JP2773718B2 - Photomask and pattern forming method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a photomask for a projection exposure apparatus, and more particularly to a photomask used for forming a fine pattern in a semiconductor device manufacturing process.

[0002]

2. Description of the Related Art At present, in a manufacturing process of a semiconductor device, an optical lithography technique is mainly used to form a pattern on a semiconductor substrate. In optical lithography,
A photomask (an exposure original plate on which a pattern consisting of a transparent area and a light-shielding area is formed by a reduction projection exposure apparatus,
When the reduction ratio is not 1: 1, it is also called a reticle, but in this specification, each is called a photomask.) A predetermined pattern can be obtained on the photosensitive resin film by a pattern.

[0003] In the conventional optical lithography technology, mainly, development of an exposure apparatus, in particular, a high N of a projection lens.
The miniaturization of semiconductor elements has been dealt with by adopting A and shortening the wavelength λ of exposure light. Here, NA (numerical aperture) corresponds to how much light the lens can collect, and the larger this value, the more light that is spread is collected, and the better the performance of the lens. And the numerical aperture NA of this lens
As the wavelength of exposure light becomes shorter, the critical resolution of the photosensitive resin film becomes finer, and it becomes possible to form a fine pattern for manufacturing a semiconductor device using this as an etching mask. .

[0004]

As described above, the resolution is improved by increasing the numerical aperture of the lens and increasing the wavelength of the exposure light. As a result, it is theoretically possible to form a fine pattern.

However, by increasing the NA of the lens and shortening the wavelength of the exposure light, the depth of focus (the range in which the deviation of the focal position can be tolerated) is conversely reduced. Therefore, it is difficult to further reduce the size in terms of the depth of focus.

Accordingly, it is an object of the present invention to provide a photomask having an improved depth of focus, thereby enabling a fine pattern of a photosensitive resin film to be formed.

[0007]

A feature of the present invention is that a photomask having a predetermined pattern consisting of a transparent region and a light-shielding region is provided by a light-shielding film selectively formed on a transparent substrate. A first transparent region in contact with the light-shielding region and formed in a band along the light-shielding region;
And a second transparent region located between the light-shielding region and the first transparent region. The exposure light from the first transparent region is smaller than the exposure light from the second transparent region. The phase is advanced , and the second
Width W (μm) of the first transparent region reaching the transparent region
Is not less than 0.82 × (λ / NA) and 2.88 ×
(Lambda / NA) or less, (where, lambda ([mu] m) is the wavelength of the exposure light, NA is the numerical aperture of the lens) photomask near a
You. Or a pattern forming method using this photomask
It is in. Here, it is preferable that the exposure light from the first transparent region is configured to have a phase advanced by 3 to 30 degrees from the exposure light from the second transparent region.

A concave portion is formed by etching from the flat main surface of the transparent substrate, the portion of the concave portion is defined as the first transparent region, and the portion of the transparent substrate where the concave portion is not formed is defined as the second transparent region. It can be a transparent area. Alternatively, a transparent film is selectively formed on a flat main surface of a transparent substrate, and a portion of the transparent substrate where the light-shielding film and the transparent film are not formed is defined as the first transparent region, and the transparent film is formed. The portion of the transparent substrate that has been formed can be used as the second transparent region together with the transparent film.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

A photomask and a method of manufacturing the photomask according to a first embodiment of the present invention will be described with reference to the drawings.
Here, as the exposure apparatus, a reduction ratio of 5 times (mask pattern size: pattern size on imaging plane = 5: 1), NA
(Numerical aperture) = 0.6, σ (coherence factor) =
An i-line reduction projection exposure apparatus of 0.6 is used, and a mask isolated pattern having a width L of 1.75 μm for forming an isolated line pattern of 0.35 μm (on the image plane) on the photosensitive resin film is shown. Shall be. Here, the isolated pattern is not a periodic pattern such as line and space. For example, when the mask pattern is a light-shielding film pattern having a width L, no other light-shielding film pattern exists within a range of 2 × L from this pattern. Things. In addition, as the conditions of the simulation described later, in addition to this, the thickness of the photosensitive resin film is 1.0 μm and the upper surface of the semiconductor substrate is silicon (Si).

