JPH0463019B2 - - Google Patents

Description

[Detailed description of the invention]

"Industrial Application Field" The present invention relates to a laser optical system element body suitable for manufacturing lenses, window members, mirrors, prisms, filters, etc. used in laser stepper devices, laser oscillation devices, laser fusion devices, etc. The present invention relates to a laser optical system element that can guarantee durability and high quality for high-output laser light in a short wavelength range. "Conventional technology" The progress in miniaturization and high integration of LSIs in recent years has been extremely remarkable, with the number of elements per chip already reaching 100,000.
We are entering the era of VLSI as described above, and along with this, the development of lithography technology for drawing integrated circuit patterns on wafers is progressing rapidly.
For example, pattern line width corresponding to 1M bit DRAM
Technologies that can draw finer line widths of 1 μm and even finer line widths of 0.8 μm, which corresponds to 4M-bit DRAM, are being developed, and all of these fine line width writing techniques can be performed using optical lithography technology. It is being said. Furthermore, in the field of lithography technology, there is an urgent need to develop submicron pattern writing technology with a pattern line width of 0.5 μm, which is compatible with 16M-bit DRAM, which will be realized in the near future. In terms of technology, it is estimated that optical lithography will become the mainstream, considering recent steady progress in optical systems, light sources, photoresists, etc. It is true that optical lithography has various conditions necessary for drawing ultra-fine line widths, such as a relatively high-intensity light source, high-sensitivity resist, and stable optical materials, but there are drawbacks. Since the exposure wavelength is large, there is a problem in that the resolution is limited by diffraction. The solution is to increase the NA (numerical aperture) of the optical system and shorten the wavelength of light. We have entered the era of high NA (numerical aperture) in optical systems, exceeding NA (numerical aperture) of 0.4, and a prototype lens with NA of 0.6 has been developed, but the depth of focus becomes shallower as the NA increases. High resolution to improve resolution
NA has reached its limit. For example, at NA0.43 and wavelength g-line (436nm),
When exposed, applying the empirical rule of depth of focus = ±0.5λ/(NA) ² , the depth of focus is approximately ±1.2 μm, which is close to the allowable limit considering the current state of resist thickness, steps, and focusing accuracy. Therefore, the next step is to consider shortening the wavelength of light. However, in order to shorten the wavelength of light, 400nm
When the following ultraviolet rays are used, the light transmittance of lenses using conventional optical glass decreases rapidly when the wavelength used is around 365 nm (i-line), in other words, light absorption and heat generation due to the light absorption occur. This causes the focal position and other characteristics of the lens to be disturbed. In order to eliminate this drawback, it has been proposed to replace the conventional optical glass with silica glass or fluorite as the lens material, but when normal ultraviolet light is transmitted through these materials, chromatic aberration occurs due to the wide light spectrum width. will occur. Therefore, it has been considered to use a laser beam with a narrow spectral width as a light source for the optical lithography, and in particular, the excimer laser is the most sophisticated laser for optical lithography. Excimer laser is a high-power pulsed laser that oscillates in a short wavelength region, mainly in the ultraviolet region. Types of excimer laser include Xe ₂ (172 nm), Kr ₂
(146nm), _Ar2 (126nm), etc., noble gas excimers,
Noble gas oxygen excimers such as XeO (538, 546nm), KrO (558nm), mercury halide excimers such as HgI (443nm), KrF (248nm), XeCl (308nm), ArF
(193nm), etc., but in terms of oscillation efficiency and gas life, KrF (248nm), XeCl (308nm), ArF
(193nm) etc. are advantageous. The wavelength of all such laser beams is 350nm.
