JP2006059889A - Thin film, optical element having the same and film-forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin film where aberrations can be reduced, coping with a wide incident angle, while fully maintaining transmission performance, and to provide an optical element in which the thin film is formed on a surface. <P>SOLUTION: In the optical element, the thin film is formed on a reflecting face, on which incident light is reflected. In the optical element, thickness of the thin film is made smaller, as the incident angle of incident light on the reflection face becomes larger. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention generally relates to a thin film formed on a surface of a substrate such as an optical element, and an optical element having the thin film, and more particularly to a film formed on the surface of an optical element such as a lens or a mirror used in a semiconductor exposure apparatus. The present invention relates to a multilayer film. The present invention is suitable for a multilayer film intended to reduce aberration when light (for example, EUV light) is incident on an optical element with a wide incident angle.

  The semiconductor technology field has been rapidly developed along with the miniaturization of integrated circuit elements. Semiconductor technology is mainly composed of lithography technology and etching technology. Of these, miniaturization by lithography technology is achieved by shortening the exposure wavelength, increasing the numerical aperture, and changing the exposure method. In recent years, a projection lithography technique has been developed in which the exposure wavelength is an EUV wavelength (10 to 15 nm), which is about one-tenth of conventional ultraviolet rays. In the EUV wavelength region, light is strongly absorbed by the substance, and therefore a refractive optical system cannot be used for the projection optical system, and a reflective optical system is used. A multilayer film for improving the reflectance is formed on the reflecting surface.

  The multilayer film has a substantially periodic structure, and the reflectivity is improved according to the Bragg condition: 2d cos θ = mλ (d: period length, θ: incident angle, λ: exposure wavelength). There is an optimum period length for the incident angle. Therefore, an inclined film whose period length (that is, film thickness) is changed depending on the reflection position is used on the reflection surface having a large incident angle difference of light within the EUV light irradiation region. A film whose period length varies depending on the reflection position is referred to as a tilted film or a tilted film.

  Since the complex amplitude reflectance of the EUV light is changed by the multilayer film, the reflection phase is changed together with the reflectance of the EUV light on the reflection surface. Therefore, the final optical system needs to be designed in consideration of the thin film. In order to give the same aberration characteristic to the rotationally symmetric object position at this time, the film thickness function is often rotationally symmetric with respect to the optical axis. However, an example in which the rotationally symmetric axis of the thin film is decentered from the optical axis has also been proposed (see, for example, Patent Document 1).

  A projection optical system including a thin film is required to have a high transmittance, a high uniformity of the transmittance, and a small wavefront aberration. However, when a multilayer film is formed on a projection optical system that sufficiently eliminates aberrations for the purpose of improving reflectivity, the reflection position of the light changes and the reflection phase changes due to the thin film. to degrade. The film thickness distribution of the reflection film uses a polynomial function of the radius from the center of rotation, but in order to suppress the deterioration of wavefront aberration due to the formation of the thin film, an even function up to the second order is often used. When the inclination of the thin film can be expressed by a simple function in this way, most of the aberrations generated by the thin film are low-order. They are mainly low-order astigmatism except for the tilt component and the defocus component. When the aberration deteriorated by the thin film is low order, a certain degree of aberration correction is possible only by correcting the surface distance of the optical system.

In order to reduce the design aberration of the projection optical system on which the thin film is formed, for example, a method (method 1) of simultaneously optimizing the design parameter of the thin film and the design parameter of the optical system is conceivable. However, this method increases the number of parameters to be optimized and is not realistic. On the other hand, redesigning only the surface spacing is relatively easy. This is because the amount of movement of the redesigned surface change is small and the change in the incident angle of the light rays on each surface is negligible, so that the redesign of the surface spacing can be performed independently of the thin film design. Therefore, a method (method 2) of designing a thin film after designing an optical system without considering the thin film is also effective. Aberrations generated at this time are suppressed by redesigning only the surface interval of the projection optical system. Method 2 is designed so that the projection optical system, which is originally complicated in design, minimizes the aberration as much as possible without forming a thin film, and the aberration generated when the thin film is formed is finely adjusted (that is, the surface interval is adjusted). ) Comes from the idea of just correcting.
US Patent Application Publication No. 2003 / 099034A1

  However, in an EUV projection optical system as shown in FIG. 9, for example, the incident angle of light differs depending on the reflection position of the reflection surface of each mirror arranged in the optical system, so that the thin film formed on each reflection surface The resulting phase difference (here, synonymous with aberration) occurs. Thin film deposition causes most of the wavefront aberration to be tilted components, defocused components, and low-order astigmatism, but these components remain even after adjusting the surface spacing of the optical system. Exists.