FIG. 1A is a plan view showing a photomask according to an embodiment, and FIG. 1B is a longitudinal sectional view taken along a line BB in FIG. 1A.

A photomask having a predetermined pattern consisting of a transparent region 10 and a light-shielding region 2 is formed by a light-shielding film 2 such as a chromium film selectively formed on a flat main surface of a transparent substrate 1 made of quartz glass. ing. The transparent region 10 is a region where the transmittance of the exposure light is 90% or more, and the light shielding region 2 is a region where the transmittance of the exposure light is approximately 0%. The photomask of the present embodiment further includes a first transparent region 11 in contact with the light-shielding region 2 and positioned along the light-shielding region 2 and having a width of W in a strip shape.
And a second transparent region 12 located between the light-shielding region 2 and the first transparent region 11 to form a transparent region 10. Exposure light from the first transparent region 11 The phase is advanced from the exposure light from the second transparent region 12.

That is, the width L is 1.75 due to the light shielding film 2.
An isolated line pattern 2 having a width of μm is formed. A transparent substrate having a width W of 0.875 μm is etched from the flat main surface from the isolated line pattern 2 to form a concave portion 3 having a depth D of 44 nm. Of the transparent region 11.
The refractive index n of the quartz glass as the transparent substrate 1 with respect to the i-line (wavelength λ = 365 nm) as the exposure light is 1.46.
Therefore, light around the isolated line pattern 2, that is, the first transparent region 11
Of the light passing through the second transparent region 12 outside the transparent transparent substrate, that is, the portion of the quartz transparent substrate which is not etched (the refractive index is 1 because the step portion is air). That is, when the phase of light passing through the second transparent region 12 is used as a reference, the first transparent region 1 around the isolated pattern
At 1, a phase difference of +20 degrees (a phase advanced by 20 degrees) occurs.

FIG. 2 shows a simulation result of the focus characteristic (relation between the focal position and the size of the photosensitive resin film pattern) of the photomask of the present invention shown in FIG. 1 (data obtained by connecting a solid line with a solid line).

FIG. 2 shows, as a comparison, a conventional photomask (in the present specification and drawings, the conventional photomask is not formed with the concave portion of FIG. 1, ie, does not generate a phase difference in the transparent region. This is a photomask in which a similar isolated line pattern is formed on the flat main surface of a quartz transparent substrate as it is) (data obtained by connecting ○ with a dotted line).

Here, "+/-" of the focal position is "+" in the direction in which the semiconductor substrate approaches the projection lens and "-" in the direction in which the semiconductor substrate moves away from the projection lens. This is the case where the focal point is at the center of the thickness of the resin film.

As shown in FIG. 2, when the photomask of the present invention is used, the focus position is on the negative side, and the focus characteristic of the isolated line pattern which has a large inclination when the conventional photomask is used is flattened. This allows accurate dimensional control. That is, even if the focal position is slightly shifted, a photosensitive resin film pattern having a size closer to an intended size is formed.

The reason why the focus characteristic is greatly inclined in the conventional mask is considered to be mainly because the photosensitive resin film has a finite thickness (normally 1 to 3 μm). That is, considering the light intensity on the image forming plane, it changes completely symmetrically depending on whether the focus position is positive or negative. Therefore, if the thickness of the photosensitive resin film is “0”, the focus characteristics are also symmetrical with respect to the positive and negative of the focal position. However, since the photosensitive resin film has a certain film thickness and a difference in the focal position within the film thickness occurs, the exposure state of the photosensitive resin film differs when the focal position is shifted in the positive or negative direction. Will be. In fact, when the thickness of the photosensitive resin film was changed, the slope of the focus characteristic also changed. The slope was reduced when the thickness was reduced, and increased when the thickness was increased.

