Microelectronic Engineering 57–58 (2001) 349–353www.elsevier.com/ locate /mee

Sensitivity, contrast and development process in electron and ionlithography

*K. Vutova , G. MladenovLaboratory of Electron Beam Technologies, Institute of Electronics, Bulgarian Academy of Sciences,

72 Tzarigradsko shosse, 1784 Sofia, Bulgaria

Abstract

The dependence of the resist solubility rate S on the average exposure dose D (or on the adsorbed energy per 1 U resistvolume) makes it possible to optimise the used sensitivity (or the dose requirements) to achieve the needed contrast at chosendevelopment conditions for an arbitrary combination of resist–developer (i.e. at a given molecular weight, resist density andradiation efficiency of the charged particles). Two cases of this relation S(D) are distinguished. In the first of them auniversal dependence S(D) is obtained during the development process with one and the same solubility rate. In the secondcase the obtained dependence S(D) is a multi-valued function of a non-linear solubility rate during the development processfor a different developing time. The contrast parameter value g (proportional to the traditionally used contrast parameter g )s d

is determined by the slope of the dependence S(D). One and the same resist–developer couple can show high sensitivity at alow contrast value and a high contrast value at lower sensitivity for different conditions. The contrast is a function of thedeveloping time and the dose in the case of a non-linear S(D) dependence. A mathematical model and results for the resistsurface evolution in electron and ion beam lithography are also presented. 2001 Elsevier Science B.V. All rightsreserved.

Keywords: Electron lithography; Ion lithography; Sensitivity; Contrast; Solubility rate

1. Introduction

Critical microstructure dimensions of 50–150 nm will be mastered in the manufacturing ofmemories and micro-processors during the next few years. Electron and ion lithography are thetechniques which avoid the drawbacks of this challenge. Understanding of the features of the resistmodification and the remaining material is important to optimise electron beam direct write and ionprojection lithography exposure and the connected development processes. Generally, the resistsensitivity to ion exposure is higher than to electron exposure [1,2]. The comparison is not precise dueto the different conditions and the peculiarities of the figures of merit used. The correct selection ofthe exposure and the development conditions in order to ensure the necessary resolution and the resist

*Corresponding author. Tel.: 1359-2-750-757; fax: 1359-2-975-3201.E-mail address: vutova@ie.bas.bg (K. Vutova).

0167-9317/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S0167-9317( 01 )00527-5

350 K. Vutova, G. Mladenov / Microelectronic Engineering 57 –58 (2001) 349 –353

contrast is the general task of the nano-image’s technology designer. In the field of high resolutionelectron beam and ion beam lithography it is very important to predict the developed profiles in theresist.

2. Sensitivity–contrast relation of the polymer resists (and the developers)

The property which undergoes changes upon irradiation largely because of cross-linking or scission,is solubility. Researchers evaluate changes of the resist thickness d during the development process ina chosen solvent (dissolution removal of the irradiating area for the positive resists and the opposite— removal of the non-irradiated area adjacent to the irradiated pixels in the event of the negativeresists) versus the dose D (for the development time). One can evaluate the contrast and the sensitivityof the resist using the dependence d(D).

The sensitivity to radiation (electron and ion beams) is measured as the minimal dose D for the0

development of the exposure image. The contrast parameter normally is defined as a ratio of thenormalised thickness remaining and the dose interval between the initial exposure dose D and the1

21dose D . The value of the contrast parameter is g 5 [lg(D /D )] because the removed normalised0 d 1 0

thickness is 1. Sometimes the contrast is also defined as the slope of the dependence d(D) at 0.5 of theremoved normalised thickness.

The sensitivity and the contrast are described as independent properties. The values of thesecharacteristics are estimated in a lot of papers for the average molecular weight of the used resist(measured for example in a.u.) and for a chosen developer. Data for the sensitivity and for thedeveloper effect on the contrast g of PMMA in the case of electron submicron lithography are givend

in Refs. [3,4]. A similar effect of the used developer on the contrast value g for electron lithographyd

of amil acetate layers is shown in Ref. [5] and in the event of PMPS and Novolak in Ref. [6]. In Ref.[1,7] data for the sensitivity and for the contrast g in the case of some ion beam resists are given.d

Fig. 1. Dependence of the solubility rate S on the dose D in the case of electron exposure for different developers. The resistthickness is 0.5 mm, the development time is: a, 15 s; b, 30 s; c, 60 s; d, 120 s; e, 240 s; f, 480 s; j, 960 s; h, 1920 s.

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It can be understood now that these parameters for a combination of resist–developer are mutuallyconnected. This connection can be seen from the dependence of the solubility rate S or the relativevalue S /S on the exposure dose D (where S is the solubility rate of the resist after the exposure and0

S is this rate without exposure). This suggestion was presented in our previous papers [8,9]. Fig. 10

represents the dependence S(D) in the case of electron exposure of polymethyl methacrylate (PMMA)and the following developers — pure methyl-isobutyl-ketone (MIBK), pure isopropyl alcohol (IPA),MIBK/IPA 1:1 solution and methyl-ethyl-ketone (MEK)/ IPA 1:1 solution. This figure shows that thesolubility rate S of the resist is a function of the adsorbed energy in the resist. Experimentalinvestigations also show that S depends on the radiation efficiency of the charged particles in theresist, the resist density and the molecular dispersion. The relation S(D) can be experimentallyobtained for a given resist thickness and irradiated particle energy. The calculated ranges andabsorbed energy distribution of the irradiating species can easily convert this dependence in the morephysically important dependence S(E /V ), where E /V is the absorbed energy per 1 U resist volume.

