JPH0616483B2 - Projection optics - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a projection optical apparatus for projecting a mask pattern onto a projection object by a projection optical system, and particularly to the position of the image plane of the projection optical system and the projection object. The present invention relates to a projection optical device in which a deviation from the surface position is detected by using an oblique incident light type focus detector.

(Background of the Invention) In recent years, a reduction projection type exposure apparatus (hereinafter referred to as a stepper) has become widespread as an apparatus for exposing and transferring a circuit pattern drawn on a mask onto a wafer. This stepper uses a reduction projection lens that reduces the pattern image of the mask to ⅕ or 1/10 and projects it on the wafer. The wafer is placed on the two-dimensional moving stage and the wafer is exposed once. After the stage is moved by a fixed pitch, the exposure is repeated. Reduction projection lens used for this kind of stepper (hereinafter simply referred to as projection lens)
Is required to be bright and have high resolution. Therefore, the numerical aperture (numerical aperture) becomes large and the depth of focus becomes extremely shallow. Although the value varies depending on the lens type and the lens configuration, it is at most about several μm. Therefore, when projecting a pattern image of a mask onto a wafer, an apparatus for detecting the position of the surface of the wafer and detecting the focus so that the pattern image is accurately focused on the wafer is disclosed in, for example, Japanese Patent Laid-Open No. 56-42205. No. This device detects the position of the wafer in the optical axis direction of the projection lens by irradiating the surface of the wafer with an image-forming light beam obliquely and detecting the light receiving position of the reflected light from the wafer surface. . In such an apparatus, when the mask pattern image is focused on the wafer, it is necessary to make adjustments at the time of manufacturing the apparatus so that the light receiving position of the reflected light of the image forming light flux becomes the origin. If this adjustment is performed accurately, thereafter, by vertically moving the wafer in the direction of the optical axis of the projection lens so that the light receiving position is aligned with the origin, a focused pattern image is projected on the wafer.

However, the focus position of the projection lens (the position of the image plane) changes in the optical axis direction, although it is a minute amount, due to the environment in which the projection optical device is used, such as changes in temperature and atmospheric pressure. So-called focus shift occurs. Therefore, just by aligning the vertical position of the wafer with a conventional oblique incidence type focus detection device, the wafer surface and the image plane of the projection lens can be stably aligned for a long period of time ( It had the drawback of not being able to focus.

(Object of the Invention) The present invention solves the above-mentioned drawbacks and the pattern image of the mask projected on the object to be projected such as a wafer is always in focus even when the oblique-incidence light type focus detection device is used. The aim is to obtain a projection optics which is maintained.

(Summary of the Invention) The present invention provides a projection optical system (6) for projecting a pattern formed on a first substrate (R) such as a reticle onto a predetermined image plane (P ₁ ), and a predetermined image plane (P _1). ) Side, the surface of the second substrate (wafer W or reference plate) has a predetermined image plane (P ₁ ).
The second substrate is held so as to be substantially parallel to the holding means (Z) and is movable in the optical axis direction of the projection optical system (6).
Illumination light (14) is obliquely projected onto the surface of the second substrate from the outside of the stage 11) and the projection optical system, and a change in the light receiving position of the reflected light (15) from the surface is detected to obtain a predetermined result. A deviation amount (level of the detection signal Sd) of the second substrate in the optical axis direction with respect to a reference position (a position where the detection signal Sd from the PSD 45 becomes a zero point) preset so as to substantially match the image plane (P ₁ ) Oblique-incidence light type focus detection means (20 to 31, 43 to 46) for detecting and first control means (10, 10) for controlling movement of the holding means (11) in the optical axis direction based on the detected shift amount. 42) and the improvement of the projection optical device.

