Silylation of novolac based resists: Influence of deepultraviolet induced crosslinkingMaaike Op de Beeck and Luc Van den hove Citation: Journal of Vacuum Science & Technology B 10, 701 (1992); doi: 10.1116/1.586435 View online: http://dx.doi.org/10.1116/1.586435 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/10/2?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Negativetone deepultraviolet resists containing benzylic crosslinkers: Experimental and simulation studies of thecrosslinking process J. Vac. Sci. Technol. B 12, 3851 (1994); 10.1116/1.587453 New siliconrich silylating reagents for drydeveloped positivetone deepultraviolet lithography J. Vac. Sci. Technol. B 11, 2789 (1993); 10.1116/1.586603 Silylated acid hardened resist process: A deep ultraviolet surface imaging technique J. Vac. Sci. Technol. B 8, 1497 (1990); 10.1116/1.585104 The silylation processes for positive and negative deep ultraviolet resists J. Vac. Sci. Technol. B 8, 1488 (1990); 10.1116/1.585102 Silylation processes based on ultraviolet laserinduced crosslinking J. Vac. Sci. Technol. B 8, 1476 (1990); 10.1116/1.585100

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Silylation of novolac based resists: Influence of deep .. ultravlolet induced crosslinking

Maaike Op de Beecka) and luc Van den hove [MEC vzw, KapeldreeJ 75, B-3001 Leuven, Belgium

(Received 3 June 1991; accepted 7 January 1992)

Most surface imaging resist schemes are based on the selective diffusion of a Si-containing compound into the resist. In the diffusion enhanced silylated resist process hexamethyldisilazane is being incorporated in the exposed regions during the silylation treatment. Upon deep ultraviolet irradiation (at 248 nm), novolac based resists will however crosslink to some extent. The crosslinking reaction influences the degree of silicon incorporation in the exposed resist and hence plays an important role in the imaging properties. The crosslinking reaction has been studied in detail in this work for two types of Plasmask deep-ultraviolet resists, along with its consequences on resist imaging. A decrease in silicon incorporation due to crosslinking puts a serious limitation on the useful process window for Plasmask 150u, but the process window is not limited by crosslinking for Plasmask 301 u. A thorough investigation of the influence of crosslinking on the pattern deformation is also carried out.

I. INTRODUCTION

Subhalf micron lithography is increasingly more impor­tant, as feature sizes below 0.5 J-Lm need to be delineated for the production of 64 M dynamic random access memories (DRAM). I-line steppers with high numerical apertures (NA) have been developed, which offer subhalf micron resolution capabilities. However, because of the high NA, the focus latitude for the smallest feature sizes is insuffi­cient for production applications. Since the reduction of exposure wavelength combines good resolution properties with improved focus latitude, deep-ultraviolet (DUV) li­thography is an excellent technology to fulfill the needs for subhalf micron lithography. The hardware connected with this technology is rapidly developing, with most major stepper manufacturers offering suitable exposure equip­ment. KrF excimer lasers (248 nm wavelength) are the most universally used illumination source, although several manufacturers are examining the use of more traditional light sources.

Of more direct concern, however, is the need to identify a practical resist, capable of realizing the improvements available from this technology. Conventional diazo­naphthoquinone/novolac based resists suffer from exces­sive absorption at DUV wavelengths and hence give rise to very poor profiles. An alternative approach to overcome this problem is the principle of surface imaging combined with dry development. In the so called diffusion enhanced silylated resist (DESIRE) process, I silicon diffusion is en­hanced in the exposed areas of the Plasmask resist during silylation in hexamethyldisilazane (HMDS). In the unex­posed areas, the combination of unreacted sensitizer and thermally activated crosslinking [formed during the presi­lylation bake (PSB)] effectively blocks the incorporation of silicon in these areas. This results in the selective incorpo­ration of silicon in exposed areas. During subsequent dry development in an oxygen plasma, a thin SiD2 layer forms

an in situ mask over the exposed areas, while the unex­posed areas are developed away. A negative tone resist image with straight walled profiles results. Since surface imaging is applied for this process, resist features are to­tally unaffected by the presence of reflective substrates or topography, and strong light absorption by the resist does not cause any disadvantage. In a previous publication,2 the lithographic performance of Plasmask for DUV applica­tions was evaluated. The mechanism of the DESIRE pro­cess under DUV exposure is similar to that of g or i line. However, it is widely known that novolac based resists crosslink under DUV exposure,3-5 An application of this reaction is the so called 'DUV curing' of photoresists, used to harden them prior to etching or ion implantation. In the DUV-DESIRE process, this crosslinking reaction influ­ences the Si-incorporation during silylation of DUV ex­posed areas. Hence, it will affect the process latitudes in comparison to g- or i-line exposures.

