JPH03262902A - Focusing method - Google Patents

Abstract

PURPOSE:To accurately detect the defocusing of a projection lens on the surface of a wafer by converging light beams from masks which are generated by light irradiation on wafer marks formed on the wafer and causing them to interfere with each other, and detecting the reflected and diffracted light. CONSTITUTION:Mask marks 11 and 12 provided on a mask 10 at a specific distance l are irradiated with 1st and 2nd light beams and the two diffracted light beams generated by the marks 11 and 12 are converged on a mark 31 on a wafer 30 through a projection lens 20 to interfere with each other; and the diffracted light from the mark 31 is detected to obtain a 1st optical heterodyne detection signal of frequency (f1-f2), and the position shift between the mark and wafer can be detected. Further, while the position shift is corrected, one mark 12 of the mask 10 is irradiated with a 3rd light beam and an intermediate point between the marks 11 and 12 is irradiated with a 4th light beam; and the diffracted light from the mark 12 and the 4th light beam are converged on the mark 31 through a lens 20 to interfere with each other and thus a 2nd optical heterodyne detection signal of frequency (f1-f2) is obtained to detect the defocusing on the wafer surface.

Description

Detailed Description of the Invention [Objective of the Invention] (Industrial Field of Application) The present invention is a projection exposure apparatus that projects and exposes a pattern formed on a mask onto a wafer. This invention relates to a focusing method that detects and corrects the

(Prior Art) Conventionally, in a projection exposure apparatus used for pattern transfer,
As a method for detecting the defocus of a wafer with respect to a lens system, the method shown in FIG. 3 is known. In this method, when light emitted from a light source is obliquely incident on the wafer surface, the reflected light is received by a PSD (semiconductor device detector).

As the wafer surface moves in the vertical direction, the position of the reflected light on the PSD changes. Therefore, since the output of the PSD changes, the vertical position of the wafer surface can be detected. In the figure, 1 is a mask (reticle), 2 is a projection lens, 3 is a wafer, 4 is a light source, 5 is a PSD, 6 is a Z table, 7 is a two-direction actuator, and 8 is an X table.

However, this type of method has the following problems. That is, the light used for vertical position detection is directly incident on the wafer without passing through a projection lens, and the reflected light is directly detected by the PSD. Therefore, although the vertical position of the wafer with respect to the detection optical system can be detected, it cannot be said that the positional deviation (focal deviation) with respect to the imaging point of the projection lens can be reliably measured. Therefore, it is difficult to transfer a mask pattern onto a wafer with good resolution.

(Problem to be Solved by the Invention) In this conventional method, the wafer surface is irradiated with light obliquely and the reflected light is detected by a PSD to determine the focal shift. It was difficult to transfer a pattern with good resolution onto the wafer surface.

The present invention has been made in consideration of the above circumstances, and its purpose is to accurately detect the defocus of the wafer surface with respect to the projection lens, and to provide a focal point that can contribute to improving pattern transfer accuracy, etc. The goal is to provide a matching method.

[Structure of the Invention] (Means for Solving the Problems) The gist of the present invention is to detect defocus on the wafer surface indirectly without passing through a projection lens, but to
The purpose of this method is to directly detect defocus on a wafer surface using a rough the lens method.

That is, the present invention provides a focusing method that optically detects the defocus of the wafer with respect to the projection lens and corrects the defocus prior to projecting and exposing the pattern formed on the mask onto the wafer through the projection lens. , with the relative positions of the mask and wafer aligned, different frequencies f1゜f
irradiating the first and second lights modulated by 2 at different positions on the mask at different incident angles, and at least one of the lights being irradiated onto a mask mark consisting of a diffraction grating formed on the mask,
The light from the mask resulting from these light irradiations is focused and interfered with each wafer mark made of a diffraction grating formed on the wafer through a projection lens, and the reflected diffracted light from the wafer mark is detected (f + f2). In this method, an optical heterodyne detection signal having a frequency of 1 is obtained, and wafer defocus information is obtained based on this detection signal.