The following is the wavelength of ultraviolet light used in conventional mercury lamps: g
Because the wavelength is extremely short compared to the case of the g-line (436 nm) or the i-line (365 nm), the uniformity of the refractive index of these optical materials is more than one order of magnitude higher than that of the g-line or i-line. (△n≒1×10 ^-7 ~1×
10 ^-6 , △n: refractive index variation width), but among the lens materials when the short wavelength laser beam is used as a light source, for fluorite, the uniformity of the refractive index and the maximum dimension are required. However, there are still many problems with hygroscopicity and mechanical strength during processing, and for this reason, quartz glass is the only material that can be used as a laser optical system element that can guarantee durability and high quality for laser light in the short wavelength range. I can't find it. "Problems to be Solved by the Invention" However, even if a laser optical system is manufactured using quartz glass, if the optical system is irradiated with short wavelength excimer laser light, which is high-power pulsed light, for a long time. Over time, silica glass (lenses, etc.) is damaged and distorted, causing birefringence. In addition, long-term irradiation with the short wavelength laser light causes a decrease in transmittance, an increase in the absolute refractive index, and a decrease in the refractive index. A problem arises in that the distribution changes and eventually cracks occur. In particular, for silica glass lenses for short wavelength excimer lasers, as mentioned above, there is a requirement that Δn of the refractive index distribution be 1×10 ^-6 or less, and the above-mentioned changes in the optical properties of silica glass are required. When this occurs, the optical axis and focal position of the lens fluctuate, making it extremely difficult to form fine and clear patterns. Furthermore, when irradiated with short wavelength laser light of 300 nm or less, even silica glass, which has higher optical stability than conventional optical glass, will generate fluorescence, especially in projection exposure equipment such as excimer laser steppers. Fluorescence generated from lenses and other optical systems is sensitive to the photoresist on the wafer together with the laser light, making it difficult to form a clear pattern. In view of the shortcomings of the prior art, the present invention adds new and different elements to the quartz glass described above, thereby ensuring long-term stability in refractive index, transmittance, etc., and reducing fluorescence. The object of the present invention is to provide a laser optical system element body which is particularly suitable as an optical system material for excimer lasers. "Means for solving the problem" Now, even when a laser optical system is formed using quartz glass, when the quartz glass is irradiated with excimer laser light in the short wavelength range, as the irradiation dose increases, the quartz glass As mentioned above, the refractive index of the material gradually increases and the transmittance gradually decreases. Furthermore, as explained above, fluorescence is generated in silica glass as the wavelength of the laser beam is shortened, and the degree of fluorescence generation becomes particularly strong in the wavelength range of about 300 nm or less. On the other hand, the degree of change in optical properties such as fluorescence properties, refractive index, and transmittance is determined by the energy density per pulse (J/
It is ^also known that it depends on the oscillation frequency (Hz), oscillation frequency (Hz), and total number of irradiation pulses (pules). Therefore, the present inventors used two types of synthetic quartz glass with different impurity concentration levels, namely, ordinary purity synthetic quartz glass and high purity synthetic quartz glass, and variously changed the OH group content contained in the glass structure. Prepare a plurality of formed test pieces, and apply energy density per pulse (J/cm ² ·pules) to the test pieces.
Then, we irradiated it with short-wavelength excimer laser light in the same wavelength range (248 nm) by varying the total number of irradiation pulses (pules), and investigated its fluorescence characteristics, transmittance, refractive index changes, and the presence or absence of cracks. . As a result, it was confirmed that impurity elements are one of the main causes of deterioration of optical properties such as fluorescence characteristics, refractive index, and transmittance. Especially when the impurity concentration is kept constant, changes in transmittance, refractive index, etc.
It was found that it depends on the OH group content, and specifically, by increasing the OH group content, the optical properties such as the fluorescence properties, refractive index, and transmittance can be improved. In this way, the reduction in impurity concentration and the reduction in OH
The present inventors discovered for the first time that an increase in the group content can reduce the fluorescence of laser optical systems used for short wavelength laser beams and improve the stability of transmittance, refractive index, etc. Therefore, by increasing the purity of quartz glass and regulating the OH group content, the above-mentioned object of the present invention can be achieved. Although it is not entirely clear why the OH group content affects the above-mentioned optical properties, it is thought to be as follows. When quartz glass is irradiated with a strong laser beam, the bonds between the elements that make up the glass network structure are broken, resulting in a decrease in transmittance and the appearance of absorption bands. Furthermore, the fluorescence intensity also increases. However, it is estimated that most of the breaks between these elements are repaired by the presence and movement of the OH groups themselves contained in the quartz glass or the hydrogen elements of the OH groups. The generation of cracks occurs because when a large amount of OH groups are included, the generation of absorption bands becomes smaller due to the above reasons.