II. REACTION MECHANISMS

The reaction mechanisms involved in the DESIRE pro­cess for g- and i-line exposures are somewhat different from those for DUV applications. Scheme 1 shows the reactions during the various processing steps for non-DUV expo­sures. Upon exposure the diazonaphthoquinone decom­poses to form indene carboxylic acid. During the subse­quent PSB, decarboxylation of this acid occurs, since it is not stable at elevated temperatures. Meanwhile, the unre­acted diazonaphthoquinone of the unexposed areas reacts with the novolac to form an ester, resulting in thermally crosslinked photoactive compound (PAC) and resin. 6

During silylation, the siIylating agent diffuses into the ex­posed areas and reacts with the hydroxyl groups of the novolac resin, In unexposed areas on the other hand, no silicon will be incorporated. Indeed, thermal crosslinking of PAC and resin will decrease the Si diffusion, resulting in

701 J. Vac. ScL Techno!. B 10(2), Mar/Apr 1992 0734-211 X/92/02070 1-14$0 1.00 @1992 American Vacuum Society 101

.•.... " ... -.-.-.-." ..... , '. .' ........... ' ............. -.'.-..... " ..... : ......•... , .. ,'. . .. '\.~.~.' ........... -..... ", Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.88.90.140 On: Sun, 21 Dec 2014 19:28:10

702 M. Op de Beeck and L. Van den hove: Silylation of novolac based resists

o

+ O~;(' R'

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SCHEME 1. Reaction mechanisms during the DESIRE process, g- or i-line exposure.

J. Vac. ScI. Techno •• B, Vol. 10, No.2, Mar/Apr 1992

formalion ~ indene carboxylic acid

! Pre-silylation Bake (PSB)

(XJ .'

decarboxylatiolt of iP1dene carboxylic acid

I Silylation inHMDS

(XJ .' Silicon incorporalion

702

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703 M. Op de Beeck and L. Van den have: Silylation of novolac based resists 703

an efficient blocking of silicon incorporation in unexposed resist. After silylation, an oxygen plasma is applied for the dry development of the resist.

It should be noted here that the thermal crosslinking in unexposed areas will only occur for PSB at elevated tem­peratures (over 120°C). For lower PSB temperatures, no physical crosslinking but hydrogen bridge formation be­tween PAC and resin is reported to be important in de­creasing the silicon diffusion in unexposed resist.7 It has been shown, however, that this mechanism is less effective.8 For g and i line exposed Plasmask which is sily­lated using HMDS, PSB and silylation is carried out typ­ically between 150 and 180°C.

For DUV applications, the involved reaction mecha­nisms in the unexposed areas are identical to those for g- or i-line lithography. In the exposed areas however, the reac­tion mechanisms differ if DUV exposures are applied (see scheme 2). The resin reaction under DUV irradiation is described as free radical polymerization,5 which results in polymer-polymer crosslinking. Some researchers suggest some photoinduced PAC-resin esterification,9 but others have shown that the crosslinking reaction also occurs in pure novolac resin without PAC,IO or the novolac resin with PAC completely converted to carboxylic acid,11 both indicating that the DUV -induced crosslinking is a reaction between resin polymer chains and not between PAC and resin. The diazonaphthoquinone will form indene carbox­ylic acid upon irradiation, and the PSB will result in a decarboxylation of this acid. During silylation in HMDS, the silylating agent will diffuse into the exposed resist and react with the hydroxyl groups, while Si diffusion in unex­posed areas is blocked efficiently by the thermal crosslink­ing between PAC and resin. Since the amount of OH­groups is not changed during the novolac-novolac crosslinking under DUV irradiation, the silicon concentra­tion can be identical in the case of g- or i-line exposure and DUV exposure. However, the crosslinks formed under DUV irradiation will cause important sterk hindrance for the diffusion of the silylating agent. This sterk hindrance will limit the diffusion, therefore higher silylation temper­atures are necessary, typically between 170 and 200°C. It is obvious that higher DUV exposure doses will cause in­creased resin-resin crosslinking, which will result in de­creased Si diffusion due to more severe steric hindrance. However, since diffusion increases with temperature, the influence of the steric hindrance on silicon diffusion will decrease if higher silylation temperatures are applied.

III, EXPERIMENTAL CONDITIONS

Plasmask I50u and 301u are spincoated to form a layer of approximately 1 11m thickness after development.

Both resists are studied using a modified contact print­ing system from Karl Suss, which was coupled to a Questek KrF excimer laser. Additionally, we have utilized the ASM-L PAS 5000/70 DUV stepper, fitted with a 0.42 NA Zeiss 5 X reduction lens, operating at 248 urn. It should be noted that the majority of exposures are carried out on the former system.

J. Vac. Sci. Technol. e, Vol. 10, No.2, MarlApr 1992

PSB and silylation of Plasmask is carried out with HMDS on a modified MTI vapor prime track The same temperature is used for PSB and silylation.

Dry development is performed in a MRC MIE 720 etcher and two types of dry development processes were applied. The first type is a development using a pure oxy­gen plasma (so called single stage O2 development) during which the nonexposed resist is selectively etched away. The second type of development consists of two stages: During the first step an 02-C2F 6 mixture is used as the etch plasma, which removes a thin layer of resist in all areas, whether Si is incorporated or not. During the second step the nonexposed parts are selectively etched away by a pure oxygen plasma. This second type of development is re­ferred to as two step C2F 6 dry development. Although it is slightly more complicated, two step development offers im­portant advantages concerning process latitudes, contrast, and residues.

IV. CROSSLINKING OF PLASMASK 150u

Plasmask I50u is the first DUV version of Plasmask resist, based on the same resin as used for its g- and i- line equivalents: Plasmask 150g and 150i, respectively.