In the case of positive resists the solubility rate of the irradiated areas increases while in the case ofnegative resists the solubility rate decreases. The radiation defect density or the number of themodified resist molecules is very low at lower exposure doses and one can develop the irradiatedimage at low contrast (and for a very long developing time). The contrast parameter g can be defineds

by the slope of the curve S(D) or of S /S (D). Taking into account that S5Dd /Dt, where Dd is the0

removed thickness, Dt is the developing time and d is the initial resist thickness, it can be found that0

g 5 g /(Dtd ), i.e. g is proportional to g . In the cases of higher doses (that means lower sensitivitys d 0 s d

of the resist) one can obtain a desirable irradiating image for a shorter time, which is practical, andusually at higher contrast (the relations S(D); Fig. 1). This means that one can trade sensitivity forcontrast.

The use of a solution solvent /non-solvent as a developer of electron irradiated polymer (MIBK andIPA in the case of PMMA) improves the contrast together with a loss of the sensitivity. In this case anon-linear behavior of the solution process was observed [8]. In Fig. 1 one can see two cases ofpolymeric layer solution: (i) linear resist solution with a constant solubility rate during developmentprocess and (ii) non-linear resist solution with variable solubility rate. An initial dose and a solubilityrate at every chosen development time take place in the second case. This phenomenon is connectedwith the selectivity of the solution at different molecular weights of the fractions and of the diffusionprocesses of the developer in the polymer resists.

In the case of ion exposure process a similar linear solution process was observed pertaining to ionmass [1,7,8] due to the different radiation ion efficiency to the energy transfer and subsequently, todifferent latent images. In Fig. 2 one can see the non-linear (multi-valued) solubility rate obtained in

1 1the case of He (Fig. 2a) and Ar (Fig. 2b) in PMMA. These results in the case of ion lithography areshown or the first time in this paper. A dependence of the contrast on the developing time (and theresist thickness) can be seen. The difference between the electron resist non-linear solution processand ion resist non-linear solution process is connected with the difference of the energy deposition inthe polymer in both cases. There is a high probability that the electrons will be deflected many timesfrom the initial path (up to back scattering) and that the created secondary electrons will have highenergies in the electron exposure case. As a result the resist layer up to a distance of many micronsfrom the irradiating point is changed [10]. The electronic stopping power is responsible for the resistmodification in the case of ion exposure at low doses [11]. This leads to distribution of the modifiedresist pixels (at an order of 100 nm) near and around the ion track. Ion irradiation defects, caused bynuclear energy losses, can be a reason for lower solubility rate at high doses.

352 K. Vutova, G. Mladenov / Microelectronic Engineering 57 –58 (2001) 349 –353

1Fig. 2. Solubility rate of PMMA (M5675 000) using MIBK:IPA (1:1) at 208C after exposure with 80 keV He : a, 30 s; b,160 s; c, 90 s; d, 120 s; e, 150 s; and 120 keV Ar : a, 30 s; b, 60 s; c, 90 s; d, 120 s; e, 150 s; f, 300 s.

3. Development process modelling

In our model of a development process we assume that the resist is sufficiently homogeneous andthe development process is isotropic. The main features of the model are: (i) motion of the evolvingpoints take place along the normals to the corresponding profiles; (ii) the space modification of theabsorbed energy distribution is also taken into account; (iii) in our model, we use a cubic spline in thetwo-dimensional case and a bicubic spline in the three-dimensional case to describe the developedprofiles; (iv) an original procedure is applied for the profile discretisation to increase the contouraccuracy. A general test pattern is shown in Fig. 3. The 3D profile obtained by computer simulation

27using this test pattern and shown in Fig. 4 represents the developed gap surface at a dose of 231022 1C cm . The beam is 60 keV H and the development time is 0.25 min.

4. Conclusions

Some details concerning the characterisation of micro-images patterning by electron and ionlithography processes are discussed in this paper. It is shown that the sensitivity and the contrast are

Fig. 3. Two-dimensional pattern for illustration purposes. All small gaps are 0.1 mm.

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Fig. 4. Computer simulation results for a developed surface using 1:3 solution of MIBK and IPA, applying the ideal patternshown in Fig. 3.

not independent characteristics. The sensitivity and the contrast in the cases of electron and ionlithographies can be discussed together for an arbitrary couple of resist–developer.

To compare the sensitivity of different combinations of resist–developer when using electronsand/or ions as modification radiation it is necessary to compare the dependencies of the solubility rateversus the energy per 1 U resist volume S(E /V ). The dependencies S(D) are also appropriate only inthe case of the same type of irradiating particles with the same energies. A comparison between thesensitivity (and the contrast) values for different resists at different conditions is valuable only forabove mentioned conditions and general conclusions will be speculative.

It is shown that one can observe non-linear solubility (multi-valued function) instead of linearsolubility (universal S(D) and S(E /V )) curves in every case when using less powerful developers andthere is a difference between electron and ion exposure cases. A short description of the model forcomputer simulation of the development profile kinetics in two- and three dimensions is given also.

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