Senguttuvan Nachimuthu Masahiro Aoshima

Transcription

1 U.D.C :548.55: : Senguttuvan Nachimuthu Masahiro Aoshima Shigenori Shimizu Keiji Sumiya Hiroyuki Ishibashi Recent advancement in 193 nm immersion lithography requires CaF2 lenses with very low levels of refractive index non-uniformity, stress birefringence, and laserinduced damege. CaF2 lenses are used in both the illumination and projection parts of lithography optics. Though this material has several advantages over silica glass, the latest developments in optics design, which aim to achieve high NA, include stringent specifications for CaF2. Residual inhomogeneity of the refractive index in general and stress birefringence, in particular, have strong effects on the final lithographic imaging. Efforts to improve the quality of CaF2 crystals by developing high precision annealing technology that uses a new type of annealing furnace are ongoing. This technology enabled us to obtain crystals with a stress birefringence value of 0.2 nm/cm and a refractive index homogeneity of 19 ppb. We also attempted to improve the laser resistance of the grown crystals to a level of T193 nm 0.1% by carefully optimizing scavenger addition, raw material purification, and process control. 193 nm CaF2 CaF2 CaF2 NA CaF2 CaF2 CaF2 0.2 nm/cm 19 ppb T193 nm 0.1% Large calcium fluoride (CaF2) single crystals are increasingly being used as lens material in optical lithography exposure tools for manufacturing semiconductor integrated chips. Their excellent transparency, optical homogeneity, and laser durability make them clearly superior to other materials such as fused silica and MgF2 [1-3]. Crystals oriented along (111) have better optical homogeneity, and large quantities of them with diameters greater than 200 mm are needed. However, (100) oriented crystals are also needed to compensate for the intrinsic birefringence of these crystals [4]. Large crystals are used because they increase the numerical aperture, which in turn enhances the image resolution of printed features [3,5]. Crystals with an extremely low level of stress birefringence and high refractive index homogeneity are essential since any residual stress or inhomogeneity will directly affect the image projected on the wafer. To reduce exposure time and increase the life of the lens, the lens material must be highly laser-durable and resistant to radiation damage with prolonged use. CaF2 single crystals are mainly grown using the Bridgman method, although the Czochralski method is also reported [6-9]. However, growing large, high purity single crystals along a selected orientation is very difficult, mainly due to the formation of grain boundaries during the growth of the cone part and cracks during the cooling process. Moreover, the crystal is exposed to a strong temperature gradient during growth, which induces thermal stress in the grown crystal. This induced stress causes high birefringence and low homogeneity. Many researchers now use the annealing process to eliminate the stress induced in the crystal in order to reduce birefringence and improve homogeneity. We previously reported development of the technology used to grow large crystals by the Bridgman method [10]. In this study, we report improvement in the quality of the crystals, such as low stress birefringence and excellent homogeneity, achieved by Ph.D. 7