  The present invention has been made in view of the above circumstances, and is a thin film capable of reducing aberration corresponding to a wide incident angle while maintaining sufficient transmittance performance, and an optical film on which the thin film is formed. It is an exemplary object to provide a device.

  In order to achieve the above object, an optical element as an exemplary aspect of the present invention is an optical element in which a thin film is formed on a reflective surface that reflects incident light, and an incident angle of incident light on the reflective surface is The film is formed such that the thickness of the thin film becomes thinner as the thickness increases.

  An optical element as another exemplary aspect of the present invention is an optical element in which a thin film is formed on a reflective surface that reflects incident light, and the thickness of the thin film corresponding to the central portion of the reflective surface is from the central portion. It is characterized by being formed thicker than the thickness of the thin film corresponding to the distant peripheral portion.

  In the above optical element, the film thickness may be formed in a distribution that is rotationally symmetric about the optical axis on the reflecting surface. The incident angle of the incident light with respect to the reflecting surface may be 4 ° or more and 15 ° or less. The thin film may be a multilayer film. The multilayer film may be configured by repeatedly stacking two layers including a silicon layer and a molybdenum layer. The number of repetitions may be 40.

  An exposure apparatus according to still another exemplary aspect of the present invention is an exposure apparatus that includes an illumination optical system that illuminates a reticle with light from a light source, and a projection optical system that projects a reticle pattern onto a substrate. At least one of the illumination optical system and the projection optical system has the optical element described above.

  A device manufacturing method according to still another exemplary aspect of the present invention includes a step of exposing a substrate by the exposure apparatus described above, and a step of performing a predetermined process on the exposed substrate.

  According to still another exemplary aspect of the present invention, a thin film is a thin film formed on a reflective surface of a reflective optical element that reflects incident light, and the thin film is formed as the incident angle of incident light on the reflective surface increases. The film is formed so as to be thin.

  According to still another exemplary aspect of the present invention, a thin film is a thin film formed on a reflective surface of a reflective optical element that reflects incident light, and a peripheral portion in which the film thickness of the central portion of the reflective surface is away from the central portion. It is characterized by being formed thicker than the film thickness of the part.

  A film forming method as still another exemplary aspect of the present invention is a film forming method in which a thin film is formed on a reflective surface of a reflective optical element that reflects incident light, and an incident angle of incident light on the reflective surface is large. The film is formed so that the thickness of the thin film becomes thinner as the time goes.

  A film forming method as still another exemplary aspect of the present invention is a film forming method in which a thin film is formed on a reflective surface of a reflective optical element that reflects incident light, and the film thickness of the central portion of the reflective surface is the center. The film is formed so as to be thicker than the film thickness in the peripheral part away from the part.

  Other objects and further features of the present invention will be made clear by embodiments described below with reference to the accompanying drawings.

  According to the present invention, by controlling the thickness of a thin film formed on the reflection surface of an optical element such as a mirror, an aberration can be generated on the reflection surface to cancel the aberration generated on another reflection surface. Therefore, wavefront aberration can be reduced as a whole optical system. Further, by configuring the thin film to have a rotationally symmetric distribution with respect to the optical axis, it is possible to maintain the rotational symmetry of aberration.

[Embodiment 1]
An optical element on which a thin film according to Embodiment 1 of the present invention is formed will be described with reference to the drawings. FIG. 1 is a cross-sectional view schematically showing a thin film according to Embodiment 1 of the present invention. As shown in FIG. 1, by forming a multilayer film on the optical element surface, a phase change occurs due to a change in the reflection position of the light beam. The phase change is expressed by the following equation (1) using the optical path length L1 (thick line portion in the figure), the optical path length L2 (double line portion in the figure), the incident angle φ, the period length d, and the number of periods n in the figure. Is represented by Here, the periodic length of the multilayer film at one reflection position is assumed to be constant for all layers in the depth direction. The number of periods n is a constant.

