JPS6126605B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a highly accurate alignment device that effectively utilizes two diffraction gratings arranged in parallel.

Recently, a highly accurate alignment method using two diffraction gratings has been developed by Stewart Austin, D., C.Flanders,
A book by Hanry I., Smith et al. (1) “A new interFerometric aiignment
technique “Applied Physics Letter, Vol・31
No.7 (1977) P.426 (2) “Alignment of X-ray lithography
masks using a new interferometric
technique−Experimetal results “J.Vac.Sci.
It is introduced in Technol.15 (3) May/June (1978) P.984, etc. As shown in the outline of FIG. The interference light is vertically irradiated, and the transmitted diffracted light in the +n-order direction and -n-order direction by the diffraction gratings 1 and 2 is transmitted to the receivers 4 and 5.
The light is received by each of the light beams, and the intensity of the received light is displayed on the indicator 6. In this case, if there is a grating pitch shift Δx between the diffraction gratings 1 and 2, a shift will occur in the phase of each diffracted light beam by the diffraction gratings 1 and 2, and this will cause each diffraction in the +n-order direction and the -n-order direction to occur. The lights interfere with each other. By detecting the change in light intensity due to this interference, the two diffraction gratings 1 and 2 are aligned.

On the other hand, the one shown in document (2) consists of two diffraction gratings 1 arranged in parallel, as schematically shown in FIG.
In this method, alignment is performed using the reflected diffraction light by 2, similar to that shown in document (1).

These alignment methods have the advantage of being able to perform alignment with extremely high precision defined by the grating pitch P of the diffraction gratings 1 and 2 and the wavelength λ of the coherent light.
However, when aligning with this type of system, the distance D between the diffraction gratings 1 and 2 is
When this changes, the phase of the diffracted light changes greatly due to the change in the optical path length, making it extremely difficult to determine positional deviation. Moreover, when adopted in various manufacturing devices, it is virtually impossible to avoid fluctuations in the distance D due to vibrations of the floor surface, vibrations of movable operating parts, etc. Furthermore, even if disturbance light is incident on the light receivers 4 and 5, it becomes impossible to detect it, and there are many problems in practice.

The present invention was made in consideration of the above circumstances, and
The purpose is to provide a simple and highly accurate alignment device that fully utilizes the characteristics of the diffraction grating without being affected by fluctuations in the distance between the diffraction gratings or by disturbance light.

That is, in the present invention, the distance D between the diffraction gratings is determined by modulating the phase of the diffracted light by the two diffraction gratings and determining the difference in the intensity of each interference light due to the diffracted light in the +n-order direction and the -n-order direction. By eliminating the adverse effects of fluctuations and at the same time calculating the difference in intensity, the amount of input disturbance light can be canceled out, making it possible to always perform stable and highly accurate positioning.

Here, at least one of the diffraction gratings is vibrated in the irradiation direction of the coherent light to change the distance between the gratings, or the phase of the diffracted light is modulated by modulating the wavelength of the coherent light to achieve the above purpose. Achieved.

Hereinafter, diffracted light by two diffraction gratings arranged in parallel and having the same grating pitch will be analyzed, and embodiments of the present invention will be explained with reference to the drawings. Note that although the following explanation will be made using reflected diffraction light as an example, the same applies to transmitted diffraction light.

