CN106067652B - Dual-wavelength antireflection film for excimer laser and optical film thickness monitoring system - Google Patents
Dual-wavelength antireflection film for excimer laser and optical film thickness monitoring system Download PDFInfo
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Abstract
The invention discloses a dual-wavelength antireflection film for excimer laser and an optical film thickness monitoring system for preparing the antireflection film, which are applied to an excimer laser instrument compatible with deep ultraviolet 193nm and 248 nm. A dual-wavelength antireflection film for excimer laser comprises a substrate and a multilayer film which is sequentially arranged on the substrate and is formed by alternately arranging high-refractive-index films and low-refractive-index films; the high-refractive-index film is lanthanum fluoride, and the low-refractive-index film is magnesium fluoride; the number of layers of the multilayer film is 4-10. An optical film thickness monitoring system comprises an electron beam evaporation source, a thermal resistance evaporation source and a plated substrate, wherein baffle plates are arranged between the electron beam evaporation source and the thermal resistance evaporation source and between the electron beam evaporation source and the plated substrate. The dual-wavelength antireflection film for the deep ultraviolet excimer laser not only has very low residual reflectivity of the substrate surface at the wavelengths of 193nm and 248nm, but also has the lowest absorption scattering loss.
Description
Technical Field
The invention relates to the field of deep ultraviolet excimer laser instruments, in particular to a dual-wavelength antireflection film for excimer laser and an optical film thickness monitoring system.
Background
The excimer laser is a gas laser, and the working gas of the excimer laser mainly comprises inert gas atoms of argon (Ar), krypton (Kr) and xenon (Xe), which have very stable chemical properties in normal state, and halogen atoms of fluorine (F), chlorine (Cl), bromine (Br) and iodine (I), which have more active chemical properties. The inert gas atom does not generally form a molecule with other atoms, but if mixed with a halogen atom and excited in a discharge form, an excited molecule is generated. When the excited state molecule is transited back to the ground state, the excited state molecule is reduced into atom and emits photon, and the photon is amplified by the resonant cavity and emits laser. Since such excited molecules are evanescent instantaneously and have a lifetime of only several tens of nanoseconds, they are called "excimer" and their emitted laser light is called excimer laser.
Excimer lasers have been made of more than ten kinds, the most important wavelengths are 157nm (F2), 193nm (ArF), 248nm (KrF), 308nm (XeCl), 351nm (XeF), etc., wherein the most widely used wavelengths are 193nm and 248nm in the Deep Ultraviolet (DUV), which is usually 190nm to 280nm, and the wavelength less than 190nm is called Vacuum Ultraviolet (VUV), and the wavelength more than 280nm is called Near Ultraviolet (NUV).
The deep ultraviolet excimer laser has the obvious characteristics that: first, the deep ultraviolet laser belongs to cold laser, and when it is acted on biological tissue, it is not thermal effect, but photochemical reaction, so-called photochemical reaction, i.e. when the tissue is acted on by the deep ultraviolet laser, it can break molecular binding bond and directly decompose and volatilize, and does not affect the surrounding tissue, so that it has important application in laser therapy, specially in ophthalmology and cardiovascular therapy. Secondly, the short output wavelength of the deep uv laser, especially the wavelength 193nm, is already at the deep uv and vacuum uv edges, which is the shortest wavelength that can still propagate in air with low loss, and we know that the shorter the wavelength, the higher the resolution, and therefore, is very suitable for etching precise patterns in semiconductor lithography, which is one of the most critical technologies in the fabrication of integrated circuit chips. Finally, the photon energy of the deep ultraviolet laser is very high, the photon energy at the wavelength of 193nm is 6.42eV, the photon energy at the wavelength of 248nm is 5eV, the power of a single pulse can reach hundreds of watts to megawatts, and the energy of the single pulse can reach several joules to dozens of joules, so the deep ultraviolet laser is very suitable for laser fine processing, material processing, laser weapons and the like.
The invention aims to provide a dual-wavelength antireflection film for deep ultraviolet excimer laser, which is suitable for 193nm and 248nm compatible deep ultraviolet excimer laser instruments. However, in this wavelength band, not only the highly transparent substrate material but also the applicable thin film material, especially the high refractive index material, is very limited. Furthermore, even if there are only few alternative materials, since the photon energy is very close to the electronic forbidden band gap of the material, high absorption is generated, and in addition, the wavelength is very short, the surface scattering is also very high, so the optical loss is very large, which not only causes the transmittance to be greatly reduced, but also causes the damage of high-energy laser to the thin film, and because of this, the reduction of the optical loss of the thin film becomes a difficult point of the design of the deep ultraviolet excimer laser thin film device, which is a core problem to be searched and solved by the present invention.
Disclosure of Invention
The invention aims to provide a dual-wavelength antireflection film for excimer laser and an optical film thickness monitoring system for preparing the antireflection film, which are applied to an excimer laser instrument compatible with deep ultraviolet 193nm and 248 nm.