In order to improve it, in the present invention, the reference position (the position where the detection signal Sd becomes the zero point) is arranged in the optical path of the focus detection means (20 to 31, 43 to 46), and the optical axis direction of the projection optical system (6). Through the projection optical system and the movable optical element (parallel plate glass 24) that relatively shifts the light receiving position of the reflected light from the second substrate (wafer W or reference plate) so as to be displaced to the first substrate. (R) and the second substrate (wafer W or reference plate) reciprocating the contrast change (bottom / peak value PB) of the imaging light flux, the holding means (1
Focus detection means (5, 12, 40, 50-) for detecting the focus (Z ₀ ) of the surface of the second substrate and the predetermined image plane (P ₁ ) by moving 1) up and down for detection. 54, and steps 100 to 111), and the deviation to be detected by the focus detection means (20 to 31, 43 to 46) when the surface of the second substrate is located at the focal point (Z ₀ ) detected there. Second control means (26, 42, 62, 63) and steps 120 to 125 for controlling the drive of the movable optical element (24) so that the amount (level of the detection signal Sd) becomes a constant value (for example, zero point). ) And. By the operation of the second control means, the reference position (detection signal Sd
The positional relationship in the optical axis direction between the position (at which is a zero point) and the predetermined image plane (P ₁ ) is calibrated.

(Embodiment) FIG. 1 is a schematic configuration diagram of a projection optical apparatus according to an embodiment of the present invention. The light from the light source 1 passes through the optical fiber bundle 2, the shape of the light flux is shaped by the illumination condenser lens 3, is reflected by the semi-transmissive mirror 4, and enters the observation optical system (for example, microscope objective lens) 5. The light from the light source 1 that has passed through the observation optical system 5 illuminates a transparent reticle (mask) R having a light-absorbing and light-shielding mark M. Then, the image of the mark M is projected onto the wafer W via the projection lens 6. When the projection lens 6 projects and exposes the circuit pattern on the reticle R onto the wafer W, the wavelength of light from another exposure light source that illuminates the reticle R (hereinafter, referred to as
The chromatic aberration is corrected for the exposure wavelength). Therefore, when the reticle R is illuminated with light having the exposure wavelength and the pattern surface on which the mark M of the reticle R is formed is located at a distance d ₁ from the pupil position 6a of the projection lens 6, the pupil of the projection lens 6 is projected. An image of the mark M is formed on the image forming plane P ₁ located at the distance d ₂ from the position 6a. Therefore, the wavelength of the light from the light source 1 that illuminates the mark M is adjusted so that the imaging plane P ₁ does not change from the position during projection exposure due to chromatic aberration.
That is, it is set equal to the exposure wavelength so that the reticle R and the image plane P ₁ are kept conjugate. On the other hand, an XY stage 9 that is two-dimensionally moved by a drive unit 8 is provided on the surface plate 7. A light-reflective wafer W is placed on the XY stage 9, and a drive unit 10 drives a projection lens. 6
A Z stage 11 that moves up and down in the optical axis direction is provided.
It is assumed that the surface of the wafer W is lightly coated with a photosensitive agent. In FIG. 1, the surface of the wafer W is located in a direction away from the projection lens 6 with respect to the image plane P ₁ (hereinafter, this state is referred to as a rear-pinned state).
The light flux l ₁ forming the optical image of the mark M is incident on the projection lens 6 and is focused on the imaging plane P ₁ and then reflected on the surface of the photosensitive agent on the wafer W or the surface of the wafer W itself. The light enters the projection lens 6 again. The surface of the photosensitizer and the surface of the wafer W itself will be simply referred to as the surface of the wafer W hereinafter.
The reflected light beam l ₂ passes through the transparent portion around the mark M of the reticle R again, and passes through the observation optical system 5 and the semi-transmissive mirror 4.
It reaches the light receiving surface of the television camera (photoelectric detection device) 12. The light receiving surface of the TV camera 12 is a reticle R by the observation optical system 5.
The rear surface, that is, the conjugate relationship with the pattern surface is maintained. Therefore, the mark M observed by the observation optical system 5
The light image of is always formed on the light receiving surface of the television camera 12 as indicated by a light beam l ₃ in the figure. Further, when the mark M is projected onto the wafer W by using the light of the exposure wavelength, the pattern surface and the image plane P ₁ have a conjugate relationship with each other by the projection lens 6, so that the image plane P ₁ and the television camera 12 are combined. It also has a conjugate relationship with the light-receiving surface. Therefore, in the rear focus state, the reflected light beam l ₂ is imaged by the observation optical system 5 on the surface P ₂ away from the light receiving surface of the television camera 12. When the wafer W is lifted from the state shown in FIG. 1, when the surface of the wafer W and the image plane P ₁ coincide (hereinafter, this state is referred to as a focused state), the reticle R and the wafer W are separated from each other. The optical path of the luminous flux l _{1 and} the optical path of the reflected luminous flux l ₂ that reciprocate between the _two coincide with each other, and the reflected luminous flux l ₂ forms an image on the light receiving surface of the television camera 12. Further, in the state where the surface of the wafer W is located on the side of the projection lens 6 with respect to the image plane P ₁ (hereinafter, this state is referred to as the front focus state), the reflected light beam l _{2 is} again reflected to the optical path of the light beam l _1. , The reflected light flux l ₂ is imaged on a virtual plane P ₃ which is retracted from the light receiving surface of the television camera 12.