FIG. 3 is a schematic diagram showing coherent light diffracted by two diffraction gratings 11 and 12. The two diffraction gratings 11 and 12 have the same grating pitch P, and are arranged in parallel with the distance between the upper surfaces of the gratings 11 and 12 being set to D. Coherent light is applied to these diffraction gratings 11 and 12 from above in the figure,
For example, the laser beam 13 is irradiated vertically. +n due to the diffraction gratings 11 and 12 of this laser beam 13
There are three main types of diffracted light in the next direction and the −n-order direction. Taking the +n-order direction as an example, the first diffracted light 14 reflected by the diffraction grating 11 is transmitted through the diffraction grating 11 and diffracted, then specularly reflected by the diffraction grating 12, transmitted through the diffraction grating 11 again, and diffracted. The second diffracted light 15, and the third diffracted light that is reflected and diffracted by the diffraction grating 12 after passing through the diffraction grating 11.
This is the diffracted light 16. Each diffracted light 14, 15, 16
When the amplitudes of are denoted by a, b, c, the first diffracted light 1
The complex amplitude Ea ⁺ⁿ of 4 is expressed as Ea ⁺ⁿ = a ...... (1). Also, the complex amplitude of the second diffracted light 15
Eb ⁺ⁿ is Eb ⁺ⁿ = b・exp i(2π2d/ λ) ......(2). Furthermore, the complex amplitude of the third diffracted light 16
Ec ⁺ⁿ is when the grating pitch length of the diffraction gratings 11 and 12 is P, and the amount of deviation between the diffraction gratings 11 and 12 is Δx. It is indicated by. Therefore, the light heading in the +n-order direction becomes a composite light (interference light) of the above three diffracted lights 15, 16, and 17, and its intensity I ⁺ⁿ is approximated as λ/P〓0, cosλ/P〓1 By doing this, the following is shown.

I ⁺ⁿ = | Ea ⁺ⁿ + Eb ⁺ⁿ + Ec ⁺ⁿ | ² = a ² + b ² + C ² + 2ab cos4πd/λ +2ac cos (4πd/λ+2πn・Δx/P) +2bc cos2πn・Δx/P……(4) Similarly, diffracted lights 17, 18, 1 in the -n order direction
Each complex amplitude Ea ^-n , Eb ^-n , Ec ^-n of 9 is Ea ^-n = a …………(5) Eb ^-n = b・exp i(2π2d/λ) ……(6) Ec ^-n =c・exp i(M-N)...(7) It is shown as follows. And these diffracted lights 17, 18, 1
The intensity I ^-n of the light which interferes with 9 and goes in the -n order direction is expressed as follows.

I ^-n =a ² +b ² +c ² +2ab cos4πd/λ +2ac cos (4πd/λ-2πn・Δx/P) +2bc cos (2πn・Δx/P)...(8) The above-mentioned documents (1), (2 ) compares the light intensities I ⁺ⁿ and I ^-n , and when they match, the diffraction grating 11,
The alignment is performed on the assumption that the deviation (Δx) between 12 and 12 is eliminated. That is, the difference between the intensity I ⁺ⁿ of the interference light in the +n-order direction and the intensity I ^-n of the interference light in the -n-order direction is I ^-n -I ⁺ⁿ = 2ac{cos (4πd/λ-2πn・Δx/
P) −cos(4πd/λ+2πn・Δx/P)} =4acsin(2πn・Δx/P) sin(4πd/
λ)... (9) In this equation, the diffraction grating 11,
The amount of deviation Δx between 12 and 12 has a constant relationship with the lattice pitch length P sin(2πn・Δx/P)=0 ...(10) That is, 2n・Δx/P=m (integer) ...(11) When there is a relationship, the above difference is 0. Furthermore, Fig. 6 shows the relationship between Δx and sin2πnΔx/P, and as is clear from this figure, when Δx=p/2n, Δx=2p/2n, Δx=3p/2n,... sin
2πnΔx/P becomes 0. Therefore, the mutual positions of the diffraction gratings can be aligned in the plane direction so that the above-mentioned difference becomes zero.

However, as is clear from equation (9) above, the difference is also 0 when sin(4πd/λ)=0. That is,
When 4πd/λ becomes 2πn, the above-mentioned difference I ⁻ⁿ −I ⁺ⁿ becomes 0 even if Δx=0, so alignment cannot be performed in this case. Therefore,
In order to avoid this point, d or λ must be set so that the above 4πd/λ is not constant, but even if d or λ is set in this way, floor vibrations and air vibrations will occur. Then, depending on the amplitude of the vibration, 4πd/λ becomes 2nπ, and as a result, the difference becomes 0, making it impossible to perform alignment.