The dual-wavelength antireflection film for the deep ultraviolet excimer laser not only requires that the wavelengths of 193nm and 248nm have very low residual reflectivity on the surface of a substrate, but also requires that the antireflection film has the lowest absorption scattering loss, so that the antireflection film can obtain the highest transmissivity, and the damage threshold of the excimer laser on the film is improved. The concept of the invention is as follows:
first, thin film materials with as small an extinction coefficient as possible were sought. For wavelengths 193nm and 248nm, the optical properties of the thin film material must be described by the complex refractive index, N, which is a function of wavelength λ and can be expressed as: n (λ) ═ N (λ) -ik (λ), where N (λ) is the refractive index and k (λ) is called the extinction coefficient, which characterizes the magnitude of the absorption scattering loss. For wavelengths 193nm and 248nm, it is first necessary to experimentally determine various thin film materials and their complex refractive indices that may be applied in this band. Secondly, selecting a high-refractive-index film and a low-refractive-index film material according to the following selection principle by using the obtained complex refractive index: 1) the difference between the refractive indexes of the high and low refractive index materials is large; 2) the refractive index of the low refractive index material is as small as possible; 3) more importantly, the extinction coefficients of both the high and low refractive index materials are sufficiently small; 4) the mechanical and chemical properties of the material can meet the actual use requirements.
Since the thin film material is applied to the wavelengths 193nm and 248nm simultaneously, and the loss of the wavelength 193nm is always larger than 248nm, the photon energy and electronic forbidden band gap of the wavelength 193nm can not be used as the basis for primarily screening the material loss. Simple treatment is carried out: because when the photon energy h v is equal to the electronic forbidden band gap EgThe short-wave absorption band edge of the material is exactly defined, and the wavelength lambda of the short-wave absorption edge of the material can be determined therefromcI.e. due to Eg＝hν＝hc/λcWherein h, v, and c are Planck constant, optical frequency, and optical speed, respectively, so that the wavelength λ of the absorption edge of short wave isc＝hc/EgThe farther away from λ toward the long wavelengthcThe smaller the optical loss. The relationship of the material refractive index n can also be used more simply if the calculation is lacking data: n is4/λcApproximately constant, for the materials of the present invention, the constant for the high index material is approximately 42, while the constant for the low index material is approximately 35. Accordingly, the invention provides a material with high refractive index, which is possibly suitable for the wavelength of 193nm, such as LaF3And Al2O3The result is a low refractive index material of predominantly MgF, which is potentially suitable for wavelengths of 193nm2And SiO2. The four materials were then subjected to experiments and test inversions to obtain complex refractive indices at 193nm and 248nm as shown in Table 1.
TABLE 1
As can be seen from Table 1, among the two high refractive index materials, although Al is present2O3The refractive index of the film is high and thus a large difference in refractive index between the high and low refractive index materials can be achieved, but unfortunately Al2O3The extinction coefficient k of (A) is too large, so LaF is preferable3(ii) a Of two low refractive index materials, MgF2Has a small refractive index and a small extinction coefficient k, and is preferably MgF2. Thus determining the LaF3And MgF2As are the preferred high and low index materials, respectively, of the present invention.
Secondly, designing an antireflection film with high transmissivity according to the determined material and complex refractive index. According to the conservation of energy: and T + R + L is 1, wherein T represents the transmittance of the antireflection film, R represents the residual reflectivity of the antireflection film, and L represents the absorption scattering loss of the antireflection film. In the visible region with longer wavelength, since L is small enough and often neglected, the transmittance can be close to 1 as long as the final design can make the reflectance small enough; however, the absorption scattering loss is large for wavelengths 193nm and 248nm, so that the transmittance can be maximized by achieving both low residual reflection at the substrate surface and low loss of the film. That is, the design in the long-wave low-loss region is only "antireflection" on the substrate surface, and is often called an antireflection film; in the deep ultraviolet of 193nm and 248nm, the design emphasis includes "antireflection" on the surface of the substrate and "low loss" of the film layer to obtain high transmittance, so the film is called an antireflection film, and is not called an antireflection film. In accordance with this idea, it is not sufficient to evaluate only the residual reflectance in the long-wavelength low-loss region to design an antireflection film while taking both the residual reflectance and the absorption scattering loss as components of the evaluation function.
According to the above idea, LaF material with high refractive index is selected3And a low refractive index material MgF2The results of designing antireflection films on four quartz substrates are shown in Table 2. From the design results, it can be found that: 1) the total number of layers of the high refractive index film and the low refractive index film of the antireflection film is an even number, wherein the odd number of layers is LaF with high refractive index3With even layers of MgF of low refractive index2(ii) a 2) As the total number of layers of the antireflection film increases, the total layer thickness increases accordingly, and the residual reflectance tends to decrease (which can be evaluated by the sum of two wavelengths), so that it is very beneficial from the viewpoint of the antireflection effect only, but since the total layer thickness tends to increase, the loss of the film increases sharply, and the final transmittance decreases on the contrary, which is an essential difference between the antireflection film of the present invention and the conventional antireflection film, in other words, at wavelengths of 193nm and 248nm, the residual reflectance cannot be considered only, and more importantly, the minimum film loss and the maximum transmittance are considered, which not only increases the transmission energy, but also increases the threshold of the antireflection film against laser damage; 3) at the wavelengths 193nm and 248nm, since the film loss is much greater than the residual reflectivity, it is reasonable to use the minimum film loss or the maximum transmittance as the criterion on the premise of ensuring sufficiently small residual reflection, and according to the criterion, the preferred scheme is to choose the 4-layer structure with the minimum total film thickness rather than the 6, 8 and 10-layer structure with the small residual reflection. Then, is the result of the 2-layer structure of the antireflection film better? The answer is negative because the 2-layer structure, although the film loss is certainly less than 4 layers, is insufficient to obtain an antireflection film of dual wavelengths due to too few design parameters, i.e., no solution for the 2-layer structure at the wavelengths of 193nm and 248 nm.