Now, in this projection optical device, a projector 13 for projecting an image forming light beam l ₄ of an image of a pinhole or a slit obliquely toward the image forming plane P ₁ and reflection of the image forming light beam l _{4 on} the surface of the wafer W are provided. The light l ₅ is received and the vertical direction of the wafer W (projection lens 6
And a light receiver 14 for detecting the position in the optical axis direction. According to the vertical position of the wafer W, the light receiver 14 photoelectrically detects that the reflected light l _{5 in} the rear focus state and the reflected light l ₅ ′ in the focused state differ from each other. Is to be detected by. The light projector 13 and the light receiver 14 detect the position of the wafer W in the vertical direction to detect the wafer W.
A focus detection system for detecting the matching state between the surface of the lens and the image plane P ₁ .

FIG. 2 is a specific configuration diagram of this focus detection system. Inside the projector 13, a light source 20 that emits light of a wavelength (for example, infrared light) that does not sensitize the photosensitizer applied to the wafer W, and a light source
Slit plate 21 illuminated by 20, and this slit plate
A lens 22 that collimates light that has passed through a slit opening formed in 21 in a direction perpendicular to the paper surface to form a parallel light flux, a mirror 23 that reflects the parallel light flux from the lens 22, and a parallel light flux from the mirror 23 Then, the parallel plate glass (plane parallel) 24 that shifts the optical axis of the light beam and emits the light beam, and the parallel light beam from the parallel plate glass 24 are condensed to form a slit plate in the image forming plane P ₁ of the projection lens 6. A lens 25 for forming the slit image of 21 is provided. The parallel plate glass 24 has a rotation axis perpendicular to the plane of the drawing, and is tilted by the driving unit 26 within a certain angle range around the rotation axis. Due to this inclination, the image forming position of the slit image by the lens 25 is changed to the image forming plane P.
_It is displaced in the direction perpendicular to ₁ . Incidentally, FIG. 2 shows a focused state in which the surface of the wafer W and the image plane P ₁ coincide with each other.

On the other hand, the photodetector 14 has a lens 27 that makes the reflected light flux l ₅ incident.
A vibrating mirror 28 that reflects the light beam from the lens 27 and changes the direction of the reflection by vibration, a slit plate 29 provided at the image forming position of the light beam from the lens 27, and a paper surface on the slit plate 29. Slits 29 extending vertically
A light receiving element 30 that receives the light that has passed through a and outputs a photoelectric signal is provided. The vibrating mirror 28 has a rotation axis perpendicular to the plane of the drawing, and the drive unit 31 causes the vibrating mirror 28 to simply vibrate at a constant angular frequency and a constant amplitude about the rotation axis.

In the configuration as shown in FIG. 2, in the focused state, the slit image of the slit plate 21 is formed on the surface of the wafer W, and the slit image on the wafer W is again formed by the lens 27.
An image is formed on 29, and the slit image is reciprocally moved with a constant amplitude on the slit plate 29 by the vibrating mirror 28. At this time,
The center of vibration of the slit image on the slit plate 29 is the slit 29.
matches a. Therefore, the surface of the wafer W is projected onto the projection lens 6
When it deviates from the image forming plane P _{1, the} center of vibration of the slit image is also displaced in the horizontal direction in the drawing with respect to the slit 29a of the slit plate 29. Of course, the direction of displacement (right or left) with respect to the slit 29a indicates the front pinned state or the rear pinned state.

