Polymers for Microelectronics - American Chemical Society

Chapter 22

Acid-Sensitive Phenol—Formaldehyde Polymeric Resists Downloaded by CALIFORNIA INST OF TECHNOLOGY on December 8, 2016 | http://pubs.acs.org Publication Date: November 23, 1993 | doi: 10.1021/bk-1994-0537.ch022

W. Brunsvold, W. Conley, W. Montgomery, and W. Moreau IBM Technology Products Division, Route 52, B 300-40E, Hopewell Junction, NY 12533

Novolak (cresol-formaldehyde) resins were converted into positive working resists by partial esterification of the phenolic groups with di-t-butylcarbonate. In the presence of an acid source, fast resists were formulated for 248 nm (8 mj/cm ), I line, electron beam, and X- ray exposure. Lithographic performance issues of poor adhesion and film shrinkage (> 25%) encountered with resists based on 100% TBOC esters of novolak were eliminated by a minimum bock content (< 20 mole%). For top surface imaging, negative type images were produced by the gas phase and by the liquid phase silylation of the novolak groups formed by the acid catalyzed removal of TBOC groups. 2

Novolak resins in conjunction with diazoquinone dissolution inhibitors have been the dominant photoresist for the last two decades (i). The novolak resin of cresol formaldehyde has served as the base soluble resin for many posiitve formulations operating in the 300-500 nm region. In the last five years, new acid catalyzed resists based on the dissolution inhibition of polyhydroxystyrene(PHOST) have been formulated for deep U V (240-260 nm) lithography (2). For example, the dissolution inhibition of novolak or PHOST can be accomplished by addition of small molecules such as diazoquinones or t-butylcarbonate (TBOC) esters of bis-phenol A (TBOCA) (2, 19).

In the presence of a photoacid, the TBOC group of TBOCA is converted to the free phenol group to induce the dissolution of the exposed region in the alkaline developer. Another example of a TBOC based resist is the TBOC ester of PHOST which undergoes an acid catalyzed removal of TBOC sites (3) to form an alkaline soluble PHOST.

0097-6156/94/0537-0333$06.00/0 © 1994 American Chemical Society Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

Downloaded by CALIFORNIA INST OF TECHNOLOGY on December 8, 2016 | http://pubs.acs.org Publication Date: November 23, 1993 | doi: 10.1021/bk-1994-0537.ch022

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POLYMERS FOR MICROELECTRONICS

The TBOC based resists are examples of resists which can be aqueous alkaline developed. The PHOST resin used in acid sensitive resists are more transparent in the deep U V region than their novolak counterparts and thus have been the focus of more investigations. Previous attempts to use diazquinone novolak resist in the deep U V region are hampered by the higher absorbance of the novolak resin and the diazoquinone photoproducts (4). Only thin films less than 500 nm thick can be used for the deep U V region (4). In the deep U V region, exposure tools which use the conventional Hg lamps require fast resists (< 15 mj/cm ) in order to provide short exposure times and sufficient throughput. To meet the photospeed requirements, resists based on the incorporation of TBOC groups onto the PHOST backbone pioneered the era of acid catalyzed amplified systems. Both negative, positive, and top surface imaging (TSI) resists based on TBOC have been formulated for the deep U V region (3, 5). Although PHOST resins based systems are more transparent than their novolak counterparts in the deep U V region, the lower costs of novolak have prompted this investigation of novolak based deep U V resist for the deep U V region. For deep U V and for electron beam lithography, three component resists consisting of a novolak or PHOST resin, an acid source(AG), and an acid labile dissolution inhibitor have been investigated. The dissolution inhibiitors mixed with novolak were of the molecular type such as TBOCA (2) or more recently of the polymeric type such as the TBOC ester of PHOST (6) or the TBOC ester of novolak (7). In the deep U V , the TBOCA-novolak formulation exhibited a low contrast of < 2 presumably due the high absorbance (B > 0.5/um) of the novolak (2). In the other cases for electron beam exposure, the TBOC-PHOST was immiscible with the novolak (6) or the TBOC-NOVOLAK formed negative images with a triphenylsulfonium triflate (7) acid generator. Finally a recent investigation of TBOC esters of novolak as electron beam resist have found negative working images are formed with 100% TBOC esters of novolak (S). Although TBOC based resist are fast positive resist system some shrinkage issues due to loss of TBOC groups have been noted such as sidewall image distortion and the loss of the film (> 25%) during conditions such as in reactive ion etching (RIE). The purpose of this work was to formulate TBOC esters of novolak (NOVOBOC) for deep U V , near U V , E-beam, and X-ray and TSI lithography. Lithographic performance issues were addressed by examining novolaks with varying amounts of TBOC groups. 2

Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

22.