Here, the fact that the focus characteristic can be corrected by the photomask of the present invention can be considered to be the same as the phenomenon that the focus characteristic is tilted when a phase error occurs in a general phase shift mask. Particularly, in a phase shift mask of the Shibuya-Levenson method, it is known that when a phase error occurs, the focus characteristic is inclined depending on the sign and magnitude of the phase error, and therefore, the phase error is suppressed within several% of the set value. Was needed. On the other hand, in the present invention, a phase error that has not occurred except in the phase shift mask is intentionally caused by improving the transparent region, and the focus characteristic is corrected by the controlled phase error.

Next, FIGS. 3 and 4 show the results obtained by simulating the cross-sectional shape of the photosensitive resin film pattern obtained by projecting and reducing exposure to the photosensitive resin film at each focal position using the photomask of the present invention, and developing. And the cross-sectional shape of the photosensitive resin film pattern to be formed is indicated by solid line hatching downward and to the right.

Similarly, FIGS. 5 and 6 show the results obtained by simulating the cross-sectional shape of a photosensitive resin film pattern obtained by projecting and reducing exposure to a photosensitive resin film at each focal position using a conventional photomask and developing. It is a figure, and the cross-sectional shape of the photosensitive resin film pattern to be formed is shown by hatching of a dotted line which is lower left.

In each of FIGS. 3 to 6, the horizontal axis represents the position X (nm) on the semiconductor substrate, and the vertical axis represents the position Z (nm) in the thickness direction of the photosensitive resin film. In addition, "+/-" of the focal position in each drawing is "+" in the direction in which the semiconductor substrate approaches the projection lens and "-" in the direction away from the projection lens, as in FIG. 0.0 μm is when the focal point is at the center of the thickness of the photosensitive resin film.

In both the conventional photomask shown in FIG. 6 and the photomask of the present invention shown in FIG.
When the thickness is 0.2 μm or more, the thickness (dimension Z) of the obtained photosensitive resin film pattern starts to decrease, and the angle of the side wall is also 90 degrees.
The photosensitive resin film pattern is used as an etching mask in the next etching step (for example, a wiring layer is formed by etching a conductive film under the photosensitive resin film pattern) because the photosensitive resin film pattern is used as an etching mask. Is not suitable for practical use. That is, the dimensional change in the etching increases.

On the other hand, if the dimensional change of the photosensitive resin film pattern is allowed to be up to 10%, the limit on the "-" side can be defined from FIGS. 5, 3 and FIG.

That is, in the conventional photomask, the depth of focus is 0.4 μm where the focal position is from −0.4 μm to 0.0 μm. On the other hand, in the photomask of the present invention, the focal depth is 0.6 μm in the focal position of −0.6 μm to 0.0 μm, and the effect of increasing the depth of focus by about 50% is obtained.

In the current manufacturing technique, the accuracy of the depth and width of the concave portion when forming the concave portion on the transparent mask substrate by etching is 2-3%. However, it is assumed that the depth and width of the concave portion are shifted by several%. The focus characteristics obtained also do not vary much. That is, even if the phase difference of 20 degrees in the above embodiment changes to this extent, almost no difference occurs. Therefore,
The photomask of the present invention has an extremely large permissible error at the time of mask production, and can easily produce an intended mask. It should be noted that this method can be used in combination with the conventional auxiliary pattern method. That is, fine auxiliary patterns are arranged on the left and right of the isolated line pattern, and a phase difference is generated between the periphery of the isolated pattern and the transmitted light outside the auxiliary pattern. For this purpose, for example, the transparent substrate between the isolated pattern and the auxiliary pattern is etched. In this combination method, the problem that the mask is difficult to manufacture still remains, but there is an advantage that the depth of focus can be further increased as compared with the conventional auxiliary pattern method.

FIG. 7 is a longitudinal sectional view showing a photomask according to another embodiment of the present invention. Note that, in FIG. 7, the same or similar portions as those in FIG.

In FIG. 7, the phase of the light transmitted through the first transmission area 11 near the isolated line pattern 2 is advanced by, for example, 20 degrees with respect to the phase of the light transmitted through the second transmission area 12. The transparent film 4 having a thickness T, such as a silicon dioxide film, is used for the second transmission region 12, and the same effect as in the photomask of FIG. 1 can be obtained in this case.