We believe that this will result in less light absorption and fewer cracks. The present invention was made based on the above-mentioned knowledge and experimental results, and its characteristics are as follows:
The laser optical system element used for laser light in a specific wavelength range of 400 nm or less, more specifically 350 nm or less, is made of a high-purity quartz glass material, and the OH group content in the structure of the glass material is reduced to at least
The point is that it is set at 300ppm or more. This makes it possible to reduce fluorescence generation in a laser optical system used for short-wavelength laser beams and improve stability of refractive index, transmittance, and the like. In addition, when using a laser beam with a shorter wavelength range and higher power, the setting value is further increased, and the element body is formed of a high-purity synthetic quartz glass material, and the OH group content in the structure of the glass material is increased. amount 700ppm
With the above settings, it is possible to provide a particularly suitable element for a high-output laser optical system of 350 nm or less. In this case, the high-purity quartz glass material means that the content of at least metal elements is below the detection limit when measured based on atomic absorption spectrometry, specifically about 0.1 to 0.01 ppm or less, depending on the type of atoms. , such as alkali metal elements such as Li, Na, K, and
The alkaline earth metal elements Mg and Ca are
It refers to a high-purity silica glass material in which the transition metal elements of Ti, Cr, Fe, and Cu and the Al element are each 0.01 ppm or less. In addition, the high purity of the synthetic quartz and the control of the OH group content can be easily adjusted by the following treatment. That is, synthetic quartz glass is synthesized, for example, by hydrolyzing silicon tetrachloride (SiCl ₄ ) gas in an oxyhydrogen flame, and especially for high purity, for example, silicon tetrachloride of high purity with an impurity concentration below a predetermined value is used. Impurities remaining in the raw material are further removed by distilling the raw material, stored in a stainless steel container lined with Teflon, and introduced into a synthesis burner through a Teflon-lined pipe, where it is converted into oxyhydrogen. By hydrolyzing and synthesizing in a flame, it is possible to produce a high-purity quartz glass material containing approximately 0.1 ppm or less of metal impurity elements. The OH group content can be increased or decreased by changing the mixing ratio of silicon tetrachloride gas and oxygen hydrogen gas during the silica glass synthesis. Note that the OH group content can also be controlled by the shape of the burner used for synthesis. "Example" First, in order to confirm the effects of the present invention, a plurality of test pieces for excimer laser irradiation experiments were prepared by the following manufacturing method. First, silicon tetrachloride, a raw material with a low concentration of impurities, is distilled, and then high-purity silicon tetrachloride, which is stored in a stainless steel container with a Teflon lining, and silicon tetrachloride of normal purity, which is not subjected to the distillation process, are separated. Each of these is introduced into a synthesis burner through a Teflon-lined pipe, and when reacting in an oxyhydrogen flame, the mixing ratio of the silicon tetrachloride gas and oxygen hydrogen gas is changed,
Multiple types of quartz glass ingots with different OH group contents are manufactured. When we measured the impurity concentration of such silica glass ingots, we found that in ingots using silicon tetrachloride of normal purity, Mg was 430 ppb and Fe was 430 ppb.
is 100ppb, Al is 90ppb, Cu is 10ppb, and other metal elements are below the detection limit, namely Li, Na, K, and Ca.