Now, FIG. 3 is a block diagram of a control system of this apparatus. An image signal from the television camera 12 is input to a signal processing circuit (imaging detection device) 41 via a camera control unit (CCU) 40. The signal processing circuit 41, based on the image signal, a television camera of an image (superposed image) in which the optical image of the mark M and the reflected image of the mark M on the wafer W are superposed.
The detection signal S _a which indicates the amount of the Z stage 11 to be moved according to the image formation state is detected by detecting the image formation state on the 12 light receiving surfaces.
Is output. The main control unit 42 has a microprocessor (hereinafter referred to as a host MPU), and based on the detection signal Sa, makes the image formation state of the superimposed image the best, that is, makes the contrast of the superimposed image the highest. Then, the drive signal Sb for positioning the wafer W in the vertical direction is output to the drive unit 10. Also, the main controller 42 is X
The drive unit 8 of the Y stage 9 also outputs a signal Sc for two-dimensional positioning. The drive unit 31 of the vibrating mirror 28 inputs an AC signal of a constant frequency generated from the oscillator 43,
The vibrating mirror 28 is simply vibrated in accordance with the frequency of the AC signal. The photoelectric signal of the light receiving element 30 is amplified by the amplifier 44, and the photoelectric signal and the AC signal from the oscillator 43 are input to the phase synchronous detection circuit (PSD) 45. PSD45 is an oscillator
The photoelectric signal is synchronously detected by using the AC signal of 43 as a reference signal. The low-pass filter (LPF) 46 removes harmonic components from the signal detected by the PSD 45, and inputs the signal to the main controller 42. The output signal of the LPF 46, that is, the detection signal Sd becomes zero when the vibration center of the slit image matches the slit 29a, and the vibration center is, for example, the slit.
This is a so-called S-curve signal that has a positive polarity when displaced to the left in FIG. 2 with respect to 29a and has a negative polarity when displaced to the contrary. However, the zero point of this S-curve signal is obtained when the surface of the wafer W coincides with the reference position in the optical axis direction set in advance by the arrangement of the light projector 13 and the light receiver 14 of the oblique incident light type focus detection device. However, the reference position is not always always exactly on the image plane P ₁ of the projection lens 6. Further, the main controller 42 sends a drive signal Se that determines the rotation angle of the parallel flat glass 24,
Output to the drive unit 26. The drive signal Sb is the detection signal S
Not only is it output based only on a, but the detected signal S
It is also output according to d. Which of the two signals Sa and Sd the drive signal Sb is output is appropriately selected by the host MPU included in the main controller 42.
Further, the host MPU controls the arithmetic operation necessary for outputting the drive signals Sb, Sc, Se and the output timing thereof, and totally controls the sequence of the entire apparatus.