TABLE 2
And thirdly, how to manufacture the dual-wavelength antireflection film meeting the design requirement. In the deep ultraviolet, the most critical issue is film thickness monitoring. In the prior art, two methods are commonly used for monitoring the thickness of a deep ultraviolet film: firstly, the film thickness is obtained by controlling the film quality by adopting a quartz crystal, but the method has larger control error and is not very suitable; and secondly, a set of photoelectric film thickness control instrument is independently established, but due to the fact that a deep ultraviolet light source-optical system-receiver is completely different from a visible light region, cost is high, and time consumption is long. The invention proposes to solve this problem by a baffle method, if the central wavelengths of 193nm and 248nm are 220nm, the visible light control wavelength is 440nm, the baffle is just used to block half of the evaporated molecular flow, and the design and manufacture of the baffle are simple and accurate. Therefore, the original optical film thickness control system of the film plating machine does not need to be changed, and the film thickness control of the antireflection film in the deep ultraviolet region can be realized by using the photoelectric control of the existing visible light region only by adding a baffle plate below the substrate to be plated.
In order to achieve the purpose, the invention adopts the following specific technical scheme:
a dual-wavelength antireflection film for excimer laser comprises a substrate and a multilayer film which is sequentially arranged on the substrate and is formed by alternately arranging high-refractive-index films and low-refractive-index films;
the high refractive index film is lanthanum fluoride (LaF)3) The low refractive index film is magnesium fluoride (MgF)2)；
The number of layers of the multilayer film is 4-10.
In the invention, the dual-wavelength antireflection film for the excimer laser with the structure has smaller film loss and higher transmissivity, and is very suitable for an excimer laser instrument compatible with deep ultraviolet 193nm and 248 nm.
The following are preferred technical schemes of the invention:
the substrate is fused quartz or calcium fluoride (CaF)2)。
The double wavelengths are 193nm of argon fluoride laser wavelength and 248nm of krypton fluoride laser wavelength.
The high refractive index film LaF3Complex refractive index at wavelength 193nm of 1.72-i 7X 10－4Complex refractive index at wavelength 248nm of 1.68-i 3X 10－4；
The low refractive index film MgF2Complex refractive index at wavelength 193nm of 1.44-i 4.7X 10－4Complex refractive index at wavelength 248nm of 1.41-i 1X 10－4；
The total number of film layers (namely the total number of film layers of the high refractive index film and the low refractive index film) of the multilayer film is even, wherein the odd layers are lanthanum fluoride (LaF) from the substrate to the outside3) The even number layer is magnesium fluoride (MgF)2)。
The total number of the layers of the multilayer film is 10, the total thickness of the layers is 255.2nm, the layers face outwards from the substrate, and the odd layers are lanthanum fluoride (LaF)3) The even number layer is magnesium fluoride (MgF)2) The thicknesses of the 1 st to 10 th layers are as follows: 15.34, 23.17, 10.06, 34.50, 30.93, 21.16, 10.46, 40.17, 31.61, 37.80 in nm.
The total number of the layers of the multilayer film is 8, the total thickness of the layers is 194.6nm, the layers face outwards from the substrate, and the odd layers are lanthanum fluoride (LaF)3) The even number layer is magnesium fluoride (MgF)2) The thicknesses of the 1 st to 8 th layers are as follows: 5.03, 8.66, 40.64, 22.67, 3.92, 44.04, 31.94, 37.70 in nm.
The total number of the layers of the multilayer film is 6, the total thickness of the layers is 181.4nm, the layers face outwards from the substrate, and the odd layers are lanthanum fluoride (LaF)3) The even number layer is magnesium fluoride (MgF)2) The thicknesses of the 1 st to 6 th layers are as follows: 44.51, 24.15, 3.41, 40.89, 31.16, 37.23 in nm.
The total number of the layers of the multilayer film is 4, the total thickness of the layers is 169.2nm, the layers face outwards from the substrate, and the odd layers are lanthanum fluoride (LaF)3) The even number layer is magnesium fluoride (MgF)2) The thicknesses of the 1 st to 4 th layers are as follows: 16.47, 83.38, 30.31, 38.99 in nm.