That is, in FIG. 7, a transparent film 4 is selectively formed on the flat main surface of the quartz transparent substrate 1, and a portion of the transparent substrate 1 where the light shielding film 2 and the transparent film 4 are not formed has a width W.
Of the transparent substrate 1 on which the transparent film 4 is formed becomes the second transparent region 12 together with the transparent film 4.

In the present invention, the effect of correcting the focus characteristic is achieved by the phase difference (the depth (D) of the concave portion or the film thickness of the transparent film) of the phase change region around the isolated line pattern, that is, the first transparent region with respect to the second transparent region. (T) and the width W of the first transparent region.

First, the influence on the width W of the first transparent region will be described. FIG. 8 shows the results of a simulation study of focus characteristics when the width W of the first transparent region of the photomask of the embodiment is changed. In the same figure, the focus characteristic of the width W = 0.875 μm on the photomask (0.175 μm on the image forming surface, that is, the surface of the photosensitive resin film) is indicated by a black circle, and the width W = 0.5 μm (the 0.
1 μm) is indicated by a triangle when the focus characteristic is narrowed,
The focus characteristic when the width W is increased to 1.75 μm (0.35 μm on the image plane) is indicated by a mark Δ.

When the width W on the mask is 0.5 μm and 1.75 μm, better focus characteristics are obtained as compared with the conventional photomask, but when the width W on the mask is 0.875 μm. The inclination of the focus characteristic has not been corrected.

In the present invention, the inclination of the focus characteristic is corrected by intentionally generating a phase error near the edge of the isolated pattern. The effect is reduced. On the other hand, if the width of the first transmission region becomes too wide, the intended phase error does not occur near the edge, and the effect is reduced. Therefore, there is an appropriate range for the width of the phase error region.

As a result of a simulation study of various cases under the exposure conditions described in the above embodiment, the width W = 0.5 μm and the width W = 1.75 μm shown in FIG. The upper limit is suitable.

Under the exposure conditions of the embodiment, the lens has an NA (numerical aperture) of 0.6, the exposure light used is i-line, and its λ (wavelength) is 365 nm = 0.365 μm. Therefore, the width W = 0.5 μm and the width W = 1.7 obtained under these exposure conditions.
Λ (μm) so that 5 μm can be applied to other exposure conditions
To normalize / NA, the lower limit W _L (μm) = 0 width W.
82 × (λ / NA), and the upper limit W _U (μm) of the width W =
2.88 × (λ / NA).

Next, the effect of the phase difference will be described. As for the phase difference, the focus characteristic changes as the phase error (difference between 0 and 180 degrees of the phase difference) increases, but if the phase error is too large, the width W of the first transmission region, which is the phase error region, becomes slightly smaller. Changes the focus characteristics greatly, the margin in mask fabrication decreases, and mask fabrication becomes difficult. On the other hand, various illumination methods are currently being studied and the coherent factors (σ) are also different, but as a result of examining various cases, the first condition for the exposure condition of the currently generally used reduction projection exposure apparatus is as follows. It is appropriate that the phase of the exposure light from the transparent area is advanced by 3 to 30 degrees with respect to the phase of the exposure light from the second transparent area. In order to obtain such a phase difference, in FIG. The depth D is determined, and the thickness T of the transparent film 4 is determined in FIG.

The refractive index of the quartz transparent substrate or the transparent film is n, the wavelength of the exposure light is λ (nm), and the phase difference is Δθ.
(Degrees), the depth D (nm) of the recess 3 or the thickness T (nm) of the transparent film 4 is λ {2 × (n−1)} ×
It is represented by {Δθ ÷ 180}.

For example, in the case of FIG. 1, the refractive index n of the quartz transparent substrate is 1.46, and the wavelength λ of the i-line of the exposure light used is 365 nm.
Since it is three times the lower limit D _L is 6.6nm next depth of depth or the recess of the recess to obtain a phase difference, 30 ° of the depth of the depth or the recess of the recess to obtain a phase difference upper limit D _U Is 6
6.1 nm. As in the case of the normal auxiliary pattern method, in the present method, the periodicity can be further improved and the depth of focus of the isolated line pattern can be increased by forming a larger number of phase change portions (a). That is, if steps (etched portions of the transparent substrate or transparent films) are periodically formed on the transparent substrate around the isolated line pattern,
The depth of focus further increases. The same effect can be obtained by narrowing the interval between the two phase change portions so that one light shielding portion is formed by the two phase change portions (b).

