JP2694045B2 - Positioning device using diffraction grating - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a semiconductor IC or LSI by, for example, aligning a mask with a wafer and then baking a mask pattern on the wafer.
The present invention relates to an aligning position by an optical heterodyne interferometry method using a diffraction grating, which is suitable for application to an exposure apparatus for manufacturing a device and an encoder for detecting a position.

[Conventional technology]

Along with the miniaturization of semiconductor IC and LSI patterns, in a device for exposing and transferring a mask pattern onto a wafer, it is indispensable to develop a technique for aligning the mask and the wafer with high precision.

Optical heterodyne interferometry has two slightly different frequencies.
A heterodyne signal is obtained by interfering two laser beams, a phase difference between a standard heterodyne signal (reference signal) and the measured heterodyne signal is obtained, and the mask and the wafer are aligned and the position is detected.

Conventionally, there is an apparatus applied to an X-ray exposure apparatus as shown in FIG. 3 as an apparatus for performing fine displacement measurement or precise alignment by using an optical heterodyne interferometry method using a diffraction grating (Japanese Patent Application No. Sho. 61-104186). In FIG. 3, reference numeral 20 denotes a transverse Zeeman effect type dual-frequency orthogonal polarization He-Ne laser light source (light source) which emits light of two wavelengths whose frequencies are slightly different from each other and whose change plane directions are orthogonal to each other. ",twenty one
Is a mirror, and of these mirrors 21 ', 2
1 ″ is configured to be excessively adjustable and constitutes an incident angle adjusting means. 22 is a cylindrical lens, 23 is a polarizing beam splitter, 24 is a prism mirror, 25 and 25 ′ are condenser lenses, and 26,
26 'is a photoelectric detector (first and second detecting means), 27 is a signal processing control section (signal processing control means), 28 is a mask stage, 29 is a wafer stage, 30 is a mask (first object),
31 is a wafer (second object), 32 is a mask diffraction grating (first
Diffraction grating), 33 is a monochromatic light incident / light diffraction extraction window, 34
Is a wafer diffraction grating (second diffraction grating), and 44 and 44 'are polarizing plates. Here, the monochromatic light incident / diffracted light extraction window 33
Is an opening provided in the mask 30 so that incident light can be directly incident on the diffraction grating 34 through the window 33, and diffracted light from the diffraction grating 34 can be directly extracted. Further, the mask stage 28 and the wafer stage 29
Constitute a moving mechanism for relatively moving the mask 30 and the wafer 31.

The light emitted from the two-wavelength orthogonal polarization laser light source 20 becomes an elliptical beam through the mirror 21 and the cylindrical lens 22, and the beam is a linearly polarized light having only a horizontal part or a vertical component by the polarization beam splitter 23 and has a frequency. Is split into two light beams having slightly different wavelengths, and the split light beams are incident on the diffraction grating 32 and the diffraction grating 34 at desired incident angles via the mirrors 21 'and 21 ", respectively. , The diffraction gratings 32 and 34 are respectively displaced in the grating line direction and are arranged in the same elliptical beam of incident light of two wavelengths, and the diffraction grating pitches of the diffraction gratings 32 and 34 are made equal. The diffracted light obtained from the diffraction grating 32 and the diffracted light obtained from the diffraction grating 34 through the monochromatic light incident / diffracted light extraction window 33 are reflected by the mirror 21,
Prism mirror 24, polarizing plates 44,44 ', condenser lenses 25,2
It is guided to the photodetectors 26, 26 'via 5', respectively, and processed by the signal processing controller 27 as a diffracted light beat signal. The signal processing control unit 27 detects the phase difference between the beat signals using one of the beat signals of the diffracted light obtained from the diffraction gratings 32 and 34 as a reference beat signal, and the phase difference becomes 0 °. The mask stage 28 or the wafer stage 29 is relatively moved so that the pattern on the mask surface is accurately overlapped with a predetermined position on the wafer surface so that the exposure can be performed accurately. It is supposed to be matched.

