JP2947920B2 - Optical device using acousto-optic element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to an apparatus for aligning two marks,
In particular, the present invention relates to an alignment apparatus suitable for aligning marks provided on a mask and a wafer used for pattern transfer for alignment adjustment and the like.

(Prior Art) In recent years, with the miniaturization of circuit patterns of semiconductor elements such as LSIs, optical projection exposure apparatuses having high resolution performance have been widely used as pattern transfer means. When transfer is performed using this apparatus, it is necessary to align the mask and the wafer with high accuracy before exposure.

As a method for performing alignment, another optical system (OFF-AXIS microscope) different from the projection optical system is used to detect a mark formed in advance on the wafer, position the wafer, and then position the wafer in the projection optical system. An OFF-AXIS system that moves to a predetermined position within the field of view with high precision and aligns with a mask that has been accurately positioned in advance, and alignment marks formed in advance on both the mask and the wafer are projected through a projection optical system. Detect and align mask and wafer directly
There is a TTL (Through The Lene) method.

The OFF-AXIS method has the advantage that the number of times of alignment is small, the time required for alignment is short, and the throughput (processing speed) is large. However, it is necessary to move the aligned wafer to a position to be transferred by an accurate distance, and it is necessary to provide an absolute length measuring system in addition to the above. It is difficult. Under such circumstances, recently, in order to perform more precise alignment, a method of directly detecting and directly aligning by detecting a mark provided on a mask and a wafer through a projection optical system, such as a TTL method, is considered to be superior. It recognized.

A method of overlaying two grating marks as one of the TTL alignment methods (G. Dubr
oeucq. 1980, ME, WR Trutna. Jr. 1984, SPIE, etc.).
As shown in FIG. 6, a one-dimensional grating mark 101 is provided on a reticle 111, and a two-dimensional grating mark 102 is provided on a wafer 112. The mark 101 on the reticle 111 by the light from the alignment light source 104
The transmitted diffracted light passes through the projection lens 103, and the wafer 1
It is led to twelve marks 102. The diffracted light reflected by the mark 102 passes through the projection lens 103 again, is diffracted again by the grating mark 101 on the reticle 111, and enters the photoelectric detector 105. Then, a relative position between the reticle 111 and the wafer 112 is detected by measuring the intensity of the diffracted light incident on the photoelectric detector 105.

Further, in the alignment method of the TTL method, an optical heterodyne method utilizing interference of two light beams having slightly different frequencies is employed. In this optical heterodyne system, as shown in FIG. 7, in order to obtain alignment light (for example, He-Ne laser light) composed of two light beams having slightly different frequencies, an AOM, that is, an acousto-optical element 12 is used.
0 and 122 are generally used. Specifically, He-Ne
The laser beam emitted from the laser light source 104 is split into two light beams, each of which is put into individual acousto-optical
Creates two beams with a frequency difference of about KHz.

The two light fluxes are used as a mark 101 on the reticle 111 and a projection lens.
103, mark 102 on wafer 112, and again projection lens 1
Through 03, a light beam detector 105 detects a detected beat signal due to the interference of the two light beams as a detection signal. When the relative position between the reticle 111 and the wafer 112 changes, the phase of the beat signal obtained here changes accordingly. Therefore, a beat signal of two light beams obtained by the photoelectric detector 105 is extracted from a position that does not include the relative position information between the reticle 111 and the wafer 112, and is used as a reference beat signal. By reading the phase difference between the reference beat signal and the detection signal, a change in the relative position between the reticle 111 and the wafer 112 is detected.

(Problems to be Solved by the Invention) In order to maximize the diffraction efficiency, the above-described acousto-optic devices 120 and 122 have the optical axis of the incident laser light and the emitted first-order diffracted light as shown in FIG. Between the optical axis and the optical axis at an angle 2α that satisfies the Bragg diffraction angle (for example, 15 mrad =
0.85 ゜). Thus, in an apparatus incorporating an optical system that includes an acousto-optic element, light that is not parallel or perpendicular to a fixed reference plane or axis, such as the plane or edge of the optical mounting base, in front of or behind the acousto-optic element. A shaft must be set up, which makes assembly of the device difficult, and high-precision adjustment is inefficient.