The present invention also provides a focusing method that optically detects the defocus of the wafer with respect to the projection lens and corrects the defocus prior to projecting and exposing the pattern formed on the mask onto the wafer through the projection lens. A wafer mark made of a diffraction grating is provided on the wafer, and first and second mask marks made of diffraction gratings are provided at at least two locations on the mask, respectively, and the first and second lights modulated at different frequencies ft and f2 are provided. are incident on different marks on the mask at the same angle, and a third light with the same frequency f2 as the second light and a different plane of polarization is made incident on the same position at the same angle as the second light. A fourth light beam having the same frequency f1 and the same polarization plane as the third light beam is incident on an intermediate position between the first and second light beams, and the diffracted light from each mask mark due to the irradiation of the first and second light beams is A portion of the light is focused and interfered on each wafer mark through a projection lens, and the reflected diffracted light from the wafer mark is detected and a first beam having a frequency of (fl f2) is detected.
Based on this optical heterodyne detection signal, the relative positional shift information of each mark is determined, and the light from the mask caused by the third and fourth light irradiation is applied to each wafer mark through a projection lens. The reflected and diffracted light from the wafer mark is detected (flf
In this method, a second optical heterodyne detection signal having the frequency 2) is obtained, and wafer defocus information is obtained based on the first and second optical heterodyne detection signals.

(Function) According to the present invention, since the defocus on the wafer surface is detected by the TTL method, the defocus can be detected more accurately than the conventional method using PSD. In other words, the conventional method can accurately detect the vertical position of the wafer surface with respect to the detection optical system, but if there is misalignment (vertical misalignment) between the detection optical system and the projection lens system, this misalignment will cause the wafer surface to change. An error occurs in vertical position detection. On the other hand, since the present invention uses the TTL method, the above-mentioned error does not occur due to its principle.

Therefore, according to the present invention, it is possible to accurately detect defocus on the wafer surface.

(Example) Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic diagram showing a reduction projection exposure apparatus used in a method according to an embodiment of the present invention.

In the figure, reference numeral 10 denotes a mask (reticle) on which a predetermined pattern is formed, and the pattern of the mask is reduced and projected onto a wafer 30 by a projection lens 20. Marks 11 and 12 consisting of diffraction gratings are formed on the mask 10.

This mask mark is one-dimensional, as shown in Figure 2.
A two-dimensional or checkered pattern may be used. wafer 30
Also, a mark 31 made of a diffraction grating is formed.

The wafer 30 is placed on a Z table 41. z actuator 42
It is placed on a sample stage 40 consisting of an XY table 43 and an XY table 43, and is movable in the XY direction and the Z direction.

The first light (p wave) for alignment is transmitted by an AOM (acoustic optical modulator) 51 to a frequency fr (for example, HMt
Similarly, the second light (p wave) is modulated by the AOM 52 at a frequency f2 (e.g. 80.1 MHz) different from fl, and then irradiated onto the mask mark 12. be done. Here, the incident angles of the first and second lights with respect to the mask marks 11, 12 are the same. The transmitted diffracted light from the mask marks 11 and 12 is irradiated onto the wafer mark 31 through the projection lens 20.

The reflected and diffracted light from the two-way mark 31 is reflected by a mirror 54, and is further reflected by a polarizing beam splitter 55 and a mirror 56.
The light is detected by the light receiving element 57 via the light receiving element 57. This gives (f
A first pre-heterodyne detection signal having a frequency of l-f2) is obtained. This first optical heterodyne detection signal is supplied to the phase meter 61 and compared with a reference signal having no mark position information.

As the reference signal, for example, light reflected and diffracted by the mask marks 11 and 12 may be used. The phase difference obtained by the phase meter 61 corresponds to the positional deviation of the mask and wafer, as will be described later.

The output of the phase meter 61 is supplied to the motor 63 via the controller 62, thereby correcting the positional deviation of the mask and wafer.

On the other hand, the third light (S wave) for focusing has a different plane of polarization from the first and second lights, and the AOM
After being modulated by 52, it is irradiated onto the mask mark 12. Then, the transmitted diffracted light from the mark 12 is irradiated onto the wafer mark 31 via the projection lens 20. here,
The second and third lights are combined into one beam by a half mirror 59. Further, the fourth light (S wave) for focusing is modulated by the AOM 53 at a frequency f1, and then is irradiated to the intermediate position between the marks 11° and 12, and is irradiated to the wafer mark 31 via the projection lens 20. Ru.

