JP2006058023A - Thin film, optical element with it, and method for forming film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin film which can reduce liquid level aberration while sufficiently maintaining a reflectivity performance, and also provide an optical element with the thin film formed on its surface. <P>SOLUTION: In this optical element, the thin film is formed on the reflective surface reflecting incident light. The thicknesses of the thin films distribute in rotational symmetry centering around a line which passes through the center of the reflective surface and is perpendicular to it and also distribute so as to include an odd function to a radius centering around a rotational symmetry axis. <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. In the projection optical system, the ratio of the outgoing light quantity (the light quantity that has passed through the projection optical system) to the incident light quantity is called the total system transmittance or transmittance, and high reflectance performance (transmittance performance) means that the total system transmittance Is high, the transmittance is uniformly distributed in the pupil, and there is no difference in transmittance for each object height.

  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.

When the film thickness distribution of the multilayer film has many inflection points in the radial direction, the surface shape of the reflecting surface also becomes a wavy shape with many inflection points, and the wavefront aberration becomes large. Therefore, the function of the film thickness distribution is preferably a function having few arrangement points. For example, when the film thickness distribution function is a polynomial function of the radius from the rotation center, the highest order is preferably about the second order or the fourth order. However, in this case, since the number of function parameters is reduced, it is difficult to increase the entire system transmittance by making the distribution of the transmittance in the pupil uniform.
US Patent Application Publication No. 2003 / 099034A1

  However, in an EUV projection optical system as shown in FIG. 13, for example, light is incident obliquely on each mirror arranged in the optical system, so that the incident angle distribution of the light tends to spread in the object axis direction. is there. Here, the object axis direction is the Y direction in the figure orthogonal to the optical axis O. The reflection phase of the light on each reflecting surface depends on the incident angle of the light with respect to each reflecting surface, and since the equiphase surface on the pupil of the optical system corresponds to the wavefront, when a multilayer film is formed with the EUV projection optical system, The slope of the wavefront, that is, the tilt component of the wavefront, may increase. The tilt component is divided into a positional shift of the light image formation position and a defocus component of the image plane. In general, in an optical system in which an object is located away from the optical axis, the displacement of the imaging position due to tilt can be corrected by magnification correction. If the object has a size and the magnification correction is performed according to the center position, the image position movement amount due to the magnification correction and the image position displacement amount due to the tilt are slightly different at other object positions other than the center position. End up. For this reason, the tilt effect cannot be corrected by only the magnification correction, and the distortion may remain as a problem. In addition, the reflecting film is required to improve the reflectivity of each surface while keeping the wavefront aberration of the projection optical system small. To improve the reflectivity, the surface shape (tilt) of the reflecting film is complicated. As a result, the wavefront aberration often deteriorates because the reflection phase has a shape with a high period with respect to the position, and it is difficult to produce a reflective film having both high reflectivity performance and wavefront aberration performance. It was.

  The present invention has been made in view of the above circumstances, and provides a thin film capable of reducing wavefront aberration while sufficiently maintaining reflectance performance, and an optical element having the thin film formed on the surface thereof. For illustrative purposes.

  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 the film thickness of the thin film is the center of the reflective surface. And is distributed so as to be rotationally symmetric about a line perpendicular to the reflecting surface and to include an odd function with respect to a radius centered on the rotationally symmetric axis.

  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 optical system according to another exemplary aspect of the present invention includes the above-described optical element, and a reflection region that reflects incident light on a reflection surface of the optical element does not include an optical axis.

  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.

  A thin film as still another exemplary aspect of the present invention is a thin film formed on a reflective surface of a reflective optical element that reflects incident light, and the thickness of the thin film passes through the center of the reflective surface to the reflective surface. It is distributed so as to be rotationally symmetric about a vertical line, and to be distributed so as to include an odd function with respect to a radius centered on the rotationally symmetric axis.

  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 thin film is the center of the reflective surface. And distributed in a rotationally symmetric manner around a line perpendicular to the reflecting surface and including an odd function with respect to a radius around the rotationally symmetric axis.

  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, it is possible to suppress the tilt component generated by the film by adding not only the conventional even function but also an odd function as a function of the film thickness, and the degree of freedom in the design of the optical system arises, resulting in reflectivity performance. Can be improved.

[Embodiment 1]
The thin film according to Embodiment 1 of the present invention will be described below. This thin film is a reflective film formed on the surface of a reflective mirror used in a reflective projection optical system. The design of the reflective projection optical system can be roughly divided into three stages. The first stage is a stage in which an optical system with high imaging performance is designed without considering the reflective film. All other performance, form, and specifications are determined without considering the reflectance of the irradiated light. The second stage is a stage in which a reflective film with high reflectivity performance is designed. At this stage, the design value of the projection optical system is not changed, and the specification of the reflection film is determined so that the reflection performance is improved. The third stage is a stage in which the imaging performance deteriorated by adding the reflective film is corrected by the optical system.