The present invention solves such problems associated with the conventional acousto-optic element, and makes the most of the optical axis of the alignment apparatus including the acousto-optic element perpendicular or parallel to the reference axis, thereby making the apparatus efficient and low-cost. It is an object of the present invention to provide a positioning device that enables accurate assembly and high-precision adjustment.

(Constitution of the Invention) The present invention is characterized in that, in an optical device for modulating two light beams used in an optical heterodyne system by an acousto-optic element, the acousto-optic element is arranged to be inclined by a blog diffraction angle α with respect to a reference axis. This is an optical device using an acousto-optic element.

(Embodiment) Hereinafter, an optical projection exposure apparatus incorporating a positioning apparatus according to an embodiment of the present invention will be described with reference to the drawings. The optical projection exposure apparatus of the first embodiment has a TTL type alignment device as shown in FIG. That is, the reflecting mirror 3 and the beam splitter 2 are positioned on the optical axis O _{1 of} the He-Ne laser light source 1.
Is arranged. Optical axis O ₂ transmitted through beam splitter ₂
A first acousto-optic element 4 is arranged on the upper side, and a second acousto-optic element 8 is arranged on the optical axis O ₃ reflected by the beam splitter 2 and further reflected by the reflecting mirror 6. First and second
Acoustic driver 11 is connected to the optical element 4 and 8, the modulated light of each acoustic respective frequencies f ₁ from the optical element 4, 8, f ₂ is outputted.

More specifically, as shown in FIG.
When the Bragg diffraction angle of the second and third acousto-optic elements 4 and 8 is α, the optical axis O ₁ reflected by the reflecting mirror 3 is equal to the first and second axes.
₂ for each of the emission optical axes OO ₁ OO _{2 of the} acousto-optic elements 4 and 8
The reflecting mirror 3, the first and second acousto-optical elements 4, 8 are arranged so as to be inclined by α. That is, tilting by α the perpendicular of the first reference axis mm of the acousto-optic device 4 'relative to the optical axis O _1. Similarly, to tilt only α to the perpendicular of the reference axis nn 'of the second acousto-optic element 8 with respect to the optical axis O _3.

In this way, by making the exit optical axes OO ₁ and OO ₂ parallel to the edge of the attached base 9 on which the first and second acousto-optic elements 4, 8 and the like are attached, many parts of the device can be removed. It can be parallel or perpendicular to the reference axis.

In front of the first and second acousto-optical elements 4 and 8 on the optical axes O ₂ and O ₃ , beam splitters 10 and 12 are arranged, respectively. First and second light beam displacement optical systems 14 and 16 are arranged on the reflection optical axes O ₄ and O _{5 of the} beam splitters 10 and 12.

The first and second optical displacement optical systems 14 and 16 are the same optical system. As shown in FIG. 3, the declination θd = 1 ° and the prism apex on the optical axis O ₄ (O ₅ ). θ _W = about 2 °, refractive index n = 1.
51462 (θd = (n−1) · θ _W ) 2 wedge-shaped prisms 1
01 and 102 are arranged so as to be rotatable about the optical axis O ₄ (O ₅ ), and two plane parallels 104 and 10 are arranged in front of this. The plane parallel 104 is held rotatably about a horizontal axis XX ′ orthogonal to the optical axis O ₄ (O ₅ ), and the plane parallel 106 is held about a horizontal axis YY ′ orthogonal to the optical axis O ₄ (O ₅ ). It is held rotatably.

The optical axis O ₄ (O ₅ ) is, as shown in FIG.
After passing through the light beam displacement optical type 14 and 16, first alignment marks 22 made of a diffraction grating of pitch P _R provided on the mask 20
Reach Wherein the pitch P _R is, ± 1-order diffracted light of the first and second acousto-optic device 4, 8 is determined so that incident on the first alignment marks 22.