CrossHnking of Plasmask I50u is studied by investiga­tion of the silicon content of exposed resist after silylation. Infrared (IR) spectroscopy measurements are carried out on exposed wafers for different silylation temperatures. The integrated absorbance (with linear background sub­traction) for the Si-C stretching-vibration peak (870-835 em - I) is calculated, which is a measure for the Si content in the resist filmo The result is shown in Fig. 1: The Si content is plotted versus the exposure dose for different silylation temperatures. It is obvious that the silicon con­tent increases with increasing silylation temperatureso This graph clearly highlights the reduced Si incorporation un­der high DUV exposure doses. The dose from which crosslinking becomes so important that the amount of sil­icon drops, is referred to as the crosslinking threshold. This crosslinking threshold moves towards higher exposure doses for higher silylation temperatures.

The silicon incorporation is also studied by Rutherford backscattering spectroscopy (RBS) measurements. Figure 2 compares the silicon content for Plasmask 150u under different exposure conditions: (a) 50, (b) 200, and (c) 1400 mJ/cm2

• All samples are silylated at 185°C. Case (b) corresponds with a process condition resulting in equal lines/spaces for 0.5 pm features. Case (a) represents an underexposed and case (c) a strongly overexposed condi­tiono These graphs confirm the influence of crosslinking: the depth of the silylated area and the atomic weight-per­cent of silicon are reduced under high DUV exposure doses.

In the RBS graphs of Fig. 3, the silicon content for Plasmask 150i under i-line exposure and for Plasmask ISDu under DUV exposure are compared. For both cases, the silylation temperature was 185 ·C, and the exposure doses are optimized to obtain equal lines/spaces for 0.5 pm features. From the RBS graphs, it is dear that the atomic weight percent of silicon in the silylated areas is the same

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704 Nt Op de Beeck and L. Van den hove: Silylation of novolac based resists

no exposure

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SCHEME 2. Reaction mechanisms during the DESIRE process, deep UV exposure,

J. Vac. Sci. Technol. e, Vol. 10, No.2, Mar/Apr 1992

o I

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It·

1 Pre-silylation Bake (PSB)

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705 M. Op de Beeck and L. Van den hove: Silylation of novoiac based resists 70S

210°C

500 1000 1500 2000 2500 3000

Exposure dose (mJ/cm2)

FIG. 1. Silicon content of Plasmask 150u vs exposure dose for different silylation temperatures OR measurements).

for i-line DUV exposures, and it was found that this atomic weight percent corresponds with the reaction of all hy­droxyl groups with silicon.7 From this it can be concluded that the amount of hydroxyl groups is not significantly changed by the polymer-polymer crosslinking. The depth of the silylated area is much smaller in the case of DUV exposure, so the total amount of incorporated silicon is less compared to i~Iine exposures. This decrease in silylation depth is probably caused by the fact that the DUV exposed area is more shallow than the i-line exposed area, due to the high absorbance ofDUV light by the resin. It should be noted here that it is not required to have a very deep sHy­hated layer for dry development, the silicon concentration is at least as important.

Energy (MeV) 1.2

1.0 1.1 1.2 1.3 1.4 I

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FIG. 2. RBS measurements showing the silicon concentration for different exposure conditions for Plasmask 150u (silylation at 185 ·C).

J. Vae. Sct Techno!. e, Vol. 10, No.2, Mar/Apr 1992

Energy (MeV) 1.2 ...--_---'1T'.O'---_-..c;1...:.,1 __ ....:1,.,.2'--_-..c;1..o:,3 __ ....:1-r.4'--_-,

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FIG. 3. RBS measurements showing the silicon concentration in Plasmask 150u for i-line and DUV exposures under standard process conditions.

V. CONSEQUENCES Of CROSSLINKING Of PLASIVIASK 150u ON RESIST PATTERNING

To investigate the influence of crosslin king on resist pat­terning, the resist profiles of 3 pm LIS are studied for different exposure doses. An wafers are silylated at 180°C and are developed using the so caned two step C2F 6 dry development. IR spectroscopy measurements are carried out on flood exposed parts of these wafers, they are per­formed before and after the first step of the dry develop­ment process. The results are plotted in Fig. 4: clearly the silicon content drops due to the first nonselective dry de­velopment step, which removes silylated resist as well as nonsilylated resist. After development, scanning electron microscopy (SEM) photographs of 3 /lorn LIS are taken as shown in Fig. 5. The correlation between the silicon con­tent of the resist and the resulting resist profile is obvious. If the silicon content is very low, no imaging takes place.

2..0

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FIG. 4. IR measurements showing the silicon content of Plasm ask 150u vs exposure dose for different dry development conditions (silylation at 180·C).

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106 M. Op de Beeck and L. Van den hove: Silylation of novolac based resists

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FIG. 5. Influence of silicon content on resist imaging for different exposure doses. Process conditions: silylation at 180°C and two steps dry development. The SEM photographs show 3 /Lm LIS features.