Next, a positioning method using the positioning device thus configured will be described with reference to FIG. In FIG. 4, 35 is a mask diffraction grating (first diffraction grating), 36 is a wafer diffraction grating (second diffraction grating), 37 and 38 are two wavelengths of incident light whose frequencies are slightly different from each other, and 39 and 40. Is diffracted light (optical heterodyne interference diffracted light), 41 is a mask (first object) made of a transparent thin film, 43 is an opaque thin film for forming the diffraction grating 35, and 42 is a wafer (second object). Also, B ₁
−B ¹ ′ is the grating line direction of the wafer diffraction grating 36, B ₂ −B ₂ ′
Is the grating line direction of the mask diffraction grating 35, and A ₁ −A ₁ ′ is B ₁ −
A grating pitch direction perpendicular to B ₁ ′, A ₂ −A ₂ ′ is a grating pitch direction perpendicular to B ₂ −B ₂ ′, C ₁ −C ₁ ′ is a direction perpendicular to the grating plane of the wafer diffraction grating 36. , C ₂ −C ₂ ′ are mask diffraction gratings 35
Is the direction perpendicular to the lattice plane of. In the example of FIG. 4, the pitches of the mask and the wafer diffraction gratings 35 and 36 are made equal to P, and the mask diffraction grating 35 is B ₂ -B ₂ so as not to overlap with the grating surface of the wafer diffraction grating 36. The takeout window 33 (not shown in FIG. 4) is provided above the wafer diffraction grating 36 in the vertical direction above the wafer diffraction grating 36.

Then, the incident direction of the incident light 37, 38, the mirror 21 ',
By adjusting the angle of 21 ″, the diffraction grating 3 can be aligned with the direction C ₁ −C ₁ ′ (or C ₂ −C ₂ ′) perpendicular to the diffraction gratings 35 and 36.
5,36 ± 1st-order diffracted light angle θ ₋₁ = sin ⁻¹ (λ ₁ /
P), θ ₁ = sin ⁻¹ (λ ₂ / P). Here, the wavelengths of the incident lights 37 and 38 are λ ₁ and λ ₂ , respectively, and the frequency difference Δf is a value of several kHz to several hundreds kHz. Letting the speed of light be c, Δf = | c / λ ₁ −c / λ ₂ | and Δf << c, so θ ₋₁ ≈ θ ₁ (or λ = λ
₁ ≈λ ₂ ).

By doing so, the incident lights 37 and 38 incident on the diffraction gratings 35 and 36 are respectively in the directions (C ₁ -C ₁ ′ or C ₂ -C ₂ ′ direction) perpendicular to the grating surface in the diffraction gratings 35 and 36, respectively. −1st-order diffracted to be combined into an optical system to become optical heterodyne interference diffracted lights 39 and 40, respectively. These optical heterodyne interference diffracted lights 39, 40 are diffracted lights diffracted by different diffraction gratings 35, 36, but since the incident angles of the incident lights 37, 38 are symmetrical with respect to the vertical direction of the grating surface, The diffraction gratings 35 and 36 are arranged in the vertical direction (C ₁ -C ₁ ′ or C ₂ -C ₂ ′ direction) and in the grating line direction (B ₁ -B ₁ ′ or B ₂ −B ₂ ′ direction), respectively. However, the change in the optical path length of the incident light 37 and the incident light 38 until they enter the diffraction grating 35 or the diffraction grating 36 becomes equal,
The phase difference of the beat signals obtained from the diffracted lights 39 and 40 is not affected by the phase shift with respect to the displacement of the diffraction gratings 35 and 36 in the vertical direction and the grating line direction. That is, the phase difference between the beat signals obtained from the diffracted lights 39 and 40 is the diffraction grating 35 and the diffraction grating 36.
In the pitch direction (A ₂ -A ₂ ′ or A ₁ -A ₁ ′ direction), that is, only in accordance with the relative displacement amount. Then, when the diffraction gratings 35 and 36 are aligned in the grating line direction, the phase difference of the beat signals obtained from the diffracted lights 39 and 40 becomes 0 °, and the alignment is completed. Note that the relative displacement of the diffraction gratings 35 and 36 with respect to the pitch direction is
If x and the phase difference of the beat signal are Δφ, there is a relation of Δφ = 2π · Δx / (P / 2), and the phase difference Δφ is 1/2 of the pitch of the diffraction grating.
Changes with the relative displacement amount of.