Below the mask 20, a reduction projection lens 24 and a mask 20
A wafer 26 to be printed is placed. The optical axes O ₄ and O ₅ having passed through the first alignment mark 22 pass through the reduction projection lens 24 and reach the second alignment mark 30 provided on the wafer 26 and having a pitch _{PW and} a diffraction grating. Where pitch
P _W is determined such that the first-order diffracted light of the light beam incident along the optical axes O ₄ and O ₅ by the second alignment mark 30 matches the vertical optical axis of the reduction projection lens 24.

A reflecting mirror 40 is arranged between the reticle 20 and the reduction projection lens 24 and at a position conjugate with the second alignment mark 22 with respect to the reduction projection lens 24. Second alignment mark 22
From the mirror 40 to the reflecting mirror 40, and the optical axis O ₆ reflected by the reflecting mirror 40
Above, a positioning signal detector 42 is disposed in front of the reflecting mirror 40.

Further, a beam splitter 60 is arranged on the optical axis O ₂ in front of the beam splitter 10, while a reflecting mirror 61 is arranged on the optical axis O ₃ and in front of the beam splitter 12, and the reflected optical axis of the beam splitter 60 is And the transmitted optical axis are in line with the optical axis
O ₇ Reference signal detector 62 in front of the beam splitter 60 on the optical axis O ₇ is arranged.

In the above configuration, the signals detected by the alignment signal detector 42 and the reference signal detector 62 are beat signals having a frequency difference Δf = | f ₁ −f ₂ |. For example, when f ₁ = 80 MHz and f ₂ = 79.9 MHz, Δf = | 80 MHz−79.9 MHz | = 100 KHz.

On the other hand, as shown in FIG.
The phase phi _s beat signal _detected, the phase of the beat signal detected by the reference signal detector 42 and phi _r by. Phase difference between phases φ _s and φ _r Δφ = φ _s −φ
_r , the first alignment mark 22 is formed by optical heterodyne.
And the optical relative position of the second alignment mark 30 can be calculated.

Incidentally, the phase phi _s to be detected with high accuracy, the injected modulated light from the first and second acousto-optic device 4, 6, the first alignment marks 22, the entrance pupil of the reduction projection lens 24 (FIG. (Not shown), it is necessary to properly overlap the diffracted light from the second alignment mark 30 by accurately entering the second alignment mark 30 to obtain a sufficient level of alignment signal.

That is, as shown in FIG. 5, a laser beam having a frequency f ₁ irradiates the mark area of the second alignment mark 30 at an incident angle α ₁ without any excess or shortage, and a laser beam having a frequency f ₂ is incident at an angle α ₂ ( = Α ₁ ), the second alignment mark 30 is illuminated to the mark area without any excess or shortage, and each reflected diffracted light beam is reflected in a direction perpendicular to the wafer 26, and a sufficient level of alignment signal is obtained. .

On the other hand, when the laser beam of the laser beam and the frequency f ₂ of the frequency f ₁ is not aligned signal of sufficient level is obtained by the second alignment mark 30, the wedge-shaped prism 101,1
The light beam can be deflected by rotating 02 around the optical axes O ₄ and O ₅ .

That is, the vertex angles of the wedge-shaped prisms 101 and 102 are A, the angle between the respective deviating directions is 2θ, and the refractive index is n. A laser beam passing through the optical axes O ₄ and O ₅ through the wedge prisms 101 and 102 passes through the wedge prisms 101 and 102 in an intermediate direction of the deflected directions of the optical axes O ₄ and O ₅ , and an angle δ = (n = 1) ) · A · 2sinθ.

Further, by rotating the plane parallels 104 and 106 about the axes XX 'and YY', the light beam can be translated or shifted. For example, the shift amount h when the plane parallel rotation angle is θ, the thickness t, and the refractive index n are: Is required.