Acid-Sensitive Polymeric Resists

BRUNSVOLD ET A L .

335

Table 1. NOVOBOC (Novolak) Resin Precursors


Novolak

M

w

meta cresol 19.5K 3.4K meta cresol 8.3K 1.7K meta/para 7.5K 1.2K bis-phenol A 3.7K 0.9K cresylic acid 17.9K 2.3K para cresol 3.7K 0.7K

M /M w

5.8 4.9 6.3 3.8 7.6 5.6

n

248 nm 254 nm T .°C A/um A/um g

119 110 94 79 105 81

0.53 0.52 0.43 0.41 2.15 0.33

0.36 0.38 0.34 0.28 1.92 0.33

EXPERIMENTAL Various novolak resins, Table 1, were obtained from commercial sources(Schenectady Chemical, Reichold, and Rhone-Poulec) and characterized for their absorbance(A), molecular weight by gel permeation chromatography, and glass transition (T ). Various amounts of TBOC groups were introduced on the phenolic site of the novolaks by reaction with di-t-butylcarbonate using a trace of amine as a catalyst (9). Photoacid generators (PAG) of the triflic acid type were selected for deep UV(PDT) and I line (DPMT) from derivatives of N-hydroxymaleimide-triflates (70). g

For top surface imaging, after deep U V exposure, silylation of the image was performed in the gas phase using N,N-diethylaminotrimethylsilane (DEATS) or in the liquid phase (HMCTS) using hexamethylcyclotrisilazane (22). Photospeed, resolution, and contrast were determined using 50 KeV I B M EL-3 electron beam, Brookhaven synchrotron X-ray radiation, G C A I line stepper, Perkin Elmer 500(deep U V mode) and A S M excimer laser deep U V stepper.



336


% FREE PHENOLIC OH in NOVOLAK RESIN

Figure 1. Dissolution rate in 0.263 N T M A H of meta/para novolak esterified with varying amounts of TBOC. RESULTS AND DISCUSSION The acid catalyzed removal of TBOC groups from PHOST is a highly efficient reaction for forming positive or negative images. For the case of positive working resists, the presence of many TBOC groups left on remaining resist, can significantly lower or alter the physical, thermal and reactive ion etch properties of the resist. In addition, the masking of phenolic sites by TBOC may reduce the adhesion to silicon type surfaces. Image distortion can occur due to the loss of TBOC groups during post expose bake. A reduction in the number of TBOC groups could enhance the overall lithographic performance of the resist. Thus we sought to determine the minimum of TBOC esters on the novolak for dissolution inhibition. Using a fixed developer concentration of 0.263 N tetramethylammonium hydroxide(TMAH), the minimum TBOC of the novolak to suppress the dissolution of the resin was in the range of 15-20 mole%. For a 55/45 meta/para cresol novolak of 7K M , the minimum TBOC content to provide a unexposed film thinning rate of < 500A nm/min., Figure 1, was 17 mole%. After exposure to generate an acid and subsequent heating, the TBOC ester of the phenol group of PHOST or novolak is removed to reform the phenol structure. The exposed region rapidly dissolves in the alkaline developer. In the case of a novolak resin with 100% TBOC content, Figure 1, the TBOC level has to be lowered to < 10 mole% in order for the exposed region to dissolve faster than 6000A°/min. The net loss of TBOC group is of the order 90 mole%. If an initial lower TBOC content of 20 mole% is used, a net smaller conversion (10 w

0


22.

BRUNSVOLD ET A L .