J. Vac. Sci. Techno!. S, Vol. 10, No.2, Mar/Apr 1992

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707 M. Op de Beeck and L Van den hove: Silylation of novolac based resists

zons3

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J. Vac. Sci. Technol. B, Vol. 10, No.2, Mar/Apr 1992

707

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708 M. Op de Beeck and L Van den hove: Silylation of novo lac based resists 708

For silicon contents higher than a certain threshold value, there is proper imaging since the silylated area is thick enough to withstand the oxygen plasma during the dry development step. For intermediate levels of Si, there is some imaging but since the silylated area is etched away before the dry development is finished, the resist surface will be damaged. As a result, two silicon content thresh­olds can be defined, which determine four exposure dose thresholds (see Fig. 4).

In Fig. 5, the correlation between the silicon content of the resist and the resist profile after development is ex­plained by plotting the image intensity for each exposure dose and by comparing it with the different exposure thresholds.

For very small exposure doses (zone 1), the silicon con­tent is too low for imaging. For slightly higher doses (zone 2), the silicon content is not high enough yet, so the sur­face of the resist is seriously damaged. (The line structure, which can be seen on the corresponding SEM photograph, is most likely due to ringing effects since the exposures are carried out on a contact printer.) For exposure doses be­tween T2 and T3 (zone 3), the amount of Si incorporation is sufficient to provide proper imaging. Exposures between T3 and T4 result again in damage of the resist surface, although the borders of the features remain undamaged. This can be explained by considering the aerial image: the resist edges correspond with the part of the intensity profile between thresholds T2 and T3, so providing proper imag­ing. Above T3 some crosslinking occurs. Hence, less Si is incorporated resulting in damaged areas after develop­ment. Finally, exposure doses higher than T4 (zone 5) result in imaging of the resist edges only.

It is obvious that one should choose the exposure con­ditions in such a way that the image intensity profile stays below threshold T3 to be sure of proper imaging. But un­der practical circumstances, this is not always that easy. Figure 6 shows the image intensity profile for printing 0.5 JIm LIS next to a wide resist line (10 JIm in width) and a SEM photograph of this situation: the 0.5 !J.m lines are printed properly, but large resist features suffer from resist damage. If the exposure dose was chosen lower, the large resist feature would not be damaged anymore, but the 0.5 !J.m LIS would be underexposed.

It should be mentioned here that exposure thresholds are dependent on several process parameters, such as the silylation temperature and the dry development process which is applied. Because of selectivity differences of the several dry development processes, the two silicon content thresholds will differ for each development process, mak­ing zone 3 for proper imaging (between T2 and T3) the widest for the single stage O2 development. Since the sily­lation temperature has a strong influence on the silicon content, zone 3 corresponding with proper imaging will widen with increasing silylation temperature. As can be seen in Fig. 1, the influence of crosslinking is very small if a silylation temperature of 210 °c is chosen.

J. Vac. Sci. Techno!. B, Vol. 10, No.2, Mar/Apr 1992

~ -------------------->-13 ~ CD c: W T2

TI

DOC

FIG. 6. Difficulties whelllooking for the optimum process conditions: 0.5 p,m LIS close to big resist features.

VI. SILYLATION TEMPERATURE WINDOW FOR PLASMASK 150u

It is obvious that processing conditions should be cho­sen for which zone 3 is sufficiently large. A two step de­velopment process has important advantages over the sin­gle stage development process, but it decreases zone 3. So all the problems should be solved by choosing very high silylation temperatures; unfortunately this is not possible! If silylation is performed at very high temperatures, the unexposed parts of the resist will contain silicon too, since the thermal crosslin king of the resist is not severe enough to avoid silicon incorporation at such diffusion conditions. This phenomenon is illustrated in Fig. 7, the so called 'endpoint curves' of Plasmask 150u. These 'endpoint curves' are used to determine the minimum and maximum silylation temperatures based on silicon diffusion selectiv­ity. During dry development, the CO-emission signal is used to determine the endpoint of this process step, since the intensity of this signal corresponds to the etch rate of the resist. If silicon is present in the resist layer, the inten-

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109 M. Op de Beeck and L. Van den hove: Silylation of novolac based resists 709

~I ,

exposed

unexposed

o~' --,--~/--~~~~~ lilQ 120 140 160 180 200 220

siiylation temperature (0C)

FIG. 7, Determination of ihe silylation temperature window by the end­point curves for Plasmask 150u.

sity of the CO-emission signal wiII drop due to the de­creased resist etch tate. As soon as the Si containing layer is removed, the i.ntensity will increase again. Therefore, the amount of Si incorporation is proportional to the time dur­ing which the development is retarded. To determine the silylation temperature window for Plasmask I50u, flood­exposed and unexposed wafers are dry developed after si­lylation at various temperatures. The delay time caused by silicon incorporation was calculated from the CO-emission signal, and the result is plotted in Fig. 7. The minimum useful silylation temperature is the temperature for which the Si incorporation in the exposed parts is sufficiently high to provide proper imaging (here around 150°C). The max­imum temperature is defined by the point where unexposed resist starts to incorporate silicon, which will result in res­idue formation in unexposed areas. So the maximum sily­lation temperature is around 175°C for one stage O2 de­velopment and 5-10 DC higher for two steps developments.

