Negative-tone high-resolution photocatalytic resist for x-ray lithography

Microelectronic Engineering 9 (1989) 575-578 North-Holland

575

Neaative_tone R. Dammel, Hoechst

K.-F.

AC,

H. Huber,

Wssel,

Corporate

H. Oertel,

Fraunhofer-Institute

J. Lingnau,

Research,

J. Theis,

P.O.Box

800320.

D-6230

Frankfurt/M

80. Fed. Rep.

of Germany,

and J. Trube, for Microstructure

Technology,

Dillenburger

StraBe

53. D-1000

Berlin

33. Fed. Rep. of Germany.

A negalive-tone photocatalytic resist malerial /or X-ray lithography wirh high resolution and good process stabiliry is presented. It is based on acid-catalysed crosslinking of a novolak matrix wirh a melamine type compound. Design considerations are discussed for this three componenr sysrem. and process optimizarion by means of orthogonal experiment design is described.

High-resolution lithography, whether at optical or at Xray wavelengths, has traditionally been the domain of positive-tone photoresists. While negative-tone resists, especially those of the polymer-crosslinking type, offer inherently higher processing stability, improved thermal reflow temperatures, and -relative to the non-crosslinked polymershigher etch resistance, the resolution obtainable with them is severely limited: the classical negative-tone photoresists, which are solvent-developed, are prone to swelling of images, snaking of lines, and quite often insufficient solvent exhibit contrast. Moreover, development is more and more seen as a drawback in a world of increasing environmental consciousness, and the concomitant capital investment for solvent reclamation can be substantial. Positive-tone photoresist design has in the last decade responded to the need for high-temperature stable lithographic patterns by providing the image reversal technique [I], in which a positive latent image of, e.g., a line is transformed into a cross-linked latent line pattern by the two steps of baking and flood-exposure. During development, the flood-exposed positive photoresist is dissolved in the developer, revealing the insoluble , crosslinked line pattern. These images show the high resolution characteristic of positive photoresists with none of the disadvantages of negative-tone resists, such as swelling and snaking. Why is it that these defects can be avoided in the image reversal scheme? The answer appears to be that development is effected not by an organic solvent, but by aqueous base, in which the (novolak) matrix resin of the image-reversal resist is soluble. If the crosslinked image is sufficiently hydrophobic, no image-swelling, and no loss occurs. In designing a negative-tone of resolution, photoresist for X-ray lithography, where the highest resolution is paramount, it therefore is vital to choose a type of resist chemistry which is compatible with development in aqueous base. In the resist material induced differentiation phenolic matrix resin

0167.9317/89/$3.50

described in this paper, radiationis effected by crosslinking of a with an acid-activated crosslinking

0 1989, Elsevier Science Publishers B.V. (North-Holland)

agent. Since the acid acts as a catalyst for the crosslinking, the resist makes use of the concept of “chemical amplification” [2]. This type of chemistry, sometimes also called “acid-hardening chemistry”, is not new [3]; the design task was therefore “merely” to adapt it to the resolution and sensitivity demands of X-ray lithography. As in the positive-tone X-ray resist described earlier [4], and now known under the experimental product name of RAY-PF, each of the three main functions of a resist has been assigned to a separate component which may be individually optimized for best performance, resulting in a three component system (3CS). It is instructive to compare the demands placed on component performance for positive and negative 3CS-systems. In a positive-tone system of the dissolution inhibition type, such as RAY-PF, the dissolution rate of a matrix resin is lowered by the addition of a dissolution inhibitor, which is transformed into dissolution enhancing products as a result of irradiation (Fig.la). In RAY-PF, this step is effected in a chemically amplified reaction, in which a strong Brgnsted acid, released from an acid generator upon irradiation, catalyzes the hydrolysis reaction of an acid-labile dissolution inhibitor, such as an acetal or an orthoester. The same dissolution rate enhancement of the exposed resist by the photoproducts is observed in diazonaphthoquinone type resists, resulting in a high differential dissolution rate. One may conjecture that the success of these resists is, at least in part, due to this phenomenon. If differential solubility is to be maximized in a negativetone resist, in which the dissolution rate of the exposed resist will have to be virtually zero for lithographic reasons, it is the solubility of the unexposed resist coating which will have to be enhanced relative to the matrix resin (Fig. lb). Again, the three main resist functions are assigned to one specialized component each: for the matrix resin, a novolak type material is chosen for its high dry etch resistance and its developability in aqueous base with absence of swelling. The acid generator is optimized for yield of acid upon irradiation, and the crosslinking agent is chosen for high reactivity. Moreover, the latter two

R. Dammel et al. /Negative-tone

576

3-Component

(b)

Fig.1:

Negative

resist

principle, it was the melamine type crosslinkers which were found to exhibit the requisite spectrum of properties. With these materials, crosslinking occurs via electrophilic attack of a cationic intermediate on the ortho or para position of an aromatic ring, here a member of a novolak chain. Ether formation, i.e. attack on the oxygen atom of a phenolic group, is apparently only a minor side reaction in this case (see Fig. 2). In all photocatalytic systems, differentiation takes place in a dark reaction which may continue beyond the end of irradiation. While for RAY-PF, the dark reaction occurs at room temperature, the crosslinking of novolak chains by hexamethylolmelamine ethers (see Fig. 2) requires elevated temperatures, i.e. a bake step. That is, in RAYPF the activation energy of the hydrolysis reaction is lowered below the energies available at room temperature by the acid catalyst; for the negative-tone 3CS resist, the difference in activation energies is such that the catalyzed reaction proceeds at observable speeds above 90°C, compared to the uncatalyzed reaction which only occurs above 13O’C. It very quickly became evident that negative-tone resist materials designed along these lines could equal positive-

Resist

Performance Principles of Three Component Resists (a) positive tone dissolution inhibitor (b) negative tone crosslinking type

photocatalytic

type

must follow the additional constraint of enhancing matrix resin dissolution: thus the crosslinker must be chosen for lack of matrix resin inhibition, possibly even enhancement of dissolution, and the acid generator is designed to be highly soluble in aqueous base developer. While virtually all crosslinking agents commonly used in acid-hardening chemistry, such as epoxides, resol type or formaldehyde liberating materials, were found to work in

tone resists in resolution and lithographic performance. Fig. 3 shows a comparison of a test pattern consisting of nominally 0.4 cm lines , imaged in RAY-PF and in an experimental negative-tone resist. SEM inspection of the mask shows that the pattern is reproduced faithfully in both cases. Once a promising resist formulation has been found, there remains the tedious job of optimizing the process conditions. In a linear grid search the number of required experiments grows exponentially with the number of parameters, so that daunting numbers are reached very quickly. Rather than evaluate literally thousands of

OH‘

Fig.2

Photochemically induced acid-catalyzed crosslinking of novolak binder matrix derivatives leads to reduced solubility of the resist in aqueous-alkaline

by melamine developers.

571

R. Dammel et al. /Negative-tone photocatalytic resist

3

2

1

input

5

Fig.3: Fig.4: Fig.5

m

4

level

4

I

Nominally 0.4 urn mask structures in positive tone RAY-PF (1.) and experimental negative tone resist (r.). Lithographic miminum dose (function average) vs. input level for the parameters of Tab. 1. Contrast function average vs. input level for the parameters of Tab. 1.

contrast curves in a full factorial experiment, we decided to resort to orthogonal process optimization. “Generally speaking, the purpose of orthogonal design is to study the relationship between process or product parameters (input parameters) and their corresponding output functions by selecting certain representative combinations of the input parameter level settings. These level settings fit into certain orthogonal tables. By following the orthogonal tables, the maximum amount of information can be gained using the minimum number of at 4 different experiments.“[5] In the case of 5 arameters ! may be used, requiring levels, the orthogonal table Ll64 only 16 experimental runs, instead of 45=1024. From the results of the 16 experiments, output function averages mirror the influence of the are computed which parameters studied. It is important to realize that e.g. the contrast output function must not be confused with actual contrast values. The parameters chosen for systematic evaluation (cf. Figs. 4 and 5) were -prebake temperature at a fixed prebake duration of two minutes on a hotplate -post exposure bake (PEB) duration (hotplate) -post exposure bake (PEB) temperature (hotplate) -developer strength (normality) -development time. Fig. 4 shows the effect of these parameters on the sensitivity output function average, Fig. 5 that on the contrast (y-value) function average. Sensitivity, somewhat arbitrarily represented here as the minimum dose Dl for 90% film retention, depends very little on prebake temperature once this parameter has exceeded 100°C. As expected, increasing developer concentration and development times have the effect of reducing sensitivity (increasing Dl) with the influence of developer strength being by far the stronger one. Also not unexpectedly, increasing PEB time and temperature

2 input

1

. prebok?

Tab.1:

Parameters

T

+ PEB t

for Input

3 level

parameter .PEB

Levels

T

AdW

4 cont.

xde”.

time

1 to 4

1 2 Input Level ________--_-----------~~~~~~~~~_~-----------------

3

110 90 100 Prebake (‘C) 90 110 100 PEB-Temp.(‘C) 60 300 PEB time (set) 180 0.4 0.2 0.3 Dev. cont. (N) 90 30 60 Dev. time (set) __________________________________________________