The preferable scheme of the excimer laser dual-wavelength antireflection film is a 4-layer film structure, and the total thickness of a multilayer film is 169.2 nm; the thicknesses of all film layers from the substrate to the outside are as follows: 16.47, 83.38, 30.31, 38.99 in nm. This preferred solution makes available: single-sided transmission of 99.40% at a wavelength of 193nm (ArF); the single-side transmittance at a wavelength of 248nm (KrF) was 99.82%.
The manufacturing technical scheme of the excimer laser dual-wavelength antireflection film preferably adopts a baffle plate method so as to realize the film thickness control of the antireflection film in the deep ultraviolet region by using the original photoelectric extreme value control of the visible region. If the central wavelengths of 193nm and 248nm are 220nm, the visible light control wavelength of 440nm is selected to control the thickness of the film, and at the moment, half of the evaporated molecular flow is just blocked by the baffle plate, so that the design and the manufacture of the baffle plate are simple and accurate. Further, the 8-grade baffle plate is adopted to replace the 2-grade baffle plate, so that the film growth structure can be improved.
An optical film thickness monitoring system comprises a light source, a focusing lens, a film thickness monitoring sheet, a light filter, a receiver, an electron beam evaporation source, a thermal resistance evaporation source, a workpiece clamp disc and a plated substrate, wherein baffle plates are arranged between the electron beam evaporation source and the thermal resistance evaporation source and the plated substrate to shield a part of film material molecules when the electron beam evaporation source and the thermal resistance evaporation source respectively evaporate, and the required deep ultraviolet film thickness is obtained on the substrate.
The baffle is circular, and the middle of the baffle is provided with a baffle central round hole.
The baffle is divided by a bisection method, and a half of the baffle provided with the baffle central circular hole is cut off.
The baffle is divided by an octant method, the circumference of the baffle provided with the baffle central round hole is divided into eight equal parts, and four spaced parts are cut off from the eight parts.
Compared with the prior art, the invention has the beneficial effects that:
1) the excimer laser antireflection film in the prior art is only suitable for the condition of single wavelength, such as an excimer laser antireflection film with the wavelength of 193nm, an excimer laser antireflection film with the wavelength of 248nm and the like, and obviously, the antireflection films can not be suitable for the newly proposed excimer laser instrument with dual-wavelength of 193nm and 248 nm. However, not only is the difficulty of designing and manufacturing the dual-wavelength excimer laser antireflection film greatly increased, but also, as can be seen from fig. 2 and 4, the film loss of the dual-wavelength excimer laser antireflection film is much greater than that of a single-wavelength antireflection film, which is a great challenge to the laser damage of the film. The reason for this is mainly because the total thickness of the single-wavelength antireflection film can be relatively thin, for example, for a wavelength of 193nm, the total thickness of the antireflection film is only 110nm, which is reduced by about 1/3 from the thinnest total thickness of 169.2nm in the dual-wavelength antireflection film of the present invention. Obviously, designing and preparing the low-loss dual-wavelength antireflection film is extremely important for the application of deep ultraviolet excimer laser.
2) The antireflection film design of the prior art generally only evaluates the residual reflectivity, but in the deep ultraviolet excimer laser film of the present invention, it is impossible to obtain the lowest optical loss by evaluating only the residual reflectivity, and for this reason, the present invention proposes to use the minimum film loss or the maximum transmittance as a criterion for design evaluation on the premise of ensuring sufficiently small residual reflection. By using the criterion, the transmission energy of the deep ultraviolet excimer laser instrument can be increased, stray light can be reduced, and the contrast and the resolution can be improved.
3) The prior art for preparing the deep ultraviolet film usually adopts a quartz crystal film thickness controller, but the control principle of the instrument is based on the film quality to obtain the film thickness, so the instrument is closely related to the film density, that is, the film thickness is closely related to the preparation parameters, and larger film thickness errors are easily caused. For this reason, for high-precision film thickness control, it is necessary to obtain the optical thickness by employing photoelectric control based on optical interference. Unfortunately, in the deep ultraviolet band, the optoelectronic control system is completely different from the near ultraviolet and visible light regions, so it is a costly and time-consuming task to develop a new deep ultraviolet optoelectronic film thickness control system. The invention proposes to solve this problem by using a baffle method, wherein if the central wavelengths of 193nm and 248nm are 220nm, the visible light control wavelength is 440nm, and the baffle is used to block half of the evaporated molecular flow, so that the design and manufacture of the baffle are simple and accurate. Therefore, on the premise of not changing the original visible light film thickness control system of the film coating machine, the film thickness control of the antireflection film in the deep ultraviolet light region can be realized by using the photoelectric control of the visible light region as long as a baffle is added below the substrate to be coated.