That is, if one of the diffraction gratings is moved at a constant speed, then sin2πnΔx/P
changes as shown in Figure 7a. On the other hand, if d changes as shown in FIG. 7b due to floor vibration at this time, sin4πd/λ becomes as shown in FIG. 7c. Therefore, as is clear from equation (9), the difference I ^-n -I ⁺ⁿ in this case is 4ac the product of Figure 7a and Figure 7b above.
Therefore, the difference I ^-n -I ⁺ⁿ becomes as shown in FIG. 7b. As is clear from the figure, many points such as point B where the difference I ^-n -I ⁺ⁿ = 0 appear in addition to the true alignment point A, and as a result, alignment cannot be performed.

Therefore, in the present invention, as a first means, the phase of the diffracted light is modulated by vibrating the diffraction gratings 11 and 12 in the irradiation direction of the laser beam 13 and constantly changing the distance d between the diffraction gratings in a vibrational manner. . Then, out of the difference in the interference light intensity of this phase-modulated diffracted light, only the modulation component is extracted, and the output thereof is rectified and smoothed to obtain a differential output, thereby removing the fluctuation component of the distance d due to disturbance vibration, An attempt is made to find a singular point that depends only on the diffraction gratings 11 and 12. As a second means, by changing the wavelength λ of the laser beam 13 itself, the phase of the diffracted light is modulated to find a singular point in the interference light intensity that depends only on the deviation Δx between the diffraction gratings 11 and 12. That's what I do.

The above principle will be explained in more detail as follows. That is, for example, the diffraction gratings 11, 12
As shown in FIG. 8a, one of the piezoelectric vibrators 2
Assuming that modulated vibration is added by 3 and disturbance vibration is added by floor vibration as shown in FIG. 8b, the grating spacing d, which is the distance between the diffraction gratings 11 and 12, changes with time. (See Figure 8c). Therefore, sin4πd/λ shown in FIG. 7c, which is a function of interstitial case d, changes as shown in FIG. 8d. That is, this sin4πd/λ is directly affected by the modulated vibration by the piezoelectric vibrator 23 and the disturbance vibration caused by the floor vibration, and undergoes phase modulation. On the other hand, the sin2πnΔx/P shown in FIG. 7a
does not include the grid spacing d as a variable. and,
The amount of deviation Δx between the diffraction gratings 11 and 12 is Δx=
k・t (k is a constant), that is, Δx is time t
Since it is a function of t and has a directly proportional relationship to time t, it becomes a monotonous sine wave as shown in FIG. 8e. In other words, sin2πnΔx/P is completely unaffected by variations in the lattice spacing d. Thus, according to the above equation (9), when the difference I ^-n -I ⁺ⁿ , that is, the product of the waveform of FIG. 8 d and the waveform of FIG. 8 e, is calculated, the waveform shown in FIG. 8 f is obtained. get. In this waveform, the difference I ^-
n . At the point where I ⁺ⁿ = 0, the lattice spacing d
The fluctuations due to modulation vibration and disturbance vibration are canceled out and become 0 (especially at the point where sin2πnΔx/P becomes 0, I ^-n -I ⁺ⁿ is always 0). Therefore, a filter circuit 36 (when the vibration frequency of the ^{vibrator 23} is f, the frequency band f to 2f (has a bandpass characteristic of 3
7, it is output to the smoothing circuit 38 (the eighth
(See Figure g). Then, the smoothing circuit 38 outputs a signal SC from which the modulated vibration component has been removed (see Fig. 4).
(see Figure 8h). In this signal SC, only the true alignment point A is 0, so it is possible to detect only the true alignment point A and perform alignment using the signal SC. .