  In the present invention, by adding not only the conventional even function but also an odd function as a function of the film thickness, it is possible to suppress the tilt component generated by the film (first effect), and there is a degree of freedom in design. The effect that the reflectance performance can be improved (second effect) can be achieved.

  The film thickness distribution of the reflective film is a rotationally symmetric distribution shape about a predetermined rotational symmetry axis. When the film thickness at the position of the radius r from the rotational symmetry axis of the function of the film thickness distribution (hereinafter referred to as the film thickness function) is d, d is expressed as a function of the absolute value of the radius r. For example, when the film thickness function is composed of a function of a second order or lower (including an odd function) of a polynomial, the relationship between the film thickness d and the radius r is expressed by the following equation (1). Here, C0, C1, and C2 represent coefficients.

... (1)
As can be seen from equation (1), the film thickness function of the reflective film including the odd function is not smooth at the rotation center position, or the differential coefficient is discontinuous. In this case, the differential coefficient of the shape of the reflected wavefront is also discontinuous at the rotation center position. Therefore, the C1 term is not used on the reflection surface that uses the rotation center position of the reflection film as the light irradiation region (reflection region). That is, C1 = 0. FIG. 1 shows an example in which the case of using the term C1 is compared with the case of not using it. FIG. 1A is a schematic diagram showing an example of the thickness distribution shape of the reflective film when the term C1 is used, and FIG. 1B is a case where the term C1 is not used (that is, C1 = 0). It is a schematic diagram which shows the example of the film thickness distribution shape of a reflective film of (in the case of).

[Example 1]
The effect when the thin film (reflection film) according to the present invention is applied to an EUV projection optical 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. The wavelength of EUV light is 13.5 nm. FIG. 2 shows an optical path diagram of the EUV projection optical system 1 according to the first 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.

  In Example 1, the rotational symmetry axis of each reflective film coincides with the optical axis O. The reflecting surfaces of the mirrors constituting the EUV projection optical system are arranged in order from the mask MS as the projection original plate toward the wafer W as the projection target in order from the first reflecting surface M1, the second reflecting surface M2, and the third reflecting surface. A surface M3, a fourth reflecting surface M4, a fifth reflecting surface M5, and a sixth reflecting surface M6 are used. Since the first reflecting surface M1, the third reflecting surface M3, and the fourth reflecting surface M4 do not include the rotation center position of the reflecting film in the reflecting region, these reflecting surfaces have a film thickness distribution composed of an even function and an odd function. Apply a graded membrane. The film thickness d at the position of the radius r from the rotational symmetry axis is expressed by the following formula (2). Here, C0, C1, and C2 are coefficients, and i is a reflection surface number.

... (2)
In addition, since the second reflecting surface M2, the fifth reflecting surface M5, and the sixth reflecting surface M6 include the rotational center position of the reflecting film in the reflecting region, these reflecting surfaces have an inclined film having a film thickness distribution composed of an even function. Apply. The film thickness d at the position of the radius r from the rotational symmetry axis is expressed by the following formula (3). Here, C0, C1, and C2 are coefficients, and i is a reflection surface number.

... (3)
Table 1 is a table showing design values of the reflecting surfaces of the mirrors used in the EUV projection optical system 1. The first reflecting surface M1, the third reflecting surface M3, and the fourth reflecting surface M4 have a first-order term (that is, C1 ≠ 0 and include an odd function), the second reflecting surface M2, the second reflecting surface M2, and the second reflecting surface M2. The fifth reflecting surface M5 and the sixth reflecting surface M6 do not have a primary term (that is, C1 = 0). A to G in the table indicate aspheric coefficients, and C0 to C1 correspond to the coefficients shown in Expression (2) and Expression (3).

Further, in order to confirm the effect of the EUV projection optical system 1 of Example 1 including the term C1 (primary term), the first reflecting surface M1, the third reflecting surface M3, and the fourth reflecting surface M4 are used as Comparative Example 1. It was assumed that there was no C1 term in each film thickness function. Table 2 shows design values of the optical system of Comparative Example 1. In the comparative example 1, the gradient function of the reflective film and the distance between the reflective surfaces are changed with respect to the EUV projection optical system 1 described above.

The tilt of wavefront aberration in each pupil of the EUV projection optical system 1 as Example 1 shown in Table 1 and the optical system as Comparative Example 1 shown in Table 2 is shown in FIG. 3, and the reflectance performance is shown in FIGS. It was shown to. FIG. 4 shows an average value of transmittance at an object height (125 to 135 mm). 5 and 6 show the distribution of transmittance for each object height in RMS and PV / MAX, respectively. By adding a first-order term to the inclination of the reflective film, it can be confirmed that the tilt can be reduced without deteriorating the reflectance performance.