2 optimizing the annealing parameters. The relationship between laser durability and the process parameters has also been studied to grow highly durable crystals. CaF2 Single crystals with diameters up to 240 mm were grown from high purity CaF2 and suitable scavengers along (111) and (100) orientations using the vertical Bridgman method under vacuum conditions [11]. Scavengers are usually metal fluorides added to the raw material to inhibit the hydrolysis process. The Bridgman furnace used for the crystal growth of CaF2 consists of a growth chamber, a crucible lowering mechanism, and a vacuum system. The growth chamber has carbon heaters surrounded by porous carbon insulators. Maximum operating temperature is about CaF2 raw materials were thoroughly mixed with a suitable scavenger and charged inside the carbon crucible. A seed crystal with a selected crystallographic orientation, such as <111> or <100>, was placed at the bottom of the crucible. The crucible charged with the raw material mixture was placed on the crucible shaft to induce crystal growth. Initially, the crucible was positioned at the bottom and the chamber was evacuated. After reaching a vacuum of about 10 4 Pa, the chamber was heated to between 1550 and This was found to be the optimum range after several temperature gradient and melting position measurements. Once the temperature was stabilized, the crucible was raised to a position where the raw materials and the top part of the seed crystal were melted. The melt was held in a uniform temperature zone for several hours to induce homogenization before lowering the crucible at a controlled rate along a temperature gradient of about 10 /cm to induce crystal growth. A post-growth cooling rate of 10~20 /h was used to produce crack-free crystals. Some of the crystals that are cut and polished up to a diameter of 220 mm are shown in. As-grown crystals usually contain high stress-induced birefringence and refractive index inhomogeneity due to the high temperature gradient used for crystal growth. However, the lens elements used in the stepper applications must have extremely low stress birefringence and very high homogeneity. The specifications commonly prescribed by the stepper industry are shown in. To meet such strict specifications, the as-grown crystals are usually subjected to an annealing process after crystal growth. Annealing is a process in which the thermal stress caused by subjection to a temperature gradient during crystal growth is relieved by heat treating the crystal at an elevated temperature and then cooling it at a controlled rate. There are two critical requirements for the annealing process: an isothermal environment where the crystal is not exposed to unfavorable temperature non-uniformities and maintenance of isothermality during cool down, i.e., when the furnace temperature is lowered, there should be no spatial variations in the crystal temperature. Taking these paramenters into consideration, we designed the annealing furnace shown in. It consists of a watercooled double wall stainless steel chamber inside which there are two kinds of carbon heaters, a cylindrical one around the crucible and disc type ones at the top and bottom of the crucible. The chamber is connected to a vacuum system to produce a vacuum of 10 4 Pa or higher. The crucible is designed so that it has a high heat capacity and so that it can be easily dismantled for crystal loading and unloading. We used computer simulations to identify the best crucible material and furnace configurations [12]. Various crucible materials such as fine, porous, and glassy carbon (GC) were investigated. The simulated results of the temperature distribution inside the entire volume of the crystal as contained in various crucible materials are shown in. According to the study, porous carbon provides the best thermal uniformity for the annealing of CaF2. Since CaF2 is easily oxidized by reaction with residual moisture or air in the 8

3 chamber, annealing is usually carried out in a reduced atmosphere. The temperature gradient of the furnace measured without a crucible is shown in. As can be seen, the furnace has two temperature zones, one with high uniformity for annealing CaF2 and the other with a gradient suitable for the scavengers. CaF2 single crystals grown along (111) and (100) were cut into discs with diameters of 130 to 220 mm and thicknesses of 30 to 60 mm. These crystals were polished to a mirror finish on both sides and measured for birefringence and refractive index homogeneity (details of the measurement will be discussed later in this report). After the measurements, each crystal was placed inside the annealing furnace. A suitable amount of scavenger was also introduced into the furnace at a lower temperature zone. The chamber was then evacuated and the temperature of the furnace gradually increased to After keeping the crystal at the annealing temperature for a period of time, the temperature was gradually decreased at a controlled rate. The total annealing process takes about one month. Birefringence, or double refraction, is the division of a ray of light into two rays (the ordinary ray and the extraordinary ray) when it passes through certain types of crystals, depending on the polarization of the light. This is explained by assigning two different refractive indices to the material for different polarizations. The birefringence is quantified by: n = ne no (1) where no and ne are the refractive indices for the ordinary and extraordinary rays, respectively. Stress birefringence inside a lens (crystal) creates different refractive indices for light beams with different polarization directions, which blurs the image projected through the lens. The birefringence of the crystals before and after annealing was measured using a HINDS Instruments' Exicor 450AT birefringence measurement system. This is an easy-to-use instrument designed to measure fast axis angular orientation and low-level magnitude of linear birefringence in optical materials. It allows the user to run an area scan of a flat transparent sample, creating a birefringence map of the 9