In summary, the silylation temperature window under g- or i-line exposure is only determined by the selectivity in silicon diffusion between exposed and unexposed resist. Under DUV exposure, the silylation temperature window is limited also by the crosslinking reaction. This can cause severe process latitude restrictions for the use of Plasmask 150u for DUV applications.

VII. CROSSLINKING AND PATTERN DEFORMATION

It is known that Plasm ask resist suffers from lateral pattern deformation (also called swelling) under g- or i­line exposure, if less optimal process conditions are used. (See Fig. 8)4,8,[2,13 This deformation is caused by the in­corporation of silicon, which induces stress in the resist. Both vertical and lateral swelling are observed, but the lateral swelling should be suppressed since it causes defor­mation of the resist features.

J. Vae. ScI. Technol. e, Val. 10, No.2, Mar/Apr 1992

',-............ t~ .•.•........ ,..· . ..-.·.·.->·.-.-,.,., •••• ' •••••••••• : •• -•••••••• ~~.v.·.·.·._N-._.".·._ ••• ·····_··· .•.••....

FIG. 8. J line exposed resist features suffering from swelling.

It should be noted here that vertical and lateral swelling are not strictly correlated, since lateral swelling is caused by the Si concentration in the resist and is independent of the silylation depth, while vertical swelling is mainly de­termined by the depth of Si incorporation.4

Under DUV exposure, the vertical swelling should be less because of the smaller silylation depth. Indeed, optical thickness measurements show that the vertical swelling is strongly reduced compared to the g- or i-line case. Since the Si concentration is comparable for i-line and DUV exposures (Fig. 3), the amount of lateral swelling is ex­pected to be comparable, too. However, no lateral swelling was observed for DUV exposures, as is illustrated in Fig. 9. This reduced resist expansion is explained to result from the crosslinking formed under DUV irradiation, which makes the exposed resist more rigid and therefore less sus­ceptible to stress.

The observation of lack of pattern deformation caused by crosslinking under DUV exposure, learned how to solve the swelling problem for non-DUV exposures: by applying a high softbake after coating, a small amount of PAC will form crosslinks with the resin (thermal crosslinking), making the entire resist layer more rigid. During silylation, the resist is able to sustain more stress and lateral swelling is successfully suppressed. 4

VIII. APPLICATION OF CROSSLINKING UNDER DUV~EXPOSURE: POSITIVE TONE IMAGES

The crosslinking properties of Plasmask 150u can be used for printing positive tone images. This has been firstly proposed in Ref. 6 for e-beam exposures and has later been used in the process called PRIME3

,13 for DUV applica­tions. For this process, two exposures are necessary: one DUV exposure through a photomask, and a second g- or i-line flood exposure. The patternwise DUV exposure will induce some crosslinking. As a result of the flood exposure, Si will readily diffuse in the non-DUV exposed parts. Dur­ing silylation of the resist at rather low temperatures, the silicon incorporation will be high in the areas which re-

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710 M. Op de Beeck and L. Van den hove: Silylation of novolac based resists 710

FIG. 9. DUV-exposed PIasmask 150u does not show any swelling.

ceived only one exposure, since non-DUV exposures do not require such high silylation temperatures as DUV. The double exposed areas are crosslinked, and at the lower si­lylation temperatures which can be used for g- or i-line exposures, the crosslinking is sufficient to block the silicon diffusion completely. During the subsequent dry develop­ment, the exposed resist is developed away, while an in situ formed SiOz mask protects the unexposed area from ero­sion. It is important to note that the DUV exposure doses required to prevent Si diffusion are lower than above since lower silylation temperatures are being used.

Figure 10 shows the IR spectroscopy measurements car­ried out on wafers which received a double exposure: first ag-line flood exposure of 150 mJ/cm2

, followed by a DUV exposure. The DUV exposure dose was varying from wafer to wafer, the g-line exposure dose was the same for all samples. Silylation was carried out at 165°C. Again, there are two Si-content thresholds as was the case in the stan-

1.2

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$ 0.0

-0.211 100 200 300 400

DUV exposure Dose (mJ/cm2)

500

1 proper

Imaging

1 attack of

reSist

1

SUn:ca

Imaging

FIG. 10. IR measurements showing the silicon content of Plasmask 150u vs DUV expos,ure .dose for a positive tone process (g-line fiood exposure of 150 mJ/cm" pnor to DUV exposure, silylation at 150 'c).

J. Vac. Sci. Technol. B, Vol. 10, No.2, Mar/Apr 1992

>­CD ... Q) c: UJ

ODD FIG. 11. Illumination and corresponding resist profile for a positive tone process.

dard DESIRE process. But in this case, the Si-content thresholds define only two exposure dose thresholds T3 and T4, as is clear in Fig. 10. In Fig. 11 the image intensity profile for small LIS structures is plotted and compared with the two exposure thresholds T3 and T4. Since T3 and T4 are very close to each other, the damaged resist part is extremely small. Again, the thresholds are influenced by process conditions such as silylation and dry development.

Since Si incorporation will occur in the i- or g-line ex­posed areas, which are not crosslinked, lateral pattern de­formation can exist if the process is not optimized with respect to swelling.