FIG. 4 is a circuit block diagram showing a specific example of the signal processing circuit 41. In FIG. 4, a high speed analog-to-digital converter (hereinafter referred to as ADC) 50 digitally converts the level of the image signal from the CCU 40. The sampling pulse generator 51 generates a pulse signal SP consisting of a pulse corresponding to each pixel in the horizontal direction during one horizontal scanning period of the television camera 12 in response to the horizontal synchronizing signal SH from the CCU 40. The ADC 50 samples the image signal in response to each pulse of the pulse signal SP, and converts the sampled level into a digital value. Digital data (hereinafter, simply referred to as data) D1 exchanged by the ADC 50 is a randomly accessible memory circuit (hereinafter, referred to as RAM) 52.
To enter. The address generation circuit 53 counts each pulse of the pulse signal SP and generates the address signal Ad of the RAM 53. Therefore, the RAM 52 sequentially updates the address in response to the pulse signal Sp, and stores the data D1 at each sampling at the updated address. A microcomputer (hereinafter, referred to as a CPU) 54 managed by a host MPU of the main control unit 42 sequentially reads the data D1 stored at each address of the RAM 52 to determine a rising or falling state of the image signal level. , That is, the contrast of the superimposed image is detected. Further, the CPU 54 erases the data stored at each address in the RAM 52, and clears the count value in the address generation circuit 53 to zero or an initial value, and starts the operation of the signal processing circuit 41. And a start signal St. The start signal St is input to the sampling pulse generator 51 and the address generation circuit 53. The sampling pulse generator 51 receives the start signal St and then outputs the horizontal synchronizing signal SH.
When receiving, the pulse signal SP is generated. Further, the address generation circuit 53 generates the address signal Ad according to the pulse signal Sp while the start signal St is input, but when the start signal St is not input, the CPU generates the address signal Ad.
Address signal Ad from 54 via address bus ADB
And the CPU 54 of the RAM 52 can access it. Further, the address generating circuit 53 determines that the data D1 for one scanning line has been taken in at the end point of one scanning of the horizontal scanning line, that is, when the count value of the pulse signal SP reaches a predetermined value, and outputs the acquisition end signal CR. Output to CPU 54.
In the above configuration, in order to simplify the following description, one horizontal scanning line of the television camera 12 has 1024 pixels, and the ADC 50 has 8 bits (that is, the data D1 is 8 bits).
RAM52 is used, and RAM52 is 1 Kbyte (1024 ×
A device with a capacity of 8 bits or more shall be used. Also C
The PU54 stores data such as calculation results and constants,
A memory is provided to store the program.

Now, FIG. 5 shows the detection signal S provided in the main controller 42.
It is a circuit block diagram of a processing circuit of d. Detection signal Sd
Is a switching signal S1 from the host MPU in the main control unit 42.
Is input to the switch 60, which performs the switching operation in response to. The differential circuit 61 outputs to the switch 60 a difference signal corresponding to the deviation of the detection signal Sd from the zero level (ground potential). The switch 60 outputs the detected signal Sd and one of the difference signals thereof to an analog-digital converter (hereinafter referred to as ADC) 62 according to the switching signal S1. The ADC 62 receives the detected signal Sd or the difference signal as digital data D2.
To the host MPU and also to the latch circuit 63. The latch circuit 63 switches whether to latch the data D2 according to the signal S2 from the host MPU. Then, the data D2 output via the latch circuit 63 becomes the above-mentioned drive signal Se. In such a configuration, the switch 60 is a plane parallel
The host MPU selects the difference signal of the differential circuit 61 when adjusting the 24 angles (calibration) and the detection signal Sd when detecting the vertical position of the wafer W (focus detection). Can be switched.

Next, the operation of this device will be described. 6 and 7 are flowcharts for explaining the operation of the signal processing circuit 41 and the host MPU, and FIG. 8 is a time chart of each signal of the signal processing circuit 41. FIG. 9 is a diagram showing an example of a superimposed image formed on the light receiving surface of the television camera 12 in the rear focus state shown in FIG.

The mark M optical image M'of the reticle R is as shown in FIG.
It has an elongated slit shape and has edges Eg ₁ and Eg ₂ extending in a direction orthogonal to the horizontal scanning line SL of the television camera 12. The scanning line SL and the edges Eg ₁ and Eg ₂ are not necessarily orthogonal to each other, but here they are orthogonal for convenience. Since the observation optical system 5 forms an optical image M ′ of the mark M on the light receiving surface of the television camera 12, the edges Eg ₁ and Eg ₂ are imaged in a focused state with extremely high contrast. The reflected image of the mark M, which is back-projected through the wafer W, is defocused (non-focused) at the position of the reticle R. Therefore, the reflected image does not exactly overlap with the optical image M'on the light receiving surface of the television camera 12, and becomes a blurred reflected image Md that spreads around the edges Eg ₁ , Eg _2, etc. with low contrast. Since the mark M has a light-shielding property, the reflected image Md appears dark around the optical image M ′, and the optical image M ′ itself becomes black because the mark M is a light absorber. On the other hand, since the illumination light is reflected by the wafer W in portions other than the light image M ′ and the reflection image Md on the scanning line SL, it becomes whitish. Therefore, the contrast around the edges Eg ₁ and Eg ₂ of the light image M ′ is detected, and the best contrast is obtained, that is, the reflected image Md and the light image M ′ are exactly coincident with each other. Good.