FIG. 4 is a schematic diagram showing an embodiment of the apparatus of the present invention constructed based on the above principle. In the figure, a movable table 2 is provided movably on a base 21.
A sample 24 is placed on the sample 2 with a piezoelectric vibrator 23 interposed therebetween. The movable table 22 is moved under position control by a table moving device 25, and the vibrator 23 is energized by a vibrator power source 26 to vibrate in thickness. This thickness vibration causes the sample 24 and the diffraction grating 27 provided on the upper surface of the sample 24 to vibrate in the laser beam irradiation direction, which will be described later. On the other hand, above the sample 24, a second diffraction grating 28 is disposed parallel to the diffraction grating 27 and supported by a support base 29. They are arranged parallel to these moving directions. Moreover, each diffraction grating 27, 28 has
Those having the same lattice pitch length P are used.
A laser beam of a predetermined wavelength emitted from the laser device 30 is perpendicularly irradiated onto the diffraction gratings 27 and 28 arranged in this manner. The diffracted lights in the +n-order direction and the -n-order direction by the diffraction gratings 27 and 28 are received by the optical sensors 31 and 32 arranged equiangularly with respect to the optical axis of the irradiated laser light, respectively. The light reception signals from these optical sensors 31 and 32 are inputted to a differential amplifier 35 after passing through preamplifiers 33 and 34, and the difference between them is calculated. After unnecessary frequency components are removed from this differential output signal through a filter circuit 36, it is smoothed through a rectifier circuit 37 and a smoothing circuit 38 in order, and is displayed on an indicator 39. Further, the smoothed signal is fed back to the table moving device as table movement control information.

According to the apparatus having such a configuration, interference light of diffracted light in the +n-order direction and -n-order direction by the diffraction gratings 27 and 28 of the laser beam or the diffraction light of the diffraction gratings 27 and 2
The optical sensor 3 receives a signal with an intensity corresponding to the deviation of 8.
As mentioned above, the light is received at the 1st and 32nd. Then, the difference is determined by the differential amplifier 35. At this time, the disturbance light components that have entered the optical sensors 31 and 32 are canceled out and removed by the differential operation of the amplifier 35. Therefore, the adverse effects of ambient light can be removed. Thereafter, the differential signal from the differential amplifier 35 from which unnecessary components such as disturbance light have been removed is input to the filter circuit 36. This filter circuit 36 has a band-pass characteristic of a frequency band f to 2f, where f is the vibration frequency of the vibrator 23, and as a result, vibration components generated when the table 22 moves or due to disturbances are filtered out. removed. Therefore, the adverse effects of floor vibration etc. are eliminated. Thereafter, by rectifying and smoothing the signal from which each unnecessary signal component has been removed, the output of the smoothing circuit 38 receives a direct current corresponding to the amplitude sin (4π/λ(do+Δdsin2πft)} shown in FIG. 13 above). A level signal can be obtained. By displaying this DC level signal value on the indicator 39, the positional relationship between the diffraction gratings 27 and 28 is indicated. Also, according to the DC level signal value, the table moving device 25 controls the movement of the table 22 and positions the table 22 so that the signal value becomes zero.As a result, the diffraction grating 28 (sample 24) is aligned with the diffraction grating 27.

Thus, according to the present apparatus, extremely highly accurate positioning can be performed using the diffraction gratings 27 and 28 without being affected by disturbance light or at the same time, without being adversely affected by floor vibrations or the like. Moreover, this can be achieved by a simple technique of vibrating one of the diffraction gratings 27 in the direction of laser beam irradiation. Therefore, its configuration and control are simple and highly practical.