[Example 2]
Considering an odd function in the film thickness function of the reflective film increases the parameters, so that the reflectance performance can be improved. In the second embodiment, as in the first embodiment, an inclined film having a film thickness distribution including an odd function is applied to the first reflecting surface M1, the third reflecting surface M3, and the fourth reflecting surface M4 of the EUV projection optical system 1. This is to improve the reflectance. Table 3 is a table showing design values of the reflecting surfaces of the mirrors according to Example 2. As in the case of the first embodiment, the second reflecting surface M2, the fifth reflecting surface M5, and the sixth reflecting surface M6 do not have a primary term (that is, C1 = 0). A to G in the table indicate aspheric coefficients, and C0 to C1 correspond to the coefficients shown in Expression (2) and Expression (3).

As Comparative Example 2 with respect to Example 2, it was assumed that there was no C1 term in each film thickness function of the first reflecting surface M1, the third reflecting surface M3, and the fourth reflecting surface M4. Table 4 shows design values of the optical system of Comparative Example 2.

The reflectance performance of the EUV projection optical system 1 as Example 2 shown in Table 3 and the optical system as Comparative Example 2 shown in Table 4 are shown in FIGS. FIG. 7 shows an average value of transmittance at an object height (125 to 135 mm). FIGS. 8 and 9 show the transmittance distribution for each object height in RMS and PV / MAX, respectively. It can be confirmed that the reflectance performance can be improved without deteriorating the wavefront aberration by adding the first order term to the inclination of the reflective film.

[Embodiment 2]
FIG. 10 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, an embodiment of a device manufacturing method using the above-described exposure apparatus S will be described with reference to FIGS. FIG. 11 is a flowchart for explaining how to fabricate devices (ie, semiconductor chips such as IC and LSI, LCDs, CCDs, etc.). 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. 12 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 a figure for demonstrating the film thickness distribution of the reflecting film which concerns on Embodiment 1 of this invention, Comprising: (a) is a schematic diagram which shows the case where the term of C1 is used for a film thickness function, (b ) Is a schematic diagram showing a case where the C1 term is not used in the film thickness function. It is an optical path figure of the EUV projection optical system which concerns on the Example of this invention. 6 is a graph showing tilt of wavefront aberration in each pupil of Example 1 and Comparative Example 1. It is the graph which showed the reflectance performance in each pupil of Example 1 and Comparative Example 1, and shows the average value of the transmittance | permeability in object height (125-135 mm). It is the graph which showed the reflectance performance in each pupil of Example 1 and Comparative Example 1, and shows the distribution of transmittance for each object height in RMS. It is the graph which showed the reflectance performance in each pupil of Example 1 and Comparative Example 1, and shows the distribution of transmittance for each object height in PV / MAX. It is the graph which showed the reflectance performance in each pupil of Example 2 and Comparative Example 2, and shows the average value of the transmittance | permeability in object height (125-135 mm). It is the graph which showed the reflectance performance in each pupil of Example 2 and Comparative Example 2, and showed the distribution of the transmittance | permeability for every object height by RMS. It is the graph which showed the reflectance performance in each pupil of Example 2 and Comparative Example 2, and shows the transmittance distribution for each object height in PV / MAX. 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. 12 is a detailed flowchart of step 104 shown in FIG. 11. It is an optical path figure of the conventional EUV projection optical system.

Explanation of symbols

S: exposure apparatus 1: EUV projection optical systems M1 to M6: reflecting surface 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 (9)

An optical element in which a thin film is formed on a reflecting surface that reflects incident light,
The film thickness of the thin film is distributed rotationally symmetrically about a line passing through the center of the reflecting surface and perpendicular to the reflecting surface; and
An optical element that is distributed so as to include an odd function with respect to a radius around the rotational symmetry axis.   The optical element according to claim 1, wherein the thin film is a multilayer film.   The optical element according to claim 2, 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 3, wherein the number of repetitions is 40.   5. The optical element according to claim 1, wherein a reflection region that reflects incident light out of a reflection surface of the optical element does not include the optical axis. Optical system. 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 4. Exposing the substrate with the exposure apparatus according to claim 6;
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,
The film thickness is distributed rotationally symmetrically about a line that passes through the center of the reflecting surface and is perpendicular to the reflecting surface; and
A thin film distributed so as to include an odd function with respect to a radius around the optical axis. A film forming method for forming a thin film on a reflective surface of a reflective optical element that reflects incident light,
The thickness of the thin film is distributed rotationally symmetrically about a line passing through the center of the reflecting surface and perpendicular to the reflecting surface; and
A film forming method, wherein an odd function is distributed with respect to a radius around the rotational symmetry axis.