The flowchart of FIG. 6 shows a program for matching (focusing) the surface of the wafer W and the image plane P1 of the projection lens 6. The steps 100 to 111 of the program will be described below.

[Step 100] First, the Z stage 11 is set to the initial position. Here, the position where the wafer W is most lowered as shown in FIG. 1 is set as the initial position, and the Z stage 11 is driven according to the drive signal Sb from the main controller 42.

[Steps 101 and 102] Next, the host MPU outputs a start command to the CPU 54, and in response thereto, the CPU 54 outputs a clear signal CL, and RA
M52 and address counter 53 are cleared. This allows
The address 0 of the RAM 52 becomes accessible. Then, the start signal St is continuously inverted from the logical value "L" to "H". After that, as shown in FIG. 8, when the horizontal synchronizing signal S _H is input, a pulse signal SP is generated from the input point, and the image signal from the CCU 40 is sequentially sampled in response to the pulse and digitized data. D1 is R
The data is stored in order from the 0th address of AM52. As shown in the histogram in FIG. 8, the data group stored in the RAM 52 has the lowest level in the part of the optical image M ′, and the parts on both sides thereof are gentle because the reflection line Md is blurred (low contrast). The level becomes higher.

[Steps 103 and 104] Thus, at the end of the horizontal scanning period, the pulse signal SP
When 1024 pulses are counted by the address generation circuit 53,
The address generation circuit 53 outputs the end signal CR. Therefore, the CPU 54 monitors the end signal CR and outputs the end signal C.
When R is input, the start signal St is set to the theoretical value "L".
Flip to. As described above, the image signal generated in one horizontal scanning is divided into 1024 pixels, the luminance level of each divided pixel is converted into digital data, and the RAM 52
It is stored in addresses 0 to 1024 of the above.

[Step 105] Next, the CPU 54 selects a data string of the edges Eg ₁ and Eg _{2 of the} optical image M ′ from the data group in the RAM 52, and cuts the trailing and rising portions of the time-series image signal. Specifically, as shown in FIG. 10 (a), the data from the 0th address to the 1024th address in the RAM 52 are examined, and the address AdO corresponding to the edge Eg ₁ and the address Ad ₂ corresponding to the edge Eg ₂ are detected. Then, seek an address Ad ₁ intermediate between the addresses AdO and Ad ₂ .
Then, the address 0 to the address Ad ₁ is the falling section of the image signal, and the addresses Ad ₁ to 1024 are the rising section.

[Step 106] Next, the CPU 54 first-order differentiates the data sequence in the RAM 52 with respect to the falling section and the rising section. This differentiation can be processed at high speed, for example by passing the data sequence through a numerical filter. Specifically, as shown in FIG. 10 (b), the differential waveform data is stored in the RAM 52 at addresses other than 0 to 1024. Here, although the envelope of the waveform is shown, it is actually a discrete data string like the data D1 shown in FIG.

[Step 107] Next, the CPU 54 causes the peak value P _v on the differential waveform in the falling section to be generated.
And the bottom (lowest) value B _v, and the absolute value P of the difference
Calculate B (| P _v −B _v |). Do the same for the rising section. This value PD is stored in the memory of the CPU 54. The value PB becomes larger as the rising and falling edges of the image signal become steeper, and becomes smaller when the falling and rising edges of the image signal are gentle as shown in FIG.

[Steps 108, 109] Next, the CPU 54 determines whether or not the Z stage 11 is located at the limit point (upper limit) of the rise, and if the upper limit is not reached yet, the Z stage 11 is moved by a unit amount, for example, 0.1 μm. The data for increasing the value is output to the host MPU of the main control unit 42. As a result, the drive unit 10 operates and the Z stage 11
Is increased by 0.1 μm.

After that, the same operation is repeated again from step 101, the Z stage 11 is moved up by 0.1 μm, peak-bottom detection is performed, and the increased position is stored together with the value PB. This is performed up to the upper limit position of the Z stage 11.