Drawings
FIG. 1 is a test refractive index (a) and extinction coefficient (b) curve for a deep ultraviolet excimer laser antireflection film material, wherein (a) in FIG. 1 is a test refractive index curve for a deep ultraviolet excimer laser antireflection film material, and (b) in FIG. 1 is an extinction coefficient curve for a deep ultraviolet excimer laser antireflection film material;
FIG. 2 is a graph showing the reflectivity (solid line) and loss (dotted line) of a deep ultraviolet excimer laser single-wavelength antireflection film of the prior art, wherein (a) in FIG. 2 is a graph showing the reflectivity (solid line) and loss (dotted line) of a deep ultraviolet excimer laser single-wavelength antireflection film of the prior art at a wavelength of 193nm, and (b) in FIG. 2 is a graph showing the reflectivity (solid line) and loss (dotted line) of a deep ultraviolet excimer laser single-wavelength antireflection film of the prior art at a wavelength of 248 nm;
FIG. 3 is a diagram showing the relationship between the film thickness and refractive index of a dual-wavelength antireflection film for deep ultraviolet excimer laser according to the present invention, wherein (a) in FIG. 3 is a diagram showing the relationship between the film thickness and refractive index of a dual-wavelength antireflection film for deep ultraviolet excimer laser having 10 total film layers according to the present invention; FIG. 3(b) is a diagram showing the relationship between the film thickness and refractive index of the deep ultraviolet excimer laser dual-wavelength antireflection film having 8 total film layers according to the present invention; FIG. 3(c) is a diagram showing the relationship between the film thickness and refractive index of the deep ultraviolet excimer laser dual-wavelength antireflection film having 6 total film layers according to the present invention; FIG. 3(d) is a diagram showing the relationship between the film thickness and refractive index of the deep ultraviolet excimer laser dual-wavelength antireflection film having 4 total film layers according to the present invention;
FIG. 4 is a graph comparing the reflectance (solid line) and loss (dotted line) curves of the deep ultraviolet excimer laser dual wavelength antireflection film of the present invention; wherein, fig. 4(a) is a graph comparing the reflectivity (solid line) and loss (dotted line) curves of the deep ultraviolet excimer laser dual-wavelength antireflection film with 10 total film layers according to the present invention; FIG. 4(b) is a graph comparing the reflectance (solid line) and loss (dotted line) curves of the deep ultraviolet excimer laser dual wavelength antireflection film having 8 total film layers according to the present invention; FIG. 4(c) is a graph comparing the reflectance (solid line) and loss (dotted line) curves of the deep ultraviolet excimer laser dual wavelength antireflection film having 6 total film layers according to the present invention; FIG. 4(d) is a graph comparing the reflectance (solid line) and loss (dotted line) curves of the deep ultraviolet excimer laser dual wavelength antireflection film having 4 total film layers according to the present invention;
FIG. 5 is a graph showing a comparison of transmittance curves of the deep ultraviolet excimer laser dual wavelength antireflection film of the present invention; wherein, fig. 5(a) is a graph comparing transmittance curves of the deep ultraviolet excimer laser dual wavelength antireflection film with 10 total film layers according to the present invention; FIG. 5(b) is a graph showing a transmittance curve comparison of a deep ultraviolet excimer laser dual wavelength antireflection film having 8 total film layers according to the present invention; FIG. 5(c) is a graph showing a transmittance curve comparison of a deep ultraviolet excimer laser dual wavelength antireflection film having 6 total film layers according to the present invention; FIG. 5(d) is a graph showing a transmittance curve comparison of a deep ultraviolet excimer laser dual wavelength antireflection film having 4 total film layers according to the present invention;
FIG. 6 is a schematic diagram of the relative positions of a baffle and a substrate to be coated and the structure of an optical film thickness control system;
fig. 7 is a schematic view showing the shape of a baffle plate used in the present invention, wherein (a) in fig. 7 is a schematic view showing the structure of a baffle plate divided by a bisection method, and (b) in fig. 7 is a schematic view showing the structure of a baffle plate divided by an octadivision method.
Detailed Description
In order to implement the present invention, it is necessary to find a high refractive index thin film material and a low refractive index thin film material which are expected to be used in the deep ultraviolet band. Since the loss at a wavelength of 193nm is always larger than 248nm, the material can be applied to a material having a wavelength of 193nm, and can be applied to a wavelength of 248nm without fail. Thus, the LaF of the high-refractive-index material can be obtained by screening by taking the photon energy with the wavelength of 193nm and the band gap of the electronic forbidden band as the basis for primarily screening the loss of the material3And Al2O3While low refractive index materials are more abundant, e.g. MgF2、AlF3、CaF2And SiO2Etc. but MgF is selected based on a combination of mechanical, chemical and optical properties2And SiO2Is more reasonable. Then toThe two high refractive index materials and the two low refractive index materials are subjected to single-layer and multi-layer film evaporation experiments and optical constant test inversion, and fig. 1 shows refractive index (a) and extinction coefficient (b) curves obtained by testing the four thin film materials in the deep ultraviolet band, wherein the optical constants of the two high refractive index films at a wavelength of 193nm are as follows: for LaF3Refractive index of 1.72 and extinction coefficient of 7X 10－4(ii) a For Al2O3Refractive index of 1.83 and extinction coefficient of 6X 10－3. While the optical constants at a wavelength of 193nm for the two low refractive index films are: for MgF2Refractive index of 1.44 and extinction coefficient of 4.7X 10－4(ii) a To SiO2Refractive index of 1.57 and extinction coefficient of 5.6X 10－4. Similarly, the optical constants at a wavelength of 248nm of the two high refractive index films are as follows: for LaF3Refractive index of 1.68 and extinction coefficient of 3X 10－4(ii) a For Al2O3Refractive index of 1.78 and extinction coefficient of 1X 10－3. While the optical constants at a wavelength of 248nm for the two low refractive index films are: for MgF2Refractive index of 1.41 and extinction coefficient of 1X 10－4(ii) a To SiO2Refractive index of 1.53 and extinction coefficient of 1.1X 10－4. For high index films, a high index of refraction means that a small residual reflection can be obtained. From the optical constants tested, although Al2O3Has a refractive index greater than that of LaF3However, since it is much more important to obtain a low extinction coefficient (low loss) than a high refractive index, it is clearly preferable to use LaF with a much smaller extinction coefficient3. For low index films, a lower index of refraction means less residual reflection, apparently at MgF2And SiO2In the case of a similar extinction coefficient of (2), MgF having a lower refractive index is preferable2. In the following examples, LaF is used as a high refractive index material3And a low refractive index material MgF2The process is carried out.