4 sample. The crystals were polished on both sides for the measurement, which was carried out using a He-Ne laser beam at 633 nm with a pitch of 2 mm. To reduce the stress birefringence to the level specified in, a number of experiments were carried out to identify optimized annealing conditions for (100) and (111) oriented crystals. It was found that the (100) oriented crystals usually have higher birefringence with four-fold symmetry than the (111) oriented crystals, which should have a three-fold symmetry due to their cubic crystal structure. Accordingly, different annealing conditions were used for crystals oriented in different directions. The crystal growth process was also improved to obtain better as-grown crystals. Typical examples of birefringence mapping for (100) and (111) crystals before and after annealing are shown in. The mean stress birefringence values before and after annealing are 6.2 and 0.8 nm/cm for the (100) oriented crystal and 1.6 and 0.2 nm/cm for the (111) oriented crystal. For this kind of lens to be effective, the spatial homogeneity of the material s refractive index must be excellent. The main causes of inhomogeneity in the crystal are the melting process, which can cause a chemical composition gradient, structural disorder, and permanent stress due to the temperature gradient during solidification and subsequent cooling. Homogeneity is generally evaluated using a phasemeasuring interferometer by integrating over the light path in the crystal. To suppress surface irregularity, the crystal sample is sandwiched between two oil on plates that are coated with immersion oil (oil with the same refractive index as the sample). Since homogeneity is measured by evaluating the wavefront deviations using an interferometer, the measurement accuracy of the interferometer is given in nm wavefront deviation. The optical inhomogeneity is usually represented in parts per million (ppm) or in parts per billion (ppb) calculated directly from the root mean square (RMS) or peak-to-valley (PV) values over the sample thickness by the formula, n = (wavefront deviation wavelength used) / sample thickness 2 A series of annealing experiments was carried out to study the effects of annealing on the resultant homogeneity of the crystals. Reducing the inhomogeneity level of crystals that are highly inhomogenous in the as-grown form was found to be extremely difficult. The main cause of inhomogeneity is the sub-grain boundaries that form during the crystal growth process. Therefore, we focused on optimizing the process used to grow the crystals in order to improve their quality. We realized that the as-grown crystals with an inhomogeneity greater than 100 ppb show only a little improvement, whereas crystals with an inhomogeneity value of about 70 ppb or less show tremendous improvement upon annealing. The actual homogeneity mapping data of two (111) oriented crystals before and after annealing are shown in. The crystal with poor homogeneity shows a nerve-like structure in the 10