[Problems to be solved by the invention]

By the way, in the above conventional apparatus, since the mask 30 and the wafer 31 are arranged close to each other by several tens of μm, the incident light 37, 38 or a part of the diffracted light generated by the incident light is generated by the mask, The light is multiply reflected on the wafer surface and emitted in the same direction as the diffracted lights 39 and 40.

This will be described with reference to FIG. FIG. 5 (a) shows the state of multiple reflection at the mask diffraction grating portion, and FIG. 5 (b) shows the state of multiple reflection at the wafer diffraction grating portion. The diffracted light 50 shown by the broken line in FIG. 5 is diffracted by the mask diffraction grating 35 and diffracted by the wafer surface, and the diffracted light 51 is transmitted by the mask diffraction grating 35, reflected by the wafer surface and transmitted by the mask diffraction grating 35. Diffracted light, diffracted light 52
Is the diffracted light that is reflected and diffracted by the wafer diffraction grating 36 and then reflected by the mask and wafer surfaces, and diffracted light 53 is the diffracted light that is reflected and diffracted by the wafer diffraction grating 36 after being reflected by the mask and wafer surfaces. In FIG. 5, only the diffracted light reflected once by the mask and the wafer surface is shown.
There is diffracted light. Further, although the state of multiple reflection of the incident light 37 is shown, the same can be said for the left and right symmetrical incident light 38.

As described above, in the above-described conventional alignment apparatus, the unwanted diffracted light 50, 51 or the unnecessary diffracted light 52, 53 interferes with the desired diffracted light 39 or 40. Then, these multiple interference diffracted lights become sensitive to a minute variation of the gap G, that is, the gap G with respect to the direction perpendicular to the grating surface of the mask and the wafer diffraction grating,
Intensity fluctuation occurs with the fluctuation of the gap G of λ / 2 as a cycle. Accordingly, the intensity fluctuation appears as a fluctuation in the amplitude of the beat signal, destabilizing the phase difference detection signal and deteriorating the positioning accuracy.

[Means for solving the problem]

In order to solve the above-mentioned problem, the present invention provides an optimum method for the distance (gap) G between the first object and the second object to be aligned such that the effect of multiple interference is minimized. The diffraction grating pitch is set by theoretical calculation, and the alignment is performed by minimizing the influence of the amplitude fluctuation of the beat signal. That is, when the distance between the first and second diffraction grating surfaces is G and the wavelengths of the two wavelengths generated from the light source are λ ₁ and λ ₂ , respectively, the grating pitch P of the first and second diffraction gratings is The following equation P = (2λ ₁ G / n) ^1/2 or P = (2λ ₂ G / n) ^1/2 (where n is an odd number) is set.

[Action]

By setting the diffraction grating pitch P of the first object and the second object according to the gap G of the alignment device by the above equation, the effect of multiple interference is reduced, and as a result,
The influence of the amplitude fluctuation of the beat signal becomes small, the signal processing by the AGC circuit, etc. becomes easy, a stable phase difference signal is obtained, and highly accurate alignment becomes possible.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2.

FIG. 1 shows an embodiment of an alignment apparatus according to the present invention,
That is, it is a diagram showing a main part of an X-ray exposure apparatus for manufacturing a semiconductor IC or LSI. The overall structure of the alignment device of this embodiment is basically the same as that shown in FIG.

FIG. 1A shows a mask diffraction grating portion, and FIG. 1B shows a wafer diffraction grating portion. In FIG. 1, reference numeral 1 is a mask diffraction grating (first diffraction grating), 2 is a wafer diffraction grating (second diffraction grating), and 3 and 4 are slightly different in frequency from each other and have wavelengths of λ ₁ and λ ₂ , respectively. Incident light, 5 and 6 are diffracted light (optical heterodyne interference diffracted light), 7 is a mask (first object) made of a transparent thin film, 8 is an opaque thin film for forming the mask diffraction grating 1, and 9 is a wafer (first object). 2 object)
It is.

Beat frequency _{△ f = | c / λ 1} -c / λ 2 | order is compared to △ F«c the speed of light _{c, λ = λ 1 ≒ λ} 2 , and the pitch of the mask diffraction grating 1 and the wafer diffraction grating 2 By setting P according to the following equation P = (2λ G / n) ^1/2 = (2λ ₁ G / n) ^1/2 ≈ (2λ ₂ G / n) ^1/2 , glory of multiple interference is minimized. It is possible (however, n is an odd number).