FIG. 2 is a graph showing the reflectance (solid line) and loss (dotted line) curves of a deep ultraviolet excimer laser single-wavelength antireflection film of the prior art, wherein (a) is at a wavelength of 193nm and (b) is at a wavelength of 248 nm. Although the prior art single-wavelength antireflection film has a 2-layer structure, the residual reflection is still high and cannot be accepted, and a 3-layer structure can obtain satisfactory results, which is essentially different from a dual-wavelength antireflection film with an even number of total film layers. For a wavelength of 193nm, the total film thickness of the 3-layer structure is 110nm, the residual reflection is zero, and the loss is 0.379%; and for a wavelength of 248nm, the total film thickness of the 3-layer structure is 145nm, the residual reflection is zero, and the loss is 0.129%. Unfortunately, prior art single wavelength antireflection films do not meet the use requirements of the dual wavelength antireflection film of the present invention.
Similar to fig. 2, fig. 4 is a comparison of the reflectivity (solid line) and loss (dotted line) curves for a duel wavelength antireflective film of a deep ultraviolet excimer laser of the present invention, wherein (a) the number of film layers is 10, (b) is 8, (c) is 6, and (d) is 4. It should be noted that only 4 solutions are shown in fig. 4, and obviously, there are many more solutions, but since these solutions necessarily have more layers and thicker total thickness, the results will not be better than the 4 solutions shown, and thus it is not necessary to give them.
Example 1 of the invention: the number of the film layers is 10, wherein the odd number layers are high-refractive-index material LaF3The even number layer is MgF material with low refractive index2. The total thickness of the film layers is 255.2nm, and the thicknesses of the high-refractive index and low-refractive index alternate film layers are as follows from the substrate fused quartz: 15.34, 23.17, 10.06, 34.50, 30.93, 21.16, 10.46, 40.17, 31.61, 37.80 in nm, and fig. 3(a) shows the corresponding relationship between the film thickness and the refractive index. The residual reflectance and loss of this example 1 are shown in fig. 4(a), and the transmittance thereof is shown in fig. 5(a), in which the residual reflectance at a wavelength of 193nm is 0.001%, and the loss is 0.947%, so that the single-sided transmittance reaches 99.052%, and the residual reflectance at a wavelength of 248nm is 0.000%, and the loss is 0.266%, so that the single-sided transmittance reaches 99.734%. Obviously, the transmission loss of the present embodiment 1 is caused by the loss of the thin film, and the surface residual reflection is negligible.
Example 2 of the invention: the number of the film layers is 8, wherein the odd number layers are high-refractive-index material LaF3The even number layer is MgF material with low refractive index2. The total thickness of the film layer is 194.6nm, starting from the fused quartz substrate, the high refractive index and the low refractive index of each layerThe thickness of the alternate film layers is as follows: 5.03, 8.66, 40.64, 22.67, 3.92, 44.04, 31.94, 37.70 in nm, and fig. 3(b) shows the corresponding relationship between the film thickness and the refractive index. The residual reflectance and loss of this example 2 are shown in fig. 4(b), and the transmittance thereof is shown in fig. 5(b), in which the residual reflectance at a wavelength of 193nm is 0.006%, the loss is 0.749%, and thus the single-sided transmittance can reach 99.245%, and the residual reflectance at a wavelength of 248nm is 0.011%, the loss is 0.206%, and thus the single-sided transmittance can reach 99.783%. Obviously, the transmission loss of the present embodiment 2 is mainly caused by the loss of the thin film.
Example 3 of the invention: the number of the film layers is 6, wherein the odd number layers are high-refractive-index material LaF3The even number layer is MgF material with low refractive index2. The total thickness of the film layers is 181.4nm, and the thicknesses of the high-refractive index and low-refractive index alternate film layers are as follows from the substrate fused quartz: 44.51, 24.15, 3.41, 40.89, 31.16, 37.23 in nm, and fig. 3(c) shows the corresponding relationship between the film thickness and the refractive index. The residual reflectance and loss of this example 3 are shown in fig. 4(c), and the transmittance thereof is shown in fig. 5(c), where the residual reflectance at a wavelength of 193nm is 0.008% and the loss is 0.678%, so that the single-sided transmittance can reach 99.314%, and the residual reflectance at a wavelength of 248nm is 0.010% and the loss is 0.196%, so that the single-sided transmittance can reach 99.794%.
