光刻胶.docx

1、光刻胶Header for SPIE use157 nm Imaging Using Thick Single Layer ResistsMichael K. Crawforda, Andrew E. Feiringa, Jerald Feldmana, Roger H. Frencha, Viacheslav A. Petrova,Frank L. Schadt IIIb, Robert J. Smalleya, Fredrick C. ZumstegaaDuPont Central Research and Development and bDuPont iTechnologiesWilm

2、ington, DE 19880-0356ABSTRACTDuring the past year the probability that 157 nm lithography will precede next generation lithographies such as EUV or EPLhas increased, partly due to encouraging advances in the design of polymeric materials, which have sufficient transparency at 157 nm to serve as plat

3、forms for single layer photoresists. We have identified several fluorinated resins which can be developed in aqueous 0.26 N TMAH, have reasonable etch resistances (comparable to poly-parahydroxystyrene), and can be formulated to yield photoresists with optical absorbancies at 157 nm which are low en

4、ough to be used at thicknesses of 150-200 nm. We have imaged a number of these formulated resists at 157 nm with the Exitech microstepper at International Sematech, and the results for formulated resists with optical absorption coefficients (base 10) as low as 2.1 per micron are described. Keywords:

5、 157 nm photoresists, fluoropolymers, etch resistance, transparency, tetrafluoroethylene1. INTRODUCTIONIncreasingly, 157 nm lithography appears to be a viable step in the continuous evolution of optical lithography. Originally planned for the 100 nm node, it now appears that 157 nm lithography will

6、be introduced at 70 nm feature size and will be used down to the 50 nm node. At wavelengths shorter than 157 nm the difficulty of developing materials with suitableoptical properties increases rapidly with decreasing wavelength, so it may be that 157 nm lithography will be the final step in the very

7、 successful history of optical lithography.The primary challenge in the design of chemically-amplified resist resins for use at 157 nm is that of finding materials which simultaneously possess the following three properties: good optical transparency, suitable solubility in aqueous base after deprot

8、ection, and good etch resistance1. One solution, suggested early in the development of 157 nm lithography, wasto turn to fluoropolymers as the resist resins for single layer resists (SLR), since some examples of such materials werefound to be surprisingly transparent at 157 nm2. Our goal is to devel

9、op photoresists, utilizing DuPonts expertise in fluorinechemistry, which have optical absorbances in the range of 0.5-1.5 m-1, yielding optical densities of 0.1-0.3 for a resistthickness of 200 nm. Here we will describe our progress designing fluoropolymers to achieve this goal. Copolymers based on

10、TFE are one example of fluorinated resins that, when suitably functionalized and formulated, canserve as 157 nm resists3. These copolymers are typically of low molecular weight (Mn 2,000 6,000) and approximately alternating. These and related fluoropolymers are readily soluble in organic solvents, h

11、ave high glass transition temperatures, have good plasma etch resistance, and most importantly have good optical transparency at 157 nm. Introducing functionalities to impart developability in aqueous base generally decreases etch resistance and increases optical absorption at 157 nm. Here we show t

12、hat fluoropolymers can be synthesized and formulated to be soluble in aqueous base after deprotection, while maintaining the optical absorption coefficient of the formulated resist at values as low as 2.1 m-1. Furthermore, we believe that even lower optical absorption values for formulated resists a

13、re possible. Finally, we have imaged these resists at 157 nm, and the results strongly suggest that 200 nm SLR resists for 157 nm lithography are indeed achievable using fluoropolymer resins. 2.1 Polymer synthesis2. EXPERIMENTAL Caution! One of the monomers used to synthesize the materials described

14、 below is tetrafluoroethylene (TFE), a deflagrating explosive and an experimental carcinogen. All synthetic work with TFE described in this report was conducted within completely barricaded and ventilated facilities.Two basic polymer platforms have been synthesized and studied. Both include the hexa

15、fluoroisopropanol functionality toimpart aqueous base solubility. The first platform includes TFE as a comonomer, comonomers containing polycyclicentities to improve etch resistance as well as other comonomers to affect adhesion and imaging performance. The TFEcopolymers are synthesized using standa

16、rd free radical polymerization methods. The second platform is composed of copolymers containing protected and unprotected norbornene-fluoroalcohols. These polymers are synthesized by metal-catalyzed vinyl addition polymerization. 2.2 Resist formulation Formulation solvents include 2-heptanone, PGME