The reflected and diffracted light from the wafer mark 31 resulting from the irradiation of the third and fourth lights is reflected by the mirror 54 and detected by the light receiving element 58 via the polarizing beam splitter 55. As a result, a second optical heterodyne detection signal having a frequency of (ft f2) is obtained. This second optical heterodyne detection signal is supplied to the phase meter 64 and compared with a reference signal having no mark position information. The phase difference obtained by the phase meter 64 corresponds to the positional deviation of the mask and wafer and the defocus of the wafer surface, as will be described later. and,
The output of the phase meter 64 is supplied to the Z actuator 42 via the controller 65, thereby correcting the defocus on the wafer surface.

Next, the operation of positioning and focusing using the apparatus configured as described above will be explained.

First, a method for detecting a mask/wafer positional shift will be described.

Marks 11.1 on the reticle 10 separated by a predetermined distance p
2, the first and second lights modulated by AOM51.52 (
P-waves at frequencies f1 and f2 are incident, and the light diffracted in the +1st and 11th order directions passes through the projection lens 20 and is focused on the wafer 30. At the mark 31 on the wafer 30, the +1st order becomes 11th order diffraction, and the -1st order becomes +1st order diffraction, each becoming 0th order light. These can be expressed by the following formula.

It is found that

However, Δf−f2−f. Therefore, the phase becomes the following equation.

Here, al+J! 2 is a constant, Z-(nλ/p2)z
, (f
t, f2) is the length of the optical path passing between the reticle and the wafer.

The interference light between Pfl and Pr2 is given by the following equation.

P-Pf'l+Pf2 -@Also, P
The intensity IP of is calculated as lp-1p12, where L, -L
2, so ψF −2X. Therefore, if the phase ψP with respect to the reference target is measured, the reticle 10 and the wafer 3
This means that positional deviation from 0 can be detected.

Next, a method for detecting defocus on the wafer surface will be described.

The fourth light modulated by AOM53 (frequency f1+s
wave) is incident on the center of the two marks 11, 12 on the reticle 10, while the third light modulated by the AOM 52 (frequency f2.s wave) is incident on the right mark 12 on the reticle 10. The light fl passes through the reticle 10, passes through the projection lens 10, undergoes zero-order reflection and diffraction at the wafer mark 31, and
The f2 light undergoes first-order diffraction at the reticle 10 and passes through the projection lens 2.
0 and undergoes +1st order diffraction at the wafer mark 31 to become 0th order light. These can be calculated using the following formula % formula % Intensity I of the interference light S of Sfl and SF3
5 becomes the following formula.

Here, the optical path length that the fl light passes between the reticle and the wafer is Then, 2 π ψP wmX+-e zlISln α ・・
・■Here, Ip (p wave) and Is (S wave) are
The light is split by a polarizing beam splitter 55 and received by separate sensors 57 and 58.

Since ψ is a function of positional deviation and defocus, the reticle 10 or wafer 3 is first adjusted so that the positional deviation becomes zero.
By slightly moving 0 and then measuring ψP, the defocus on the wafer surface is detected.

Thus, according to the method of this embodiment, the pair of mask marks 11, 1 provided on the mask 10 at positions separated by a predetermined distance
2 is irradiated with first and second light (both p-waves at frequencies f1 and f2), and the two diffracted lights generated from the mask marks 11 and 12 are projected onto the mark 31 on the wafer 30 through the projection lens 20. By converging and interfering with the wafer mark 31 and detecting the diffracted light from the wafer mark 31 to obtain a first optical heterodyne detection signal with a frequency of (fl f2), the positional deviation of the mask and wafer is detected from the fifth equation. can do.