337

mole%) of TBOC groups to free phenol is required before onset of dissolution (> 6000A / min). A novolak with a 90 mole% TBOC content required a dose of 30 mj/cm while a TBOC with 17 mole% content required 3X lower dose. Thus, fewer chemical events are required and hence faster resists. To evaluate the imaging capability of various TBOC contents of a meta/para NOVOBOC, a Meyerhofer type plot (1, 18), Figure 2, reveals a projected R / R (dissolution rate ratio of exposed to unexposed thinning rate) of > 10 for a TBOC content of 15 mole%. When compared to novolaks of high TBOC contents, other lithographic benefits can be derived from novolaks of reduced TBOC contents. The benefits are related to the presence of more phenolic groups. The adhesion, for example, to oxide surfaces was improved (no lifting of submicron islands). However, in processing, care must be exercised in the post apply bake step as to not exceed 110°C since the phenolic group is sufficiently acidic (12, 13) to catalyze the removal of TBOC groups, Figure 3. After exposure, the TBOC based resists are baked to complete the acid catalyzed conversion to an aryl-OH group. However, with high TBOC contents, considerable film shrinkage (> 25%) of the exposed region occurs. Film cracking at the edges of the image into the unexposed region was observed for high TBOC contents (> 50 mole%). Later, in processing, the remaining resist mask can be subject to elevated temperatures such as in ion implantation or in RIE. In this case considerable film loss (> 25%) can occur. The shrinkage and loss of the resist film can be significantly reduced to < 5% for NOVOBOC with a TBOC content of < 20 mole%. In the deep U V region, novolak resins, in general, absorb a significant amount of radiation. For an effective transparency, (< 0.4/um) the novolak has to be carefully chosen or its structure modified (14). At 254 nm, when compared to PHOST, some novolaks exhibit a low absorbance window, Table 1, Figure 4. However,at 248 nm, a sharp increase is noted. Novolaks of para cresol structure are the closest match of PHOST transparency, but unfortunately because of strong intramolecular H bonds are not very soluble in alkaline developers. Novolaks based on bis-phenol A, 0

2


0

CH

CCH

3


338


DISSLN

RATE

NQVCAAK RESIN


4 -

Figure 2. Effect of TBOC content of meta/para NOVOBOC on dissolution rate of exposed resist (R) and unexposed resist rate (R ) in 0.263 N T M A H . 0

HEAT — ^ - \ H OH OB OH

H

1

OH

+

+

H+ H+ r\ r\ r\ OB OH OB OH

H

+

r\ OH

Figure 3. Schematic of thermolysis of NOVOBOC activated by acidic phenolic group.

possess an additional phenolic group which impart base solubility. NOVOBOC based on bis-phenol A and meta-para novolak were further investigated as deep UV, X-ray and electron beam resists. In the deep U V region, the photoacid generator of pthalimide-triflate (PD1) has negligible absorbance at the concentration used (5 wt%). The absorbance of the NOVOBOC of meta para novolak, Figure 5, is dominated by the phenolic group. Previous studies of electron transfer reactions of model compounds of phenol type (15,16) and of novolak resin in the deep U V (19) suggest that the energetics, Figure 6, are favorable for photoacid generation. In the deep U V the highly absorbing phenolic group of TBOC photosensitizes the generation of acid from the weakly absorbing PDT. In device lithography with resists, reflective notching and interference effects caused by substrate reflectivity, can be suppressed by using thin antire-


22.

BRUNSVOLD ET A L .

Acid-Sensitive Polymeric Resists —,

2.50

1

1

339 1

1


2.0

0>

\

1.50

/ w

\ \\

\M

/

/ / /

\

0.50

\\

\ \\~

ao 1

1

1

1

1

1

1

1

1

Wavelength, nm

Figure 4. U V absorbance spectra of 1000 nm thick films of novolaks, M-cresylic acid type, MP-meta/para, BPA-bis phenol A, and PHS-polyhydroxystyrene.

flective layers or by using multilayer resists consisting of a silicon containing resist on top of an absorbing planarizing layer (27). TSI combines the separate layers into one film and incorporates Si by silylation of the exposed resist (5). For TSI, the highest absorbing novolak of a meta cresol type (Table 1 and Figure 7) was fully esterified with TBOC, and used at a film thickness of 1500 nm. Two silylation and development schemes were used for the gas phase or for the liquid phase silylation, Figure 8. In the gas phase process, after a dose of 35 mj/cm at 248 nm, DEATS was used to incorporate Si into the novolak image structure. A negative image was formed by oxygen reactive ion etch (RIE) development, Figure 9. Liquid phase silylation process can use silylation agents such as HMCTS (22) which can also crosslink the silylated region and prevent flow of the image 2