The second embodiment of the present invention is shown in FIG. 8 in an enlarged view in the vicinity of the acousto-optic element corresponding to FIG. 2, and the configuration corresponding to the configuration of the first embodiment is the same as that of the first embodiment. And the description is omitted. Laser beam emitted from the He-Ne laser light source 1, reflecting mirror 3 along the edge and parallel to the optical axis O ₁ of the mounting base 9, a beam splitter 2, first and second acousto-optic through the reflecting mirror 200 Elements 4 and 8 are entered. Reference axes mm ', n of the first and second acousto-optical elements 4, 8
n ′ is arranged at an angle of 2α with respect to the incident optical axes O ₂ and O ₃ . Therefore, the emission optical axes OO ₁ and OO ₂ of the first and second acousto-optical elements 4 and 8 are inclined by 2α with respect to the edge of the mounting base 9. Reflection meridians 202 and 204 are disposed on the emission optical axes OO ₁ and OO ₂ , respectively, and the reflection optical axes of the reflecting mirrors 202 and 204 are parallel to the edge of the mounting base 9.

(Effect of the Invention) According to the present invention, most of the optical axes of a positioning device including an acousto-optic element can be perpendicular or parallel to a fixed reference plane, and the device can be assembled efficiently and at low cost. And high-precision adjustment is possible.

[Brief description of the drawings]

FIG. 1 is an optical perspective view of an optical projection exposure apparatus incorporating a positioning apparatus according to a first embodiment of the present invention, FIG. 2 is an enlarged view of the vicinity of an acousto-optical element of the apparatus of FIG. 1, and FIG. FIG. 4 is an optical perspective view of a light beam displacement optical system, FIG. 4 is an explanatory view showing a phase difference of a positioning signal, FIG. 5 is a side view showing a situation where a laser beam enters the first positioning mark 22, and FIG. FIG. 7 is an explanatory view of the principle of a conventional positioning optical device, FIG. 7 is an explanatory view of an acousto-optical element, and FIG. DESCRIPTION OF SYMBOLS 1 ... Laser light source 4 ... 1st acousto-optic element 8 ... 2nd acousto-optic element 10, 12 ... Beam splitter 14 ... 1st light beam displacement optical system 16 ... 2nd light beam displacement optical system 20 ... Rectile 22 first alignment mark 24 minute projection lens 26 wafer 30 second alignment mark 42 alignment signal detector 62 reference signal detector 101, 102 wedge prism 104 106 ... Plain parallel

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI H01S 3/10 H01L 21/30 507A (72) Inventor Hideo Nagai 72 Horikawa-cho, Saiwai-ku, Kawasaki-shi, Kanagawa Pref. Inventor Mitsuo Tabata 72 Horikawa-cho, Saiwai-ku, Kawasaki City, Kanagawa Prefecture (72) Inventor Tatsuhiko Higashiki 72 Horikawa-cho, Saiwai-ku, Kawasaki City, Kanagawa Prefecture Toshibauchi Corporation (56) References -227603 (JP, A) JP-A-63-173902 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01B 11/00-11/30

Claims (5)

(57) [Claims] 1. An acousto-optic device for modulating two light beams used in an optical heterodyne system by an acousto-optic device, wherein the acousto-optic device is arranged to be inclined by a Bragg diffraction angle α with respect to a reference axis. Optical device using the. 2. An optical device using an acousto-optic element according to claim 1, wherein a measurement light beam is incident on said acousto-optic element at an angle of 2α with respect to a reference axis. 3. The acousto-optic device according to claim 1, wherein a measurement light beam is incident on the acousto-optic device in parallel with a reference axis, and an emission optical axis is refracted from the acousto-optic device in parallel or perpendicular to the reference axis. 4. The acousto-optic device according to claim 1, wherein said acousto-optic device is arranged symmetrically in a pair. 5. The method according to claim 1, wherein the measuring light beam is a He-
The acousto-optic device according to Success 1, which is Ne laser light.