Note that the present invention is not limited to the above embodiments. For example, instead of vibrating the diffraction grating, the wavelength λ of the laser beam may be modulated as described above. In this case, the wavelength λ of the laser beam at instant time t
(t) is expressed as λ(t)=λo+Δλ sin 2πft (14), and alignment is possible in the same way. Further, the wavelength λ may be modulated by the laser device 30, or by interposing a wavelength modulation element in the laser beam irradiation optical path. Furthermore, the present invention can be applied not only to laser light but also to other coherent light. Furthermore, Fig. 5 shows the change characteristics of the intensity difference value with respect to the deviation of the diffraction grating, which was found by the inventor's experiments, where the characteristic shown by the broken line A is the ±1st order diffracted light, and the characteristic shown by the solid line B is ±
This is the third-order diffracted light. As is clear from this characteristic and the above theoretical formula, when there is no shift between the diffraction gratings, zero output is obtained at a point shifted by 1/2 of the grating pitch length P, but especially when a shift of P/2 occurs. It shows a sharp change in the case. Moreover, the higher the order of the diffracted light, the more clear the change is. Therefore, as shown in FIG. 4, if a plurality of optical sensors 31a and 32a each receiving the ±k-order diffraction light are used, and these optical sensors are selectively used, alignment can be performed easily and with high precision. For example, after rough adjustment is performed using the ±1st-order diffracted light, fine adjustment may be performed using the ±3rd-order diffracted light. It goes without saying that the diffraction grating may be vibrated in a non-contact manner using sound pressure from a speaker or the like. Moreover, when actually applied to various devices, this can be achieved by providing a diffraction grating in each partial area of the alignment member. It goes without saying that not only the reflective type but also the transmissive type may be used. In summary, the present invention can be implemented with various modifications without departing from the gist thereof.

As explained above, the present invention modulates the phase of coherent light diffracted by two diffraction gratings arranged in parallel and having the same grating pitch, and calculates the difference in interference intensity of each diffracted light in the ±n-order direction. After extracting only the modulation frequency component from this difference signal, it is rectified and smoothed, and alignment is performed using the output signal. Therefore, according to the present invention, it is possible to provide a highly effective positioning device that can perform simple and highly accurate positioning without being affected by external light, floor vibration, etc.

[Brief explanation of the drawing]

Figures 1 and 2 are schematic diagrams for explaining the conventional alignment principle, Figure 3 is a schematic diagram for explaining the alignment principle, and Figure 4 is a schematic diagram showing an embodiment of the present invention. FIG. 5 is a diagram showing the change in detection signal intensity with respect to the deviation of the diffraction grating, and FIGS. 6 to 8 a to 8 e are signal waveform diagrams used to explain the principle of the present invention, respectively. 11, 12... Diffraction grating, 13... Laser light (coherent light), 14, 15, 16... Diffraction light (+n
next direction), 17, 18, 19... Diffracted light (-n order direction), 22... Moving table, 23... Piezoelectric vibrator, 27, 28... Diffraction grating, 30... Laser device, 31, 32 ... Optical sensor, 35 ... Differential amplifier, 36 ... Filter circuit, 38 ... Smoothing circuit, 39 ... Indicator.

Claims (1)

[Scope of Claims] 1. First and second diffraction gratings having the same grating pitch, which are respectively attached to a pair of alignment objects while facing each other and arranged in parallel; a light source that perpendicularly irradiates coherent light onto the diffraction grating; means for modulating the phase of the diffracted light by the second diffraction grating with respect to the diffracted light by the first diffraction grating;
From the difference output obtained by extracting only the modulation component from the difference between the interference light intensity in the next direction and the interference light intensity in the −n-order direction, and rectifying and smoothing the output, the first
and means for detecting positional deviation between the second diffraction gratings. 2. The alignment device according to claim 1, wherein the means for modulating the phase of the diffracted light is performed by vibrating at least one of the first and second diffraction gratings in the irradiation direction of the coherent light. 3. The alignment device according to claim 1, wherein the means for modulating the phase of the diffracted light modulates the wavelength of the coherent light. 4. The alignment device according to claim 1, wherein the positional deviation between the first and second diffraction gratings is detected when the difference in intensity of interference light becomes zero. 5. The first and second diffraction gratings are each attached to the object to be aligned so that their mutual facing states are shifted by 1/2 of the grating pitch when the mutual positional deviation of the pair of objects to be aligned is zero. An alignment device according to claim 1.