5 post-anneal homogeneity mapping, which corresponds to the sub-grain boundaries in the crystal. The high homogeneity crystal, on the other hand, does not show such a structure. The RMS value of the homogeneity of this crystal was redced from 72 ppb to 19 ppb with high precision annealing. All the homogeneity values described here are residual values after subtraction of the Zernike 36 coefficient. CaF2 optical elements used in the stepper machine should have extremely high transmission and laser resistance at 193 nm. Pure CaF2 has excellent transmission properties without absorption bands over a wide range from 0.12 to 20 µm. Specific absorption bands appear in the spectrum if the crystal contains defects such as anionic or other metal impurities. Oxygen is considered a major impurity in CaF2 because it forms CaO at high temperatures, and CaO has strong absorption around 200 nm. Absorption can also be induced by laser irradiation. Induced absorption does not appear before irradiation, but after the crystal has been irradiated with a laser beam for a prolonged period, peaks appear due to the formation of color centers. The most common color centers are F centers, which are single electrons trapped at vacancies. Under intense irradiation, free electrons are created. They then migrate and get trapped at vacant negative sites, inducing absorption at specific wavelengths. To study the laser resistance of the crystals grown by our company, we carried out a series of experiments on crystal growth under various conditions. The parameters that were studied included raw material purity, types of scavengers and their quantity, thermal profiles for scavenger reaction, and growth chamber conditions. Among these parameters, raw material purity together with thermal profile were found to have the greatest influence on the resultant laser resistance. Laser resistance of CaF2 was determined by measuring optical transmittance across a 10 mm sample before and after irradiation with an ArF laser operating at 100 mj/cm 2. The laser dose was 10 5 shots with a repetition rate of 100 Hz. In our initial attempts, too high a concentration of the metal scavengers was added to the raw materials and residual metal impurities were found in the crystal by chemical analysis. Too small a concentration of scavengers was insufficient to complete the reaction with residual oxygen in the chamber, and the crystals showed peaks corresponding to oxygenrelated impurities. Finally, crystals were grown using high purity raw materials with the addition of the optimum amount of scavengers and with an optimized thermal profile. The laser resistance of these crystals was found to be extremely high, with almost no difference in the transmittance spectra before or after irradiation. The transmittance spectra of various crystals after laser irradiation are shown in. For comparison, a reference spectrum is provided in the figure as the transmittance for all the samples before irradiation. A careful analysis of these data showed less than a 0.1% decrease in transmittance at 193 nm after irradiation, which is well within the specifications. Single crystals up to 240 mm in diameter were grown using the vertical Bridgman method. The grown crystals were cut into discs and annealed using a high-precision anneal furnace with a temperature uniformity of 1. Parameters such as annealing temperature, annealing time, and cooling rate were varied to optimize the annealing condition. Properties such as stress birefringence and homogeneity were mesured before and after annealing. After optimization of the annealing conditions, the mean stress birefringence value of a large (111) crystal was reduced to 0.2 nm/cm from its initial value of 1.6 nm/cm. Similarly, the inhomogeneity level was reduced to 19 ppb by annealing. Both the mean stress birefringence of 0.2 nm/cm and the homogeneity level of 19 ppb are well within the required specifications of less than 0.5 nm/cm and 40 ppb, respectively. These excellent specifications have been achieved using original high precision annealing technology developed at Hitachi Chemical. The crystals also showed excellent transmittance at 193 nm with high laser resistance. The authors thank Prof. M. Ishii and Dr. K. Susa for their valuable scientific discussions. [ 1 ] T.M. Bloomstein, M.W. Horn, M. Rothschild, R.R. Kunz, S.T. Palmacci, R.B. Goodman, J. Vac. Sci. Technol. B 15 (1997) [ 2 ] J. Mulkens, J. McClay, B. Tirri, M. Brunotte, B. Mecking, H. Jasper, Proc. SPIE 5040 (2003) 753. [ 3 ] S. Owa, Y. Matsumoto, Y. Ohmura, S. Sakuma, T. Aoki, J. Nishikawa, H. Nagasawa, T. Mizutani, N. Shiraishi, K. Kido, I. Tanaka, J. Nagatsuka, Proc. SPIE 5040 (2003)

6 [ 4 ] N. Shiraishi, K. Kido, J. Nagatsuka, Y. Ohmura, T. Aoki, J. Nishikawa, H. Nagasawa, T. Mizutani, S. Owa, 4 th International Symposium on 157 nm Lithography, August 2003, Yokohama. [ 5 ] J. Mulkens, H. Jasper, B. Streefkerk, D. Flagello, H. Sewell, P. Jenkins, Proc. Semicon Japan (2003) [ 6 ] Y. Hatanaka, H. Yanagi, T. Nawata, Y. Inui, T. Mabuchi, K. Yasumura, E. Nishijima, T. Fukuda, Proc. SPIE on Optical Microlithography, 5754 (2005) [ 7 ] D. Nicora, I. Nocora, Mater. Sci. and Eng. A 102 (1988) L1. [ 8 ] A. Horowitz, S. Biderman, G. Ben Amar, U. Laor, M. Weiss, A. Stern, J. Crystal Growth 85 (1987) 215. [ 9 ] M.S. Abrahams, P.G. Herkart, J. Appl. Phys. 36 (1965) 274. [10] K. Sumiya, N. Senguttuvan, M. Aoshima, A. Gunji, H. Ishibashi, Hitachi Chemical Technical Report (in Japanese) No.43 (2004-7) 19. [11] N. Senguttuvan, M. Aoshima, K. Sumiya, H. Ishibashi, J. Crystal Growth 280 (2005) 462. [12] N. Senguttuvan, A. Gunji, M. Aoshima, K. Sumiya, K. Susa, Proceeding of 4th International Symposium on 157 nm Lithography, 2003, Japan. 12