Example 4 of the invention: the number of the film layers is 4, wherein the odd number of the film layers are made of high-refractive-index material LaF3The even number layer is MgF material with low refractive index2. The total thickness of the film layers is 169.2nm, and the thicknesses of the high-refractive index and low-refractive index alternate film layers are as follows from the fused quartz substrate: 16.47, 83.38, 30.31, 38.99 in nm, and fig. 3(d) shows the corresponding relationship between the film thickness and the refractive index. The residual reflectance and loss of this example 4 are shown in fig. 4(d), and the transmittance thereof is shown in fig. 5(d), in which the residual reflectance at a wavelength of 193nm is 0.002%, and the loss is 0.601%, so that the single-sided transmittance can reach 99.397%, and the residual reflectance at a wavelength of 248nm is 0.028%, and the loss is 0.156%, so that the single-sided transmittance can reach 99.816%. It is apparent that this example 4 has the smallest film loss and the largest transmittance.
As can be seen from the above four embodiments: 1) as the number of layers of the total film of the dual-wavelength antireflection film is gradually reduced, the thickness of the total film is correspondingly reduced, and the residual reflectivity tends to gradually increase (evaluated by the sum of two wavelengths of 193nm and 248 nm); 2) the total film layer number is reduced, the total film layer thickness is reduced, the loss of the film is sharply reduced, and the transmissivity is increased. That is, at deep ultraviolet wavelengths, it is not possible to consider only obtaining a minimum residual reflectance, and more importantly, a minimum film loss to obtain a maximum transmittance, which not only increases the transmission energy of the film, but also increases the excimer laser damage resistance threshold of the film; 3) in the deep ultraviolet band, particularly at the wavelength of 193nm, since the film loss is far greater than the residual reflectivity, the preferred scheme is undoubtedly to select a 4-layer structure with the minimum total film thickness, rather than a 6, 8 and 10-layer structure with smaller residual reflection; 4) when the number of the film layers is more than 10, the contribution to reducing the residual reflection is very little, but the loss of the film layers is greatly increased, so that the number of the film layers cannot be more than 10; 5) when the number of the film layers is less than 4, no solution exists for the 193nm and 248nm dual-wavelength antireflection film.
The most critical issue in manufacturing the above-mentioned deep ultraviolet dual-wavelength antireflection film is how to perform film thickness monitoring, and so on for the simplest 4-layer film structure. As mentioned above, the prior art solves the problem of quartz crystal control of deep ultraviolet film thickness monitoring, and the film thickness control error is large; and a set of deep ultraviolet photoelectric film thickness control instrument is independently established, which is costly and time-consuming. Therefore, the invention provides a baffle plate method to solve the problem, so that the original optical film thickness control system of a film coating machine does not need to be changed, and the film thickness control in the deep ultraviolet band can be implemented by only adding one baffle plate below a substrate to be coated. FIG. 6 is a schematic diagram showing the relative positions of the baffle and the substrate to be coated and the configuration of the original optical film thickness control system. As shown in fig. 6, the optical film thickness monitoring system is mainly composed of a light source 6, a focusing lens 7, a film thickness monitoring sheet 8, a filter 9, a receiver 10, and the like. The thickness of each film is measured by the monitor sheet 8. In FIG. 6, 1 is an electron beam evaporation source, 2 is a thermal resistance evaporation source, and LaF, a high refractive index material, is sequentially evaporated3And a low refractive index material MgF2. QuiltThe plating base 3 is placed on the work holder tray 4 (the plating base 3 is actually placed around the work holder tray 4, not only the plating base 3 shown in the figure), and the characteristics of all the plating bases are the same because the work holder tray 4 is rotated rapidly during plating. The film thickness baffle 5 is closely arranged below the substrate 3 to be plated to block a part of molecules of the film material when the electron beam evaporation source 1 and the heat resistance evaporation source 2 respectively evaporate, and the required deep ultraviolet film thickness is obtained on the substrate.
FIG. 7 shows the shape and method of making baffles used in the present invention, wherein (a) is a bisection method and (b) is an octant method. If the central wavelengths of 193nm and 248nm are 220nm, the visible light control wavelength is 440nm, and the baffle just needs to block off half of the evaporated molecular flow of the film material, so that the baffle is very simple and accurate in design and manufacture. As shown in fig. 7(a), the baffle 5 has a diameter equal to that of the workpiece holder disk 4 so as to fit under the workpiece holder disk 4; the diameter of the central circular hole 11 of the baffle is equal to that of the film thickness monitoring sheet 8, so that the control light with the wavelength of 440nm can completely pass through the hole; half of the evaporated molecular flow is blocked, and only half of the blocking plate is halved and cut off, and the other half shadow area is reserved. When the film thickness on the film thickness monitor wafer 8 is 440nm, the thickness on the substrate 3 to be plated is 220 nm. The disadvantage of this bisection method is that the film growth affects the film growth structure due to the longer time interval, and to avoid or mitigate this effect, it is proposed to use an eighth-stage baffle, which is based on a 45 ° sector baffle, as shown in the shaded area of fig. 7(b), which allows the design and manufacture to remain simple and accurate, but alleviates the disadvantages of the bisection baffle.