  Light rays from different angles of view have different incident angles with respect to the same reflection position, but the incident angle φ is defined as an average incident angle of light rays incident on the same reflection position. The phase change due to the thin film is the sum of the one that occurs based on the formula (1) and the one that originates from the thin film itself, but most of the component is occupied by the component based on the formula (1).

  As can be seen from Equation (1), the value of COSφ decreases as the incident angle increases (φ approaches 90 °). Therefore, in order to keep the phase change according to the equation (1) constant, it is necessary to increase the product nd of the periodic length and the number of periods (hereinafter referred to as the periodic product nd) as the incident angle at the reflection position increases. is there. That is, as shown in FIG. 2, it is necessary to make the thickness of the thin film in the peripheral portion of the optical element larger than the thickness of the thin film in the central portion.

  Here, the relationship between the incident angle and the periodic product nd is reversed. That is, as shown in FIG. 3, the thickness of the thin film in the peripheral portion of the optical element is made thinner than the thickness of the thin film in the central portion so that the phase is changed. By including a surface having a thin film as shown in FIG. 3 in the EUV projection optical system, the aberration of the entire system can be further reduced. The surface where the effect by this thin film is large is the following surface.

  In the present invention, the relationship between the incident angle and the periodic product nd is specified and the effect is exerted. Therefore, when assuming a rotationally symmetric inclined film on the optical axis, the incident angle is different on the same radius. It is difficult to specify the optimal periodic product. The surface on which the incident angle of light does not change so much on the same radius is a surface in which the light irradiation region does not include the optical axis (for example, in FIG. 4, the light irradiation region does not include the optical axis O. M1, M3, and M4 correspond to this.) Therefore, the present invention is more effective when applied to a surface where the light irradiation region does not include the optical axis.

  At the same time, it is more effective that the incident angle width is somewhat larger in the plane. FIG. 10 shows the incidence of incident light on the reflection surface when an attempt is made to generate a tilt of PV (Peak to Valley, that is, the difference between the maximum value and the minimum value) 0.6λ for a certain incident angle and incident angle width. A region (shaded region) in which the film thickness decreases as the corner increases is shown. For example, it is understood that the present invention should be used on a surface having an incident angle width of 4 ° or more in a projection optical system configured with an incident angle of incident light on the reflecting surface of 25 ° or less.

  The thin film according to the present invention aims to reduce the aberration of the entire optical system. On the other hand, when this thin film is formed on the reflective surface of the EUV projection optical system, its original purpose is to improve the reflectance of EUV light. Therefore, a reflective surface sensitive to EUV light reflection performance has a small degree of freedom in designing a thin film, and it is difficult to apply the thin film according to the present invention to such a reflective surface.

  Specifically, since the reflectance is generally small at the reflection position where the incident angle of light is large, the degree of freedom of the cycle length of the thin film is reduced in order to improve the reflectance. The thin film according to the present invention must generate an aberration within the range of the degree of freedom of the periodic product nd, and is difficult to exhibit the effect of reducing the aberration. Therefore, the surface to which the thin film according to the present invention is applied is preferably a surface whose incident angle does not exceed 15 °.

[Example]
The effect when the thin film according to the present invention is applied to an EUV projection optical system will be described. In this embodiment, an example in which the tilt component is reduced among the wavefront aberrations of the entire system will be described. An EUV projection optical system having a numerical aperture NA = 0.28 and a reduction ratio of 4: 1 is adopted. FIG. 4 shows an optical path diagram of the EUV projection optical system 1 according to this embodiment. A multilayer film having a gradient is formed on each optical surface (reflection surface) of an optical element (mirror) used in the EUV projection optical system 1. The multilayer film is composed of a 4.07 nm thick silicon (Si) layer and a 2.84 nm thick molybdenum (Mo) layer, and the two layers are stacked as 40 sets (40 cycles). The periodic length distribution is a rotationally symmetric distribution with the optical axis as the center, and is composed of a second-order polynomial as a function of the radius from the optical axis.