FIG. 2 shows the experimental results of the intensity change of the multiple interference diffracted light when λ = 633 nm and P = 6 μm. Horizontal axis is gap G
(Μm), and the vertical axis represents the output voltage value (V) detected by the photoelectric converter for changes in diffracted light intensity. From the above equation, n =
It can be seen that the intensity fluctuation is minimized when the gap G is approximately 18.4 μm when n = 3 and when the gap G is 85.3 μm when n = 3. That is, the effect of multiple interference is reduced under the conditions of the gap G≈28.4 μm (or G85.3 μm) and the diffraction grating pitch P = 6 μm satisfying the above formula, and the stable phase difference signal is less affected by the amplitude fluctuation of the beat signal. It is possible to obtain highly accurate alignment.

〔The invention's effect〕

As described above, according to the present invention, the influence of multiple interference due to reflection between the first object and the second object can be minimized, so that the phase difference signal is stabilized and the alignment is performed. It is possible to obtain the effect of improving the accuracy of.

[Brief description of the drawings]

FIG. 1 shows an embodiment of an alignment apparatus according to the present invention, in which (a) is a view showing a mask diffraction grating portion,
(B) is a diagram showing a wafer diffraction grating part, FIG. 2 is a diagram showing experimental results for explaining the effect of the device of this embodiment, and FIG. 3 is a general schematic configuration of an alignment device using a diffraction grating. 4 and 5 are detailed perspective views of the diffraction grating portion of the apparatus, and FIG.
(A) and (b) are explanatory views of multiple reflection in the apparatus. G ... Gap (distance between the first and second diffraction grating surfaces), P ... Grating pitch of the first and second diffraction gratings, 1 ... Mask diffraction grating (first diffraction grating), 2 ... ... wafer diffraction grating (second diffraction grating), 3 ... incident light of wavelength λ ₁ , 4 ... incident light of wavelength λ ₂ , 5, 6 ... optical heterodyne interference diffracted light, 7 ... mask (first Object), 9 ... Wafer (second object), 20 ... Transverse Zeeman effect dual frequency orthogonal polarization He-Ne laser light source (light source), 21,21 ... Mirror, 21 ', 21 "... Mirror (Incident angle adjusting means), 22 ... Cylindrical lens, 23 ... Polarizing beam splitter, 24 ... Prism mirror, 25,25 '... Condensing lens, 26 ... Photoelectric detector (first detecting means) , 26 '... Photoelectric detector (second detection means), 27 ... Signal processing control section (signal processing control means), 28 ... Mask stage, 29 ... Hastage, 30 ... Mask (first object), 31 ... Wafer (second object), 32 ... Mask diffraction grating (first diffraction grating), 33 ... Monochromatic light incident / diffracted light extraction window, 34 …… Wafer diffraction grating (second diffraction grating), 44,44 ′ …… Polarizing plate.

Claims (1)

(57) [Claims] 1. A first diffraction grating provided on a first object, a second diffraction grating provided on a second object, and the first and second diffraction gratings.
Moving mechanism for relatively moving the object, a light source for generating monochromatic light of two wavelengths having frequencies slightly different from each other, and monochromatic light of two wavelengths generated from the light source for each of the first and second diffraction gratings. Incident angle adjusting means for making the light incident at a predetermined incident angle, first detecting means for detecting optical heterodyne interference diffracted light generated from the first diffraction grating, and generating a first beat signal, Second detecting means for detecting optical heterodyne interference diffracted light generated from the second diffraction grating to generate a second beat signal, and first and second beats generated by the first and second detecting means. Sending a control signal to the moving mechanism according to the phase difference of the signal,
In a positioning device using a diffraction grating, which comprises a signal processing control means for moving the first and second objects relative to each other, a distance between the first and second diffraction grating surfaces. Is G and the wavelengths of the two wavelengths generated from the light source are λ ₁ and λ ₂ , respectively, and the grating pitch P of the first and second diffraction gratings is expressed by the following equation P = (2λ ₁ G / n) ^{1 / 2} or P = (2λ ₂ G / n) ^1/2 (where n is an odd number).