[0039]

As described above, in the photomask of the present invention, by providing a step in the transparent region, a phase error is intentionally generated to improve the focus characteristic of the isolated pattern.

Therefore, even under the exposure conditions of increasing the NA of the lens and shortening the wavelength of the exposure light in order to improve the resolution, the depth of focus is increased and the range in which the deviation of the focal position can be tolerated is widened. Thus, a predetermined fine resist pattern of the photosensitive resin film can be obtained, and a predetermined fine element pattern can be obtained on the semiconductor element using the resist pattern as an etching mask.

[Brief description of the drawings]

FIGS. 1A and 1B are diagrams showing a photomask according to an embodiment of the present invention, wherein FIG. 1A is a plan view and FIG. 1B is a longitudinal sectional view taken along the line BB of FIG.

FIG. 2 is a diagram showing focus characteristics of a photomask according to an embodiment of the present invention in comparison with a conventional photomask.

FIG. 3 is a diagram showing a cross-sectional shape of a photosensitive resin film pattern formed by a photomask according to the embodiment of the present invention.

FIG. 4 is a diagram showing a cross-sectional shape of a photosensitive resin film pattern formed by a photomask according to the embodiment of the present invention.

FIG. 5 is a diagram showing a cross-sectional shape of a photosensitive resin film pattern formed by a conventional photomask.

FIG. 6 is a diagram showing a cross-sectional shape of a photosensitive resin film pattern formed by a conventional photomask.

FIG. 7 is a longitudinal sectional view showing a photomask according to another embodiment of the present invention.

FIG. 8 is a diagram illustrating focus characteristics when the width W of the first transparent region of the photomask according to the embodiment of the present invention is changed.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 transparent substrate 2 light-shielding film, light-shielding region 3 concave portion 4 transparent film 10 transparent region 11 first transparent region 12 second transparent region

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-140743 (JP, A) JP-A-4-51151 (JP, A) JP-A-4-24637 (JP, A) JP-A-4- 29143 (JP, A) JP-A-6-266094 (JP, A) JP-A-3-259256 (JP, A) JP-A-4-50942 (JP, A) JP-A-4-50943 (JP, A) JP-A-3-269532 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G03F 1/08

Claims (5)

(57) [Claims] 1. In a photomask having a predetermined pattern including a transparent region and a light-shielding region by a light-shielding film selectively formed on a transparent substrate, the transparent region is in contact with the light-shielding region and the light-shielding region. A first transparent region formed in a band along the first transparent region, and a second transparent region located between the light shielding region and the first transparent region. The exposure light has a configuration in which the phase advances from the exposure light from the second transparent region , and
The first transparent region from the light-shielding region to the second transparent region
The range of the width W (μm) of the region is 0.82 × (λ / NA) ≦ W ≦ 2.88 × (λ / NA) (where λ (μm) is the wavelength of the exposure light, and NA is the aperture of the lens.
Photomask which is a number of units). 2. The photo-sensor according to claim 1, wherein the exposure light from said first transparent region has a phase advanced by 3 to 30 degrees from the exposure light from said second transparent region. mask. 3. A concave portion is formed by etching from a flat main surface of a transparent substrate, the portion of the concave portion is defined as the first transparent region, and the portion of the transparent substrate where the concave portion is not formed is defined as the second transparent region. Claims characterized by a transparent area
The photomask according to claim 1 or claim 2 . 4. A selectively transparent film is formed on the flat principal surface of the transparent substrate, and a portion of the transparent substrate on which the light-shielding film and the transparent film is not formed between the first transparent region, said transparent A portion of the transparent substrate on which a film is formed is the second transparent region together with the transparent film.
The photomask according to claim 1 . 5. A according to any one of claims 1 to 4
Pattern forming method using the photomask of the above.