  Table 1 is a table showing design values of the EUV projection optical system 1 used in the present embodiment. A to J in the table represent aspheric coefficients, and the aspheric surface is defined by equation (2).

  Table 2 is a table showing each design value (first example) of the thin film used in this example. C0 to C2 in the table are coefficients representing the shape of the film thickness distribution of the gradient film, and the film thickness distribution of the gradient film is represented by equation (3).

  The first example is an example in which the thin film according to the present invention is applied to the first reflecting surface (M1). Table 3 is a table showing design values (second example) of thin films designed so that aberrations are reduced on each reflecting surface. The second example is illustrated for reference, and the thin film according to the present invention is not applied to the reflective surface.

  In this embodiment, a multilayer film is considered in which films having a predetermined equal period length are stacked by the number of periods. Therefore, the period length ratio and the film thickness ratio are equal. FIG. 5 shows the relationship between the radius from the optical axis of the first reflecting surface (M1), the distribution of film thickness (period length) with respect to the radius, and the incident angle distribution.

  Table 4 is a table showing the effects of the thin film according to the present invention compared to the first example and the second example. Table 4 shows a tilt component among aberrations generated on each reflecting surface. The unit of the values in the table is λ (13.5 nm), and it indicates how much aberration occurs with respect to the length of one wavelength of 13.5 nm. It can be seen that by applying the thin film according to the present invention to the first reflecting surface (M1), the tilt component of the first reflecting surface (M1) is generated so as to reduce the aberration of the entire system. Table 5 is a table showing the optical system performance (wavefront aberration, average value of transmittance, transmittance distribution) in the first example and the second example. It can be seen that there are no restrictions on the design, film formation, and creation of the optical system and thin film as the above effects are achieved by the present invention.

[Embodiment 2]
FIG. 6 is a view schematically showing an exposure apparatus according to Embodiment 2 of the present invention. The exposure apparatus S is for exposing a circuit pattern on a reticle 51 serving as an exposure original onto a wafer 52 serving as an object to be processed. The exposure apparatus S includes an illumination optical system 54 that guides light from an EUV light source 53 onto a reticle (mask) 51 and a projection optical system 55 that projects a circuit pattern image on the reticle 51 onto a wafer 52, for example. Is done.

  The illumination optical system 54 has mirrors 54a and 54b as optical elements. The projection optical system 55 also has mirrors 55a and 55b as optical elements. On these mirrors 54a, 54b, 55a, 55b, the thin film according to the first embodiment is formed. Therefore, the aberration of the mirror is reduced, and the exposure apparatus S can perform exposure with high accuracy.

[Embodiment 3]
Next, with reference to FIGS. 7 and 8, an embodiment of a device manufacturing method using the above-described exposure apparatus S will be described. FIG. 7 is a flowchart for explaining how to fabricate devices (ie, semiconductor chips such as IC and LSI, LCDs, CCDs, and the like). Here, the manufacture of a semiconductor chip will be described as an example. In step 101 (circuit design), a device circuit is designed. In step 102 (reticle fabrication), a reticle on which the designed circuit pattern is formed is fabricated. In step 103 (wafer manufacture), a wafer (substrate) is manufactured using a material such as silicon. Step 104 (wafer process) is called a pre-process, and an actual circuit is formed on the wafer by lithography using the reticle and wafer. Step 105 (assembly) is called a post-process, and is a process for forming a semiconductor chip using the wafer created in step 104, and includes processes such as an assembly process (dicing and bonding) and a packaging process (chip encapsulation). . In step 106 (inspection), inspections such as an operation confirmation test and a durability test of the semiconductor device created in step 105 are performed. Through these steps, the semiconductor device is completed and shipped (step 107).