Similar positive tone silylation can be obtained for DUV exposures at 193 nm. to Since crosslinking is more pro­nounced at 193 nm compared to 248 nrn, diffusion is pre­vented in the exposed areas even if silylating agents of smaller molecular size are used, for example dimethylsi­lyl?imethyl-amine (DMSDMA). Some of these small sHy­latlOn agents can be used at lower silylation temperatures. Since thermal crosslinking of the unexposed resist will not

'1.00~--S;;0~0~--:1-:00~0:--:1~SO:::O--::-20::':0-::O-~25~0-O--3....JOOO

Exposure dose (mJ/cm2)

FIG. 12. Silicon content of Plasmask 301 u vs exposure dose for different silylation temperatures (IR measurements).

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711 M. Op de Beeck and L. Van den hove: Silylation ot novolac based resists 711

000 [

600 i

exposed

~ ;:)

~ -c

MWI III -c: 0 €)

in , 200

unexposed

oL---~~--~~------------~~----~ 120 140 160 180 ZOO 220 240

sHy!ation temperature eC)

Pro. 13. Determination of the siiylation temperature window by the end­point curves for Plasmask 301u.

occur at these temperatures, silicon diffusion is possible in unexposed areas. If novolac resin without PAC is used as imaging layer, silicon will diffuse even more easily in un­exposed areas, since hydrogen bridge formation between PAC and novolac will not occur. Positive tone images can be obtained, although some problems concerning residues were reported.

IX. CROSSLIN KING OF PLASMASK 301u

Since the process latitude of Plasmask I50u was limited by crosslinking, a new version of the resist (Plasmask 301u) was introduced by UCB-JSR Electronics. This resist is based on a different resin which should be less affected by crosslinking effects.

4

<:J Q)

>=3 <:J

'" ~ 0 E2 L 0

Z

1.1

(0) 50 mJlem2 (b) 100 mJlcm2 (e) 1400 mJlcm2

(e)

200 220

Energy (MeV) 1.2 1.3 1.4

240 260 280 300 Channel

FIG. 14. RBS measurements showing the silicon concentration for differ­ent exposure conditions for Plasmask 30lu (silylation at HI5 'C).

J. Vac. Sci. Technol. e, Vol. 10, No.2, Mar/Apr 1992

FIG. 15. DUV-exposed Plasmask 301u does not show any swelling.

The curves of Figure 12 are IR spectroscopy curves, showing the amount of Si content versus DUV exposure dose for different silylation temperatures. These curves are very similar to the one for Plasmask 150u: crosslinking will decrease the Si content for high exposure doses, and the influence of crosslinking is more important for lower sHy­lation temperatures.

From IR measurements and the RES measurements, it can be concluded that the influence of crosslinking looks very similar for both resists, Plasmask IS0u and 301u.

If the IR curves for silylation at 210 ·C for both resists are compared (Figs. 1 and 12), it is clear that crosslinking causes a somewhat larger decrease in Si content in the case of Plasmask 301 u. This looks contradictory to the fact that Plasmask 301u was developed especially to reduce the pro­cess window limitations due to crosslinking. But, if one compares the two curves for 210°C in the region of very low exposure doses, it is clear that there is already an important amount of silicon incorporated in Plasmask 150u for a zero exposure dose, while this is not the case for Plasmask 301u. Indeed, the 'endpoint curves' for Plasmask 150u (Fig. 7) indicate that silicon incorporation in unex­posed resist starts around 185 ·C, and therefore higher si­lylation temperatures are not possible. If we look at the 'endpoint curves' for Plasmask 301 u (see Fig. 13 ), it is ob­vious that the maximum silylation temperature is much higher in comparison to Plasmask 150u, since Si incorpo­ration starts only at 220'C for unexposed resist. For ex­posed resist, sufficient silicon incorporation is obtained from 170°C, so the silylation temperature window for Plas­mask 30Iu is shifted towards higher temperatures com­pared with Plasmask 150u. It is this shift which has very important processing consequences: since the influence of crosslinking is very small for higher silylation tempera­tures, crosslinking does not form a limitation anymore for resist imaging using Plasm ask 301u. To prove this state­ment, RBS measurements are carried out for three differ­ent exposure doses: 50, 100, and 1400 mJ/cm2

• Silylation was carried out at 210 DC. The dose of 100 mJ/cm2 corre-

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712 M. Op de Beeck and L Van den hove: Silylation of novolac based resists 712

13000

- thickness after coating + softbake

12500

exposed resist ....... 12000 E .::: VI III 11500 G) c ~ (,)

:c 11000 t-

10500 unexposed resist

10000 ,30 150 170 190 210 230

Sl!ylation temperature (OC)

FIG. 16. Resist thickness of exposed and unexposed Plasmask 301 u after silylation in function of silylation temperature.

sponds with the correct dose to print equal 0.5 ?lm LIS for this silylation condition. The results of these measurements are plotted in Fig. 14. In contrast with the results for Plas­mask 150u, the silicon content of Plasm ask 301u increases with increasing exposure dose, due to the high silylation temperature. The silicon concentration is equal for all the exposure conditions, only a small decrease is observed at the resist surface for the highest exposure dose. This silicon concentration corresponds with the reaction of the silylat­ing agent with all the available phenolic hydroxyl groups.