Claims (4)
1. A dual-wavelength antireflection film for excimer laser is characterized by comprising a substrate and a multilayer film which is sequentially arranged on the substrate and is formed by alternately arranging high-refractive-index films and low-refractive-index films;
the high refractive index film is lanthanum fluoride, and the low refractive index film is magnesium fluoride;
the double wavelengths are 193nm of argon fluoride laser wavelength and 248nm of krypton fluoride laser wavelength;
the high foldRefractive index film lanthanum fluoride has a complex refractive index of 1.72-i 7 x 10 at a wavelength of 193nm－4Complex refractive index at wavelength 248nm of 1.68-i 3X 10－4；
The complex refractive index of the low-refractive-index film magnesium fluoride at 193nm is 1.44-i 4.7 multiplied by 10－4Complex refractive index at wavelength 248nm of 1.41-i 1X 10－4；
The total number of the film layers of the multilayer film is 10, the total thickness of the film layers is 255.2nm, from the substrate to the outside, the odd layers are lanthanum fluoride, the even layers are magnesium fluoride, and the thicknesses of the 1 st to 10 th layers are as follows: 15.34, 23.17, 10.06, 34.50, 30.93, 21.16, 10.46, 40.17, 31.61, 37.80 in nm, the 10 layer film having a residual reflectance at a wavelength of 193nm of 0.001%, a loss of 0.947%, a single-sided transmission of 99.052%, a residual reflectance at a wavelength of 248nm of 0.000%, a loss of 0.266%, and a single-sided transmission of 99.734%;
or the total number of the layers of the multilayer film is 8, the total thickness of the layers is 194.6nm, from the substrate to the outside, the odd layers are lanthanum fluoride, the even layers are magnesium fluoride, and the thicknesses of the 1 st to 8 th layers are as follows: 5.03, 8.66, 40.64, 22.67, 3.92, 44.04, 31.94, 37.70, in nm, the 8-layer film had a residual reflectance at 193nm of 0.006%, a loss of 0.749%, a single-sided transmission of 99.245%, a residual reflectance at 248nm of 0.011%, a loss of 0.206%, and a single-sided transmission of 99.783%;
or the total number of the layers of the multilayer film is 6, the total thickness of the layers is 181.4nm, from the substrate to the outside, the odd layers are lanthanum fluoride, the even layers are magnesium fluoride, and the thicknesses of the 1 st to 6 th layers are as follows: 44.51, 24.15, 3.41, 40.89, 31.16, 37.23 in nm, the 6-layer film has a residual reflectance of 0.008% at a wavelength of 193nm, a loss of 0.678%, a single-sided transmittance of 99.314%, a residual reflectance of 0.010% at a wavelength of 248nm, a loss of 0.196%, and a single-sided transmittance of 99.794%;
or the total number of the layers of the multilayer film is 4, the total thickness of the layers is 169.2nm, from the substrate to the outside, the odd layers are lanthanum fluoride, the even layers are magnesium fluoride, and the thicknesses of the 1 st to 4 th layers are as follows: 16.47, 83.38, 30.31, 38.99, in nm, the 4-layer film had a residual reflectance at 193nm of 0.002%, loss of 0.601%, a single-sided transmission of 99.397%, a residual reflectance at 248nm of 0.028%, loss of 0.156%, and a single-sided transmission of 99.816%.
2. The dual wavelength antireflective film for an excimer laser as claimed in claim 1, wherein the substrate is fused silica or calcium fluoride.
3. The method for preparing a dual wavelength antireflection film according to claim 1 or 2, wherein an optical film thickness monitoring system is used, the system comprising an electron beam evaporation source, a thermal resistance evaporation source, and a substrate to be coated, and a baffle plate is provided between the electron beam evaporation source and the thermal resistance evaporation source and the substrate to be coated.
4. The method of claim 3, wherein the baffle is circular, a central circular hole is formed in the middle of the baffle, and the baffle is divided by a bisection method or an octadivision method.
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| CN105163939A (en) * | 2013-03-01 | 2015-12-16 | V·莱普特新 | Anti-reflective coating |
| CN205944718U (en) * | 2016-07-29 | 2017-02-08 | 杭州科汀光学技术有限公司 | A dual wavelength reflection reducing coating and optical film thickness monitored control system for quasi -molecule laser |
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| JPS63113502A (en) * | 1986-10-31 | 1988-05-18 | Canon Inc | Reflection preventive film |
| JPS6461702A (en) * | 1987-09-01 | 1989-03-08 | Minolta Camera Kk | Antireflecting film |
| CN1570201A (en) * | 2004-05-09 | 2005-01-26 | 中国科学院上海光学精密机械研究所 | Method for plating ultraviolet band optical film by using baffle |
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