  FIG. 8 is a detailed flowchart of the wafer process in Step 104. In step 111 (oxidation), the wafer surface is oxidized. In step 112 (CVD), an insulating film is formed on the surface of the wafer. In step 113 (electrode formation), an electrode is formed on the wafer by vapor deposition or the like. In step 114 (ion implantation), ions are implanted into the wafer. In step 115 (resist process), a photosensitive agent is applied to the wafer. Step 116 (exposure) uses the exposure apparatus S to expose a reticle circuit pattern onto the wafer. In step 117 (development), the exposed wafer is developed. In step 118 (etching), portions other than the developed resist image are removed. In step 119 (resist stripping), the resist that has become unnecessary after the etching is removed. By repeatedly performing these steps, multiple circuit patterns are formed on the wafer. According to the manufacturing method of the present embodiment, it is possible to manufacture a device with higher quality and higher integration than conventional devices at low cost.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these, A various deformation | transformation and change are possible within the range of the summary.

It is sectional drawing which shows typically the thin film which concerns on Embodiment 1 of this invention. It is sectional drawing explaining a mode that the thickness of the thin film of the peripheral part of an optical element was made thicker than the thickness of the thin film of a center part. It is sectional drawing explaining a mode that the thickness of the thin film of the peripheral part of an optical element was made thinner than the thickness of the thin film of a center part. It is an optical path figure of the EUV projection optical system which concerns on the Example of this invention. 5 is a graph showing the relationship between the radius from the optical axis of the first reflecting surface of the EUV projection optical system shown in FIG. 4, the distribution of the film thickness (period length) with respect to the radius, and the incident angle distribution. It is a schematic block diagram of the exposure apparatus which concerns on Embodiment 2 of this invention. It is a flowchart for demonstrating the device manufacturing method by the exposure apparatus shown in FIG. It is a detailed flowchart of step 104 shown in FIG. It is an optical path figure of the conventional EUV projection optical system. 6 is a graph showing a region where the thickness of the thin film becomes thinner as the incident angle of incident light on the reflecting surface increases when an attempt is made to generate a PV0.6λ tilt.

Explanation of symbols

S: exposure apparatus 1: EUV projection optical systems M1 to M6: reflecting surfaces L1, L2: optical path length 51: reticle 52: wafer (object to be processed)
53: EUV light source 54: illumination optical system 54a, 54b: mirror 55: projection optical system 55a, 55b: mirror

Claims (13)

An optical element in which a thin film is formed on a reflecting surface that reflects incident light,
The optical element is formed such that the film thickness of the thin film decreases as the incident angle of the incident light on the reflecting surface increases. An optical element in which a thin film is formed on a reflecting surface that reflects incident light,
An optical element, wherein a thin film corresponding to a central portion of the reflecting surface is formed thicker than a thin film corresponding to a peripheral portion away from the central portion.   3. The optical element according to claim 1, wherein the film thickness is formed in a distribution that is rotationally symmetric about the optical axis of the reflecting surface. 4.   The optical element according to claim 1, wherein an incident angle of the incident light with respect to the reflecting surface is 4 ° or more and 15 ° or less.   The optical element according to claim 1, wherein the thin film is a multilayer film.   6. The optical element according to claim 5, wherein the multilayer film is configured by repeatedly laminating two layers including a silicon layer and a molybdenum layer.   The optical element according to claim 6, wherein the number of repetitions is 40. An illumination optical system that illuminates the reticle with light from a light source;
An exposure apparatus comprising: a projection optical system that projects the reticle pattern onto a substrate;
An exposure apparatus, wherein at least one of the illumination optical system and the projection optical system includes the optical element according to any one of claims 1 to 7. Exposing the substrate with the exposure apparatus according to claim 8;
And a step of performing a predetermined process on the exposed substrate. A thin film formed on a reflective surface of a reflective optical element that reflects incident light,
A thin film, characterized in that the thin film is formed so that the film thickness of the thin film decreases as the incident angle of the incident light on the reflecting surface increases. A thin film formed on a reflective surface of a reflective optical element that reflects incident light,
A thin film characterized in that a film thickness of a central part of the reflecting surface is formed thicker than a film thickness of a peripheral part away from the central part. A film forming method for forming a thin film on a reflective surface of a reflective optical element that reflects incident light,
The film forming method is characterized in that the film is formed such that the film thickness of the thin film becomes thinner as the incident angle of the incident light on the reflecting surface increases. A film forming method for forming a thin film on a reflective surface of a reflective optical element that reflects incident light,
A film forming method comprising forming a film so that a film thickness of a central part of the reflecting surface is larger than a film thickness of a peripheral part away from the central part.