OCH2 N
CH2OCH3
( /
CH3OCH2
y
CH
2
+
H0CH3
CH2OCH3 CH3OCH2
CH2OCH3
/
\
CH3OCH2
Hexamethoxymethylmelamine
CH2OCH3
Iminium ion
Figure 1. Schematic of mechanism of resist reaction. (Adapted from ref. 5)
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six potential sites for reaction, and this suggests that cross-linking could occur i f more than one novolac oligomer reacted to a single H M M M . Quantitative Measurements of Resist Performance In this paper, there are a number of questions about this resist that we address. First, how is the differential dissolution rate created from the chemical-amplification reaction such that the exposed and reacted material withstands the development step with aqueous tetramethylammonium hydroxide(TMAH)? In order to answer this first major question it was necessary to address quantitatively several issues. H o w much acid is created during exposure? H o w much methanol product remains in the film after P E B ? (This is of concern because of its possible effect on the kinetics of the reaction.) H o w many cycles of catalytic reaction does the acid undergo before the desired differential dissolution rate is reached? The second question to be addressed quantitatively is what are the top surface and the sidewall roughness after the resist is developed? Analytical Techniques Used for the Quantitation. The specific details of the analytical techniques used in these studies and of the optimized processing conditions are described in a previous publications (4-5) and w i l l only be reviewed here. The X ray exposures were done on a negative-tone resist, S A L 605 (The Shipley Co.). 2
Films of 0.5 p m thickness were exposed at 125 m J / c m to a blank S i : N membrane. This dose and other resist processing parameters were consistent with statisticallyoptimized conditions for printing 0.215 Jim features. Results of the Quantitative Measurements. The moles of H M M M reacted were measured with gel permeation chromatography(GPC). If the cross-linking reaction coupled two novolac chains, one might expect the molecular weight to at least double. What was observed from a series of chromatograms for various baking times at 125 m J / c m exposure dose to S A L 605 was that the peak corresponding to unreacted H M M M was decreased while a corresponding increase in absorbance was observed throughout the novolac fraction. There did not appear to be the creation of species that were substantially higher in molecular weight. This suggests that a linking reaction had occurred, but cross-linking events were not prominent. Because unreacted H M M M is separated from the novolac resin in the chromatogram, it was possible to quantify the amount of the H M M M reacted by calibrating the peak height of the G P C against a standard of Cymel 300 (American Cyanamid, Wayne, NJ). The moles of H M M M reacted were then measured by subtracting the free H M M M i n the reacted films from the free H M M M in the unreacted films. This allowed the amount of H M M M that was reacted to be quantified, but the question of how many reactions occurred per H M M M molecule remained. This was approached by measuring the reaction product, methanol. Because the methanol could be either evolved from the resist film or retained in the film, the methanol left in the film was first quantified by G P C followed by gas chromatography(GC). This latter experiment revealed that there was no methanol left in the film after a P E B of 108°C for 60 sec - the optimized conditions used for dose to print. The amount of methanol evolved from the film could be measured by inserting a wafer in a sealed bomb, heating the system in an oven for the amount of time needed for P E B , and then analyzing the gas by G C . The amount of methanol produced from a given area of wafer at 0.5 p m thickness and detected in the gas could then be related to the amount of H M M M reacted on the identical wafer, as determined by G P C . This is shown i n Figure 2. The slope of the line was used to calculate that 1.27 ± 0.24 moles of methanol were produced/mole of H M M M reacted. In addition, 2
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Methanol detected (pmoles) Figure 2. Comparison of moles measured for constant volume of resist, m e t h a n o l : H M M M . The gray box represents the normal lithographic conditions for this resist. (Adapted from ref. 5)
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the amount of reaction produced during optimized conditions of 125 m J / c m and a P E B on vacuum hot plate is shown as the gray box i n Figure 2. A t bake times producing about double the reaction necessary for lithography, all of the H M M M is reacted approximately once, but the methanol is continually produced due to the ability for the H M M M to make multiple links per molecule. If the predominant mechanism were a cross-linking reaction, one would have expected at least 2.0 moles of methanol for each mole of H M M M reacted for this system. The fact that considerably less methanol was produced than expected for a cross-linking reaction suggests, along with the G P C , that the differential dissolution rate between the exposed and unexposed resist is created by another process rather than simple crosslinking. Using this method of quantitation, we have not yet assumed O-alkylation vs. C-alkylation. C-alkylation would also produce methanol, and would also result i n the correct amount of measured H M M M molecules reacted in the G P C experiment. However, O-alkylation has been shown to occur quantitatively over C-alkylation (2) and w i l l only be considered in the F T I R calibration. In a previous study, the amount of acid produced from 125 m J / c m exposure of the resist film was determined (5). The value was 5 x 1 0 moles/cm . If the amount of ether produced per acid during a standard hot-plate P E B could be determined, it would be possible to determine how many cycles the acid reacted before it created the required differential solubility. Although the formation of the ether peak could be monitored with F T I R at 988 c m - , the molar absorptivity of the peak was not known. However, the ether IR peak height can be calibrated by measuring the methanol produced by G C and relating the IR absorbance on the same wafer. Figure 3 shows a plot of F T I R peak height at 988 c m vs. methanol concentration produced from the film. One methanol is produced for each ether 2
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3
- 1
formed, and the x-axis is the concentration of ethers times the path length, 0.5 p m . F r o m this graph, we can obtain a molar absorptivity of the ether peak of 3.1 x 10 cm /mole. B y knowing the molar absorptivity, we can process a wafer under standard conditions on a hot plate and measure the actual ether peaks by IR. From the known acid concentration with dose, the number of reactions per acid molecule is found to be 26 ± 8. In addition, because the total number of reactions have been quantified, one can calculate the percentage of phenolic groups protected by the H M M M reactions under standard lithographic conditions. The percentage of phenols reacted is approximately 2%. 5
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Conclusions about the Mechanism from the Quantitative Measurements. Although the above quantitation steps did reveal a substantial amount of information about the reactivity of S A L 605, the remaining question is why the reaction of essentially one molecule of H M M M with one phenolic group on every other oligomer had such a profound effect on the solubility of the film. The previous suggestion of an increase of the molecular weight upon exposure and P E B clearly does not fit the data. One suggestion, advanced by others (