Ti and Cr were below 100ppb. Next, the content of OH groups in such ingots was investigated and was estimated to be 300, 500, 700, 900, and 1100 ppm. On the other hand, in the ingot using high-purity silicon tetrachloride, except for the Cl content of 25 ppm, all other impurity elements were below the detection limit. Next, when we investigated the OH group content of such ingots, we found that they were approximately 300, 500, 700,
It had 900 and 1100ppm. Furthermore, the above-mentioned normal purity silicon tetrachloride and high purity silicon tetrachloride are respectively introduced into the plasma generator,
It was transformed into quartz glass in a plasma atmosphere and an ingot was synthesized. The OH group content of the quartz glass ingot synthesized by this plasma method was 5 ppm or less. The various synthetic quartz glass ingots thus formed were cut into dimensions of 30 x 20 x 10 mm, and both sides were polished to a mirror finish to prepare nine test pieces for excimer laser irradiation experiments. Next, for each of these nine test pieces, 248 nm
(KrF) energy density per pulse is 200, 400, 600 (mJ/cm ²
pulses), and the number of irradiation pulses 1×10 ⁴ , 1×10 ⁵ , 1
Irradiation was performed under irradiation conditions consisting of a combination of ×10 ⁶ (pules). After the irradiation, each test piece was examined for changes in refractive index distribution using an interferometer, solarization using a transmittance meter, fluorescence intensity measurement using a fluorometer, and visually determining the presence or absence of cracks. . The results are shown in the table of experimental results and conditions below. As a result, there were clearly significant differences between the normal purity test piece and the high purity test piece in terms of fluorescence properties, transmittance, refractive index changes, etc., and the main cause of this was impurities contained in the silica glass. Although it was confirmed that it was an element, it was also found that it depends not only on this, but also on the OH group content in addition to the high purity of the impurity element. (Refer to Experimental Examples 1) to 6), 7) to 12)) Furthermore, in test pieces with normal purity impurity concentrations, OH
It is understandable that the stability of transmittance, refractive index, etc. improves as the group content increases, but
Even at 1100 ppm, the fluorescent properties are still strong and pose a practical problem. (See Experimental Examples 1) to 6)) Next, when comparing high-purity test pieces, it is found that when the OH group content is 5 ppm, there are problems with fluorescence intensity, transmittance decrease, refractive index change, and crack generation. If the OH group content is 300 ppm, the transmittance and refractive index are stable enough for practical use, and the fluorescence intensity and occurrence of cracks can be reduced.
(See Experimental Example 8) Furthermore, as understood from Experimental Example 10), OH
By setting the group content to at least 700 ppm, it is possible to further improve the fluorescence characteristics, transmittance, refractive index, etc. of the laser optical system, and also eliminate the occurrence of cracks. It becomes possible to provide an element suitable as an optical system for output laser light.

[Table] "Bad" in the "Transmission, refractive index" column of the "Excimer laser resistance" column in the "Experiment results list" on the previous page means that the E-prime center is relatively generated. "Good" indicates that the transmittance tends to decrease and the refractive index tends to increase, and "good" indicates that the transmittance does not easily decrease and the refractive index does not easily fluctuate. Also, in the "Excimer laser resistance" column, "Fluorescence"
In the sub-column, "strong" indicates that the red fluorescence having a peak at about 650 nm is relatively strong, and "weak" indicates that it is relatively weak. Furthermore, in the "Excimer laser resistance" column, "a lot" in the "Cracks" column means that cracks occur when irradiation is performed at a high energy density per pulse (mJ/cm ²・pulse). "Rare" means that cracks rarely occur. "Effects of the Invention" As described above, according to the present invention, by increasing the purity of quartz glass and adding a different element called OH group, it is possible to improve the optical properties and crack resistance, etc. The stability of refractive index, transmittance, etc. over time can be ensured, and an element body that is extremely suitable as an optical system material for high-power lasers can be obtained. Furthermore, the present invention is fully applicable not only to lithography equipment and other highly integrated circuit manufacturing equipment, but also to optical system elements used in laser fusion equipment and other high-power lasers.

Claims (1)

[Claims] 1. In a laser optical system element used for laser light in a specific wavelength range of about 400 nm or less, the element is formed of a high-purity quartz glass material, and the OH in the structure of the glass material is A laser optical system element characterized by having a group content of at least 300 ppm or more. 2. In the laser optical system element according to claim 1, the element is formed of a high-purity synthetic quartz glass material, and the OH group content is at least
Laser optical system element set to 700ppm or higher. 3. The laser optical system element according to claim 1, wherein the laser beam is an excimer laser.