Furthermore, with the positional deviation of the mask and wafer corrected,
The third light (frequency f2
．． At the same time, a fourth light (frequency f1, S wave) is irradiated to the middle of the marks 11 and 12, and the diffracted light from the mask mark 12 and the fourth light are transmitted through the projection lens 20. By focusing and interfering with the wafer mark 31, (
By obtaining the second optical heterodyne detection signal having a frequency of f+f2), it is possible to detect the defocus on the wafer surface from the ninth equation.

In this case, unlike defocus detection using a conventional PSD, it is possible to accurately detect defocus on the wafer surface, so the wafer surface can be accurately aligned with the focal position of the exposure optical system, and the pattern Transfer accuracy can be improved.

Note that the present invention is not limited to the embodiments described above, and can be implemented with various modifications without departing from the gist thereof. In the embodiment, the case where fl (S wave) passes through the mask and the projection lens has been explained, but as shown by the broken line in FIG. 1, f2 (
may interfere with the s-wave). Furthermore, it is also possible to omit the AOM by using a Zeeman laser that generates two lights, f1 and f2.

In addition, various modifications can be made without departing from the gist of the present invention.

[Effects of the Invention] As detailed above, according to the present invention, instead of detecting the defocus of the wafer surface indirectly without passing through the projection lens,
Since the defocus on the wafer surface is directly detected using the TTL (Through The Lens) method,
It is possible to accurately detect the defocus of the wafer surface with respect to the projection lens, and to realize a focusing method that can contribute to improving pattern shift accuracy.

[Brief explanation of drawings]

Fig. 1 is a schematic configuration diagram showing a reduction projection exposure apparatus used in an embodiment method of the present invention, Fig. 2 is a diagram showing the shape of a diffraction grating mark formed on a mask, and Fig. 3 is a conventional two-direction detection method. FIG. 3 is a diagram for explaining the method. 10... Mask (reticle), 11.12... Mask mark, 20... Projection lens, 30... Wafer, 31... Wafer mark, 40... Sample stage, 51.52.53 ...AOM. 54, 56... Mira -55...
Polarizing beam splitter, 57.58... Light receiving element, 59... Half mirror 61, 64... Phase meter. (a)

Claims (2)

[Claims] (1) In a focusing method that optically detects the defocus of the wafer with respect to the projection lens and corrects the defocus prior to projecting and exposing the pattern formed on the mask onto the wafer through the projection lens, the mask・With the relative positions of the wafers aligned, first and second lights modulated at different frequencies f_1 and f_2 are irradiated at different positions on the mask at different incident angles, and at least one of them is formed on the mask. A mask mark made of a diffraction grating is irradiated, and the light emitted from the mask is focused and interfered with the wafer mark made of a diffraction grating formed on the wafer through a projection lens, and the light is reflected from the wafer mark. Detect the diffracted light (f_1-f_2
) A focusing method is characterized in that an optical heterodyne detection signal having a frequency is obtained, and wafer defocus information is obtained based on this detection signal. (2) In a focusing method that optically detects the defocus of the wafer with respect to the projection lens and corrects the defocus before projecting and exposing the pattern formed on the mask onto the wafer through the projection lens, A wafer mark made of a diffraction grating is provided on the mask, and first and second mask mugs each made of a diffraction grating are provided at at least two locations on the mask, and the first and second mask mugs are modulated with different frequencies f_1 and f_2.
incident on different marks on the mask at the same angle, and a third light with the same frequency f_2 and a different polarization plane as the second light.
A fourth light with the same frequency f_1 as the first light and the same polarization plane as the third light is incident on the same position at the same angle as the second light. A part of the diffracted light from each mask mark due to the irradiation of the first and second lights is focused and interfered on each wafer mark through a projection lens, and the reflected diffracted light from the wafer mark is detected. Then, a first optical heterodyne detection signal having a frequency of (f_1-f_2) is obtained, and a third optical heterodyne detection signal is then obtained based on the relative positional deviation information of each mark based on the heterodyne detection signal.
The light from the mask caused by the irradiation of the fourth light is focused on and interferes with the wafer mark through the projection lens, and the reflected diffracted light from the wafer mark is detected (f_1-f_
2) A focusing method characterized in that a second optical heterodyne detection signal having the frequency is determined, and wafer defocus information is determined based on the first and second optical heterodyne detection signals.