17、A (propylene glycol methyl ether acetate), and cyclohexanone. Standard onium photoacid generators (PAGs) were used. 2.3 Optical propertiesVacuum ultraviolet (VUV) transmission measurements were made using a McPherson spectrometer equipped with a deuterium lamp. Each resist sample was spin-coated at

18、several thicknesses on Si substrates to determine the resist spin-curve. CaF2 substrates were then spin-coated at the appropriate speeds to achieve resist thicknesses between 50 and 200 nm. The VUV transmission spectra of these samples were then measured and are plotted in this paper as absorbance (

19、base 10) normalized by the film thickness.Spectral ellipsometry measurements were made at International Sematech using a Woollam VUV VASE. Resist samples were spin-coated on Si substrates for these measurements. Refractive indices and Cauchy coefficients were calculated from the ellipsometry data an

20、d used to determine resist film thicknesses.The absorption coefficients measured by direct transmission and spectral ellipsometry were generally in good agreement (at least for the ranges of absorption coefficients and resist film thicknesses we describe here).2.4 Dissolution and imaging Dissolution

21、 (contrast) curves were measured using open frame exposures made with the Exitech microstepper at Sematech. Samples were spin-coated on Si wafers and post-apply baked (PAB) at 120 C for 120 sec. An 11x11 matrix of exposuredoses was then made, after which the wafer was post-exposure baked (PEB) at va

22、rious temperatures, followed by puddledevelopment using Shipley LDW-26. The thickness of remaining resist was measured for each exposure dose using aPrometrix SM300 reflectometer. These data were used to generate contrast curves and development rates for the variousresist formulations tested.157 nm

23、images were made using the Exitech/Tropel stepper at International Sematech. The stepper has a NA of 0.6 and a of 0.7 when used with the binary mask. Images using an alternating phase shift mask were obtained with a of 0.3. Focus-exposure matrices were generated for each resist, and the resulting im

24、ages were observed using a JEOL tilt SEM, and KLA-Tencor top-down SEMs. Cross-sections were also obtained at Sematech for selected wafer regions. Standard 0.26 N tetramethylammonium hydroxide developer was used for resist processing. 2.5 Outgassing Outgassing measurements4 for several prototypical f

25、luoropolymer resin-based resists were measured at MIT Lincoln Labs by irradiating resist films with 157 nm excimer laser light and collecting in a cold trap any outgassed products for a time of10 minutes during and after irradiation. The collected gases were then analyzed by mass spectroscopy.3.1 Op

26、tical properties3.1.1 VUV absorption of resins 3. RESULTS AND DISCUSSION The VUV transmission of a resin used in a photoresist is important for several reasons. First, the resin is the majorcomponent in the formulated resist and therefore provides the largest contribution to total optical absorption

27、. Second, optical absorption by the resin does not lead to useful photochemistry (excluding the possibility of energy transfer from thephotoexcited resin to the PAG) and thus serves to decrease the resist sensitivity. Third, light absorption by the resin willcontribute to the degradation of sidewall

28、 angles. Finally, 157 nm photons absorbed by the resin may generate unwantedphotochemistry such as cross-linking reactions, or bond scissions leading to resist outgassing5. We have reported that TFE copolymers can have excellent transparency at 157 nm3. Of course, such polymers must be suitably func

29、tionalized in order to serve as chemically amplified photoresists. For 193 nm resists, carboxylic acids and esters have generally been used for this purpose6. One would expect the incorporation of carboxylic acid groups to exert anegative impact upon the optical transmission at 157 nm since these gr

30、oups contain carbon-oxygen double bonds which are known to absorb at wavelengths shorter than 200 nm due to the presence of * transitions (the lower energy oxygenlone pair transition, np *, typically appears at wavelengths between 200 and 300 nm), * transitions, and Rydberg transitions for which a l

31、one pair oxygen electron in a 2p orbital is promoted to higher oxygen atomic orbitals (such as 3s, 3p, 3d, 4s, 4p, 4d, etc.)7,8. Other acid functionalities which have been reported in the literature, such as hexafluoroisopropanolgroups9, are fully saturated and are therefore expected to absorb at sh

32、orter wavelengths than 157 nm since they do not have * transitions. Furthermore, the acid strengths of carboxylic acids (pKa 5) and fluoroalcohols (pKa 9) are also very different. These profound differences in optical properties and acidity are of obvious importance in the design of 157 nm resins.In Figure 1 we show the optical absorbance spectra of two resist resins (i.e. resins that when formulated will image at 157nm):