340


Wavelength, nm

Figure 5. Deep U V absorbance spectrum of meta/para NOVOBOC before and after post expose bake. P H O T O S E N S I T I Z A T I O N IN D E E P

DONOR + 248 nm

UV

DONOR *

—

DONOR* + ACCEPTOR"*

D

+

+A~

A + H — * HA

^ G= ^ G=

- E{ red

(E(oxid donor) 0-68 ev

acceptor)

-1.4 ev

PHENOLIC

)

-E

(donor

^

en)

- 4.1 ev

TRIFLATE

^ G = - 3.3 ev. I PDTRIFI OH

/ OH

H+

H+

r\

r\

\ H + OB OH OB OH

lonono^

OB

r\

OH

r\

OH

Figure 6. Energetics of formation of photoacid H A by energy transfer from donor phenolic group of NOVOBOC to PDTRIF (acid generator).


22.


BRUNSVOLD ET A L .

i

1.0

1

i

0.90

-i

i — r -

341

r-

A T 248 n m =

A=1.7/um


0.80 a 70 a>

o. 60

(B a

a so

a c

o (0

•Q

0.40 a

30

0.20

0.10

0.6 um

ao

§

8

8

3

Wavelength, nm

Figure 7. Deep U V absorbance spectrum of NOVOBOC (900 nm thick) used for TSI. TOP S U R F A C E IMAGING

TSi

SIMULTANEOUS SILYLATION AND DEVELOPMENT OF NOVOBOC DEATSi

PLANARIZING LAYER

EXPOSE PEB XYLENE HMCTSi

Figure 8. Schematic of TSI imaging processes for NOVOBOC, (a) gas phase silylation with DEATS followed by RIE in oxygen, (b) simultaneous liquid phase development and silylation with HMCTS followed by RIE in oxygen.



342


for high temperature lift off. In this application, a NOVOBOC (500 nm thickness) was coated on top of a polyimide layer (1000 nm thickness). The exposed NOVOBOC was baked and then developed in a mixture of xylene and 10% by wt. of (HMCTS) which simultaneously developed the unexposed region and silylated the exposed region. The final image was transferred through the polyimide layer by oxygen RIE, Figure 10. For normal deep U V type imaging, the most transparent NOVOBOC resist of bis phenol A was used and the contrast was determined to be 3.9, Figure 11. Submicron images, Figure 12, were formed at doses of < 10 mj/cm . For electron beam exposure (Figure 13) or for X-ray lithography, the opacity of the novolak is not of concern. For I line applications, the NOVOBOC resin is transparent and the acid generator of DPMT absorbs all of the radiation. For electron beam, X-ray and I line, a NOVOBOC meta-para, Table 1, was used. The contrast of each type of resist were determined to be in the range of 3-4, Table 2. Although this study of the NOVOBOC resist system has concentrated on two component formulations consisting of a NOVOBOC and acid generator (AG),the NOVOBOC resin can also be used as a dissolution inhibitor of PHOST or novolak. PHOST or novolak completely esterified with TBOC are noted to be immiscible with PHOST or novolak resins (6, 7). Partially esterified NOVOBOCS of this study are miscible with PHOST and novolak (single T and no phase separation). The good intermixing has been ascribed to efficient hydrogen bonding between the phenolic and TBOC groups (12). As an example, a meta cresol of 20 mole% TBOC content was mixed at various wt.% with a meta-para novolak. A Meyerhofer plot, Figure 14, reveals that at 8% loading, produces a R / R > 100. Lastly, for applications of the resist such as in ion implantation or as a RIE mask for metal etching, post image hardening by deep U V of diazoquinonenovolak is often employed (20). The high absorbance of the novolak or diazoquinone photoproducts limits the penetration of the radiation and also requires high doses of > 1 J/cm . The NOVOBOC resist is more transparent in the deep U V and lower doses (3X) are required. Acid induced crosslinking of novolaks has been reported (27). In this case, the additional acid generated from the A G may faciliate the novolak crosslinking. 2

0

2

SUMMARY AND CONCLUSIONS The introduction of less than 100% of TBOC leaving groups onto a low cost novolak resin improves the adhesion, photospeed, and film retention during lithographic processes. NOVOBOCS were formulated from novolaks at 15-20 Table 2. Contrast of NOVOBOC Type Positive Resists I LINE DEEP U V E B E A M (50 KeV) X-Ray CONTRAST DOSE/cm 2

3.5 27 mj

3.9 8mj

4.1 3uC

4.5 12 mj


TSi 35 mj


22.