The possibility of working at higher sHylation tempera­tures has also another benefit; the sensitivity of the resist increases with higher silylation temperatures, so the total amount of polymer-polymer crosslinking will be smaller compared to imaging with Plasmask 1S0u. This is a second reason why the influence of crosslinking is reduced in the

FlO. 17. 0,25 ILm LIS, Plasmask 301 u.

J. Vac. Sci. Technol. B, Vol. 10, No.2, Mar/Apr 1992

1.0

e- 90 .::: O.S

I~ Ui .c III - G) 'C 89 ... 'i Cl III 0.6

G)

c :2. ::::i Q) 88

" C.

Q) £ ... {/) :I

VI 0.4 aI 87 G)

:lE

0.2 11.2 0.4 0.6 O.B

86 1.11

Nominal Linewidth (jlm)

FIG. 18. Linearity of Plasmask 301u.

case of Plasmask 301u. Another advantage of smaller op­timum exposure doses is the fact that the throughput of the lithographic process step will improve.

X. SWELLING OF PLASMASK 301 u

Since the influence of crosslinking is decreased with the use of higher silylation temperatures, and since less crosslinking will occur due to lower exposure doses, one can wonder how lateral and vertical resist deformation is influenced if these higher temperatures are applied.

To investigate lateral swelling, exposed wafers are sily­lated at 200 and 210 °C, and patterns which are very sen­sitive to lateral swelling are examined in the SEM. No swelling was observed for both temperatures (see Fig. 15).

Vertical resist expansion is studied by measuring the resist thickness before and after silylation at various tem­peratures. These measurements were performed on ex­posed and unexposed wafers. The result is shown in Fig, 16. Unexposed resist will shrink during PSB and silylation due to thermal crosslinking and eventually some solvent evaporation. At very high temperatures however, unex­posed resist will incorporate some silicon which causes an increase in resist thickness. Exposed resist will shrink dur­ing PSB due to thermal crossIinking of the lower part of the resist layer, which stays unexposed since the top part absorbs all the light without bleaching. For temperatures below 160°C, the thickness of exposed resist and unex­posed resist is the same, since no important silicon incor­poration will occur. For higher temperatures, silicon incor­poration will cause vertical swelling of the resist, but the increase in thickness is still smaller compared to the thick­ness increase for i-line exposures under standard process­ing conditions.

To conclude, even by decreasing the influence of crosslinking by the use of higher silylation temperatures, there is still enough crosslinking to avoid excessive vertical swelling or pattern deformation by lateral swelling.

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713 M. 01' de Beeck and l. Van den hove: Silylatlon of novolac based resists 713

TABLE 1. Process latitudes for Plasrnask 30in.

Total depth Exposure Linewidth of focus Latitude

(lim) (lim) (%)

0.25 1.0 ±7 0.30 1.1 ±ll 0.35 1.2 ±14 0.40 1.5 ±16 0.50 1.5 ±21

XI. LITHOGRAPHIC PERFORMANCE OF PLASMASK 301u

Since Plasmask 301 u has better properties concerning the influence of crosslinking and concerning sensitivity, this resist was chosen for further lithographic evaluation.

For this evaluation, exposures were carried out on the ASM-L PAS 5000/70 KrF excimer laser stepper (NA = 0.42). The two step C2F 6 dry development process was

applied. Lines and spaces of 0.25 p,m are obtained in 0.9 p,m

thick resist (Fig. 17). No residues are seen. Ali features down to 0.25 p,m exhibit very vertical sidewalls (Fig. 18). The optimum exposure dose was 140 mJ/cm2 for a silyla­tion temperature of 210 0c.

Excellent linearity is obtained down to 0.25 p,m, as il­lustrated in Fig. 18. Table I lists the total depth of focus (DOF) for different feature sizes, using ± 10% Iinewidth variation as criterion for focus latitude. In Fig. 19 the line­width and sidewall slope is plotted versus focus, for 0.25 p,m LIS. Even for these small features, the total DOF is 1 p,m. The exposure latitude is listed in Table I for different feature sizes, using the 10% linewidth variation as crite­rion. For 0.25 f.Lm LIS, the exposure latitude is ±7% (see Fig. 20).

Exposures of Plasmask 30iu were made on different substrates: silicon, oxide, polysilicon, and aluminum. As a result of the surface imaging principle no differences were observed on all substrates. Polysilicon and aluminum wa­fers with topographical features up to 0.5 p,m high were

0.31 90 0.25 ~m LIS

0.30 89

E 0.29 'iii w

::1. CI.I .......

O~l ...

..c 88 til

.t; W ~

'i 0.27 w III 87 c Q.

:J o.2St

0 U)

86 0.25,

O.2~!.0 -0.5 0.0 0.5 85

1.lI

Focus (I!m)

FIG. 19. Depth of focus of 0.25 fJom LIS, Plasmask 30iu.

J. Vac. Sci. Techno!. 13, Vol. 10, No.2, Mar/Apr 1992

e-.:: J:: -"CI 'j III c ::i

0.350 90 0.25 Vm LIS

0.325 89

0,300 38

0.275

87 0.250

0.225 86

0.200 '-:--4---,-~--,,--,---........ ~.....I85 I f 0 120 130 140 150 160 17 II

Exposure dose (mJ/cm2)

FIG. 20. Exposure latitude of 0.25 f.Lm LIS, Plasmask 301u.