4 120 120 420 0.5 120

increases sensitivity, however with the PEB temperature dependance showing a much steeper slope. The results for the contrast output function (y-values) show a considerably more diverse behaviour than those for the Dl output function. Increasing prebake temperature has the effect of slightly lowering contrast, which appears reasonable since the differential rate of solution must be lowered by decreasing unexposed resist solubility. However, for the short (2 min) hotplate bake used, prebake temperatures of 1 IO-120°C were necessary to avoid adhesion problems. As expected, increasing PEB time has the effect of increasing contrast, since the higher speed of acidcatalyzed crosslinking will lead to a greater solubility difference when integrated over longer times. It is interesting to note that the approximately S-shaped curve is exactly what theory would predict for a saturation phenomenon. While the PEB time dependence is a monotonous function, the PEB temperature function exhibits a steep rise, and then goes through a maximum near 110 ‘C. This behaviour is also readily explained by kinetic theory since for finite reaction times, reaction will be too slow at low PEB temperatures to greatly affect solubility, whereas for high PEB temperatures the difference between catalysed and uncatalysed rates of

reaction will

decrease. It was, however, unexpected to find a Plateau at 0.3 N in the contrast vs. developer strength curve. Also, while contrast may be expected to depend strongly upon development time, the apparent existenCe Of a minimum in that curve is not explained by the Current dissoiution rate model. If these results can be substantiated, they may well require a rethinking of the dissolution mechanism for this type of resist. It is clear from the above that there are a great many ways of choosing processing conditions for this type of resist, depending on the relative value placed on, e.g., resolution and sensitivity. For a first level of optimization the following set of conditions was chosen: prebake 120 ‘C, 2 min., PEB 10s ‘C, 5 min, development in 0.3 N RAZ-Developer for 80 sec. Under these conditions, the unexposed areas of the resist were dissolved at a rate of about 0.04-0.05 rm/sec. Inspection of the contrast curves (Fig. 6) shows that even higher resolution should be obtainable with longer development times: for a 3 minute development, the y-value exceeds 8, while DI is only increased by about 20%. The extremely high contrast observed under these conditions may allow the resolution of this resist to surpass PMMA when processing is optimized for high resolution, Figs. 7 to 9 show SEMs of structures obtained at BESSY, Berlin, with an X-ray dose of 120 mJ/cm2. Imaging of mask patterns was found to be very precise. Indeed, it was not possibie to determine the resolution limit of the resist since it is obviously better than the finest mask absorber structures. Resolution is estimated to be better than 0.3 pm, presumeably better than 0.2 sm. The images show the steep side walls characteristic for X-ray exposure. The proposed negative tone resist thus provides an ideal extension of positive tone X-ray resist RAY-PF for all tone is negative where the applications those advantageous, e.g. mask replication. Moreover the almost unchanged process sequence - including as an additional step only a post exposure bake - makes it an alternative also for standard X-ray lithographic applications. Independent of the resist tone, “chemical aiTiPlifiCatiOn" meets the requirements on resist performance for the introduction of X-ray lithography as an IC manufacturing toot high sensitivity at excellent resolution.

Fig.7: 0.3 and 0.4 km line-and-space pattern printed in neg.tone resist

Contrast curves for different development times in 0.3N RAZ-Dev. (Preb.: 120°C, 2 min. PEB 105’C, 5 min)

The authors thank I. Kuna, K. Simon, and F. Gabeii for their experimental support and C.R.Lindley for checking the manuscript. References [I] V.Marriott, CM. Garza, M. Stak, SPIE m, 221 (1987), and references cited therein. (23 Cf. e.g.L.F. Thompson, C.G. Willson, M.J. Bowden, “Introduction to Microlithography”, Am. Chem. Sot. Symp. Series 219, ACS Washington, DC., 1984; J.M.J. Frechet, F.M. Houlihan, F. Bouchard, E. Eichler, A. Huh, R. Allen, S.A. MacDonald, H. Ito and C.G.Willson, in: Proceedings of Photopolymers: Principles, Processes and Materials, I (1985), and references cited therein. 131Cf. e.g. H. Rosenkranz et al., US-A-3692560 (1970); G. Buhr, DE-A-2718259 (1977); J. V. Crivello, Pot. Eng. Sci. 2, 953 (1983); A.Bruns et al.,Microelectr. Eng. 6,467 (1987); H. Liu, M.P. deGrandpre, W.E. Feely, J. Vat. Sci. Technol. B 6, 379 (1988), and references cited therein; see also [4]. [4] K.-F. Dilssel, H.L. Huber, H. Oertel, Microelectr. Eng. 3, 97 (1986); R. Dammel, K.-F. Dossel, J. Lingnau, J. Theis, H.L. Huber, H. Oertel, Microelectr. Eng. h, 503 (1987). [5] G.Z. Yin, D.W. Willie, Solid State Techn., 127 (1987).

Fig.8: Close-up of nominally 0.3 km pattern of Fig. 7.

Fig.9: 0.3 pm spaces printed in neg. tone resist.