BRUNSVOLD ET A L .


343

Figure 9. TSI image of (500 nm) in 1200 nm thick NOVOBOC.

Figure 10. Bilayer image of 500 nm in NOVOBOC/polyimide formed by liquid phase silylation/development followed by RIE oxygen transfer.


344



Figure 12. Submicron images (400 nm) formed in NOVOBOC with dose of 8.5 mj/cm at 248 nm. 2



22.

BRUNSVOLD ET A L .


345

Figure 13. Submicron images of 500 nm formed by electron beam dose of 3 microcoul/cm . 2


Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993. 0

Figure 14. Dissolution rate of exposed (R) and unexposed (R ) for three component resist of novolak (nov), % NOVOBOC (BOC), and photoacid generator (PAG).



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347

mole% TBOC content and demonstrated to be fast I line, electron beam, X-ray and TSI resist. The low amount of TBOC groups greatly reduced the film shrinkage in processing to less than 5% loss. For deep U V lithography,novolaks were chosen to be of highest transparency and for TSI of the highest opacity. In the deep U V region, evidence suggests that the phenolic group of NOVOBOC can act as a photosensitizer for acid generation. In a three component formulation, the NOVOBOC can act as a dissolution inhibitor and a fast resist of submicron resolution can be produced. For further extension, a TSI NOVOBOC resist system was demonstrated with gas phase silylation followed by RIE transfer. A bilayer Si system was also formed and processed by simultaneous liquid phase development, liquid phase silylation and RIE pattern transfer. REFERENCES 1. W. Moreau, SEMICONDUCTOR LITHOGRAPHY, Plenum Press, 1989, Chapter 2. 2. D. McKean, SPIE, 920, 61(1988). 3. H . Ito and C. Willson., Pol. Eng., and Sci., 23, 2012(1983). 4. T. Wolf, R. Hartless, A. Shugard, and G. Taylor, J. Vac. Sci., and Tech., B5, 396(1987). 5. S. MacDonald, H . Ito, H . Hiraoka, and C. Willson, SPE RETEC on Photopolymers, 1985, p.177. 6. H.Shiraishi, H . Takumi, T. Ueno, T. Sakamizu, and F. Murai, J. Vac. Sci. and Tech., B9, 3743(1991). 7. H.Ban, J. Nakamura, K. Deguchi, and A. Tanaka, J. Vac. Sci., and Tech., B9, 3387(1991). 8. A. Gozdz and J. Shelburne, SPIE Proc., 1672, 184(1992). 9. J. Frechet, E. Eichler, H . Ito, and C. Willson, Polymer, 24, 995(1983) 10. W.Brunsvold, W.Montgomery, and B. Hwang, SPIE Proc., 1466, 368(1991). 11. J. Shaw, M . Hatzakis, E . Babich, J. Paraszczak, D. Witman, and K. Stewart, J. Vac Sci. and Tech., B7, 1709, (1989). 12. H . Ito, J. Pol. Sci, 25, 2971(1986). 13. P. Paniez, D. Demattei, and M . Abadie, Microlectronic Eng., 17, 279(1992). 14. H.Bogan and K. Graziano, SPIE Proc.,1262, 180(1990). 15. N . Hacker and K. Welsh., SPIE Proc., 1466, 384(1991). 16. W. Brunsvold, R. Kwong, W. Montgomery, W. Moreau, H . Sachdev, and K. Welsh, SPIE Proc., 1262, 162(1990). 17. M . OToole, E . Liu, and M . Chang, IEEE Trans. Elec. Dev., ED-28, 1405(1981). 18. D.Meyerhofer, IEEE Trans. Elec. Dev., ED-27, 921(1980). 19. L. Schlegel, T. Ueno, H . Shiraishi, N . Hayshi, and T. Iwayanagi, Chem Mater., 2, 299 (1990). 20. J. Pacansky and H . Hiroaka, J. V A C . Sci. and Tech., 19, 1132(1981). 21. A . Knop and W. Scheib, CHEMISTRY and APPLICATIONS of PHENOLIC RESINS, Springer-Verlag, 1979. Received March 8, 1993


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