.-. VI W Q) .., til 0

~ w Q. 0 U)

coated and exposed. A resolution of 0.25 p,ID LIS was ob­tained, without any reflective notching or scum. Figure 21 shows the top view of the resist lines down to 0.25 f.Lm on aluminum steps, and Fig. 22 the sideview of 0.25 p,m LIS over an aluminum step of 0.5 fJm height.

The thermal stability and etch resistance of Plasmask 301u was also investigated. Resist lines did not show any flow after heating them for 2 min at 250°C, indicating excellent thermal stability. Polysilicon and aluminum etch experiments showed that this resist also has an excellent etch resistance, even without the use of resist hardening prior to etching.

XII. CONCLUSIONS

The crosslinking reaction upon DUV irradiation is in­vestigated in detail. The influence of crossIinking on silicon incorporation is rather similar for the two Plasmask DUV resists: Plasmask 150u and Plasmask 301u. For both re­sists, the consequences of crosslinking on resist imaging were subject of investigation.

FIG. 21. Topview of resist features in Plasmask 301u: LIS down to 0,25 jLm over aluminum topography.

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714 M. 01' de Beeck and L. Van den hove: Silylation of novolac based resists 714

FIG. 22. Plasmask 301u, 0.25 J.l-ffi LIS over aluminum topography.

For Plasmask 150u, the decrease in silicon incorpora­tion due to crosslinking puts a serious limitation on the useful process window. Nevertheless, this crosslinking re­action has also some benefits: pattern deformation is com­pletely avoided, and some imaging techniques based on this crosslinking mechanism in particular are proposed.

For Plasmask 301 u, the use of higher silylation temper­atures is possible, and therefore crosslinking does not put any limitation on the suitable process window. Although the influence of crosslinking is reduced, pattern deforma­tion is still avoided. Good lithographiC performance is ob­tained with Plasmask 301u: resolution down to 0.25 !-lm combined with excellent linearity and good process lati­tudes.

ACKNOWLEDGMENTS The authors would like to thank Anne-Marie Goethals

for many useful discussions, Bert Brijs for the RBS mea-

J. Vac. Sci. Technoi. e, Vol. 10, No.2, Mar/Apr 1992

surements, and Rudy Caluwaerts, Ronny Joblin, and Dirk Hendrickx for their help in dry development. We gratefully acknowledge the support of Douglas Ritchie, Andre Brands, and John Franssen of ASM-Lithography, for pro­viding us with the necessary exposures on the DUV step­per. We are indebted also to Bruno Roland of UCB-JSR Electronics for helpful discussions and for making avail­able the samples of Plasmask. Thanks also to Nanda Sa­marakone for careful reading of this manuscript. The au­thors also acknowledge the sponsorship of the European Community through ESPRIT Contract No. 2265 (DRY­DEL) for supporting part of this work.

a)Prcsent address: Mitsubishi Electric Corporation, LSI R&D Labora­tory, 4-1 Mizuhara, Hami, Hyogo 664 Japan.

IF. Coopmans and B. Roland, Proc. SPIE 63, 31 (l986). 2M. Op de Beeck, N. Samarakone, K. H. Baik, L. Van den hove, and D. Ritchie, Proc. SPIE 1262, 139 (1990).

'C. Pierrat, S. Tedesco, F. Vinet, T. Mourier, M. Lerme, B. Dal Zotto, and J. C. Guibert, Proceedings of the Microcircuit Engineering Confer­ence, Cambridge, United Kingdom, 1989 (Elsevier, New York, 1989), p.507.

4A. M. Goethals, D. N. Nichols, M. Op de Beeck, P. De Geyter, K. H. Baik, and L. Van de hove, Proc. SPIE 1262, 206 (1990).

5n. W. Johnson, Proc. SPIE 469, 72 (1984). 6!1. Roland, J. Vandendriessche, R. Lombaerts, B. Dcnturck, and C. Jakus, Proe. SPIE 920, 120 (1988).

7R_J. Visser, J. P. W. Schellekens, M. E. Reuhman-Huisken, and L. J. van Ijzendoorn, Proc. SPIE 171, III (1987).

oK. H. Baik, L. Van den hove, A. M. Goethals, M. Op de Beeck, and B. Roland, J. Vac. Sci. Techno!. B 8, 1481 (1990).

9K. J. Orvek and M. L. Dennis, Proc. SPIE 771,281 (1987). wM. A. Hartney, R. R. Kunz, D. J. Ehrlich, and n. C. Shaver, Proc.

SPIE 1262, 119 (1990). IIp. W. Klymko, W. T. Babie, N. R. Klymko, and A. L. Thayer, The

Proceedings of the KTI Conference, San Diego, CA, 1988 (unpub­lished), p. 209.

12D. Nichols, A. M. Goethals, P. De Geyter, and L. Van den hove, in Ref. 3, p. 515.

13c. Pierrat, H. Bono, F. Vinet, T. Mourier, M. Chevallier, and J. C. Guibert, Proc SPIE 1262, 244 (1990).

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