[Step 110] When the CPU 54 detects the upper limit of the Z stage 11 in step 108, data shown in FIG. 11, for example, is obtained in the memory of the CPU 54. In FIG. 11, the horizontal axis represents the vertical position Z of the Z stage 11 at every 0.1 μm, and the vertical axis represents the value PB corresponding to each position. Therefore, the CPU 54 sets this value PB
The position Z ₀ where the (bottom / peak value) becomes maximum (extreme value) is examined.

[Step 111] Next, the CPU 54 outputs to the main controller 42 data (detection signal Sa) for positioning the Z stage 11 at the position Z ₀ . As a result, main controller 42 outputs drive signal Sb to lower Z stage 11 so that the surface of wafer W and image plane P ₁ coincide with each other. That is, the rising and falling edges of the image signal are the steepest at the position Z _O , which indicates that the optical image M ′ and the reflected image Md exactly match (focused state).

As described above, the image plane P ₁ of the projection lens 6 and the surface of the wafer W are aligned with each other and the image of the mark M is projected on the wafer W in a focused state. Calibration of the focus detection system constituted by the device 14 is performed based on the flowchart of FIG. Hereinafter, steps 120 to 125 for that purpose will be described.

[Steps 120 and 121] The host MPU of the main controller 42 outputs the switching signal S ₁ and selects the difference signal of the differential circuit 61. Continue to host M
The PU outputs the signal S ₂ to cancel the latch operation of the latch circuit 63. As a result, the plane parasol 24 rotates under feedback control according to the difference signal, and stops its rotation when the difference signal becomes zero. This state
Shown in Figure 12. In FIG. 12, the horizontal axis represents the vertical direction of the wafer W (Z
Direction) and the vertical axis represents the magnitude (level) of the detection signal Sd. Immediately after the latch is released, it is assumed that the detection signal Sd has the level at -Sd _O as shown by the characteristic SD1. That is, if in accordance with the characteristics SD1, despite the surface of the wafer W and the image plane P ₁ is coincident with the position Z _O, -Z ₁ from the position Z _O is a true focus
Only at the position where the wafer W descends, it is detected as if the in-focus point exists. Therefore, the differential circuit 61 outputs the difference signal of the level -Sd _O , and the plane parallel 2
Change the angle of 4. Along with this, the level −Sd _O becomes smaller, and the characteristic SD1 is corrected to the characteristic SD ₂ which becomes the zero level at the position Z _O.

[Steps 122 and 123] The host MPU reads the data D ₂ from the ADC 62,
It is determined whether D ₂ has become zero, that is, whether the difference signal of the differential circuit 61 has become zero. At this time, data D
_{When 2} is completely zero, it is preferable to consider whether the noise component of PSD45 is not completely removed by LPF46 and to judge whether or not the data D ₂ has entered a level within a range including zero. To In this way, when the data D ₂ is judged to be substantially zero, the characteristic SD1 is corrected to the characteristic SD2.

[Steps 124 and 125] Next, the host MPU outputs the signal S ₂ and the latch circuit 63
Is latched, and thereafter, the latched state is maintained until the signal S ₂ is input again. Then, the drive unit 26 of the plane parallel 24 uses the latched data (drive signal Se) as a reference value and continues to maintain the angle of the plane parallel 24 that has been set thereafter. Then, the host MPU outputs the switching signal S ₁ and selects the detection signal Sd.

As described above, the calibration of the focus detection system shown in FIG. 2 is completed. When actually projecting and exposing the pattern of the reticle R onto the wafer W, the host MPU
Reads the data D2 of the detection signal Sd and outputs the drive signal Sb so that the value becomes zero, so that the wafer W is moved up and down.

Further, in the above embodiment, the reflection image Md from the wafer W is superposed on the optical image M'and observed, but the reflection flat plate is not limited to the wafer W. For example, Z stage
A reference plate having a reference mark provided on the surface of the wafer 11 at substantially the same height as the surface of the wafer W may be used. The reference plate is
Originally used for the reference alignment of the device and the drift check of the optical axis of the positioning microscope, the mark M is formed by using the high reflectance portion where the reference mark is not formed.
The projected image of may be reflected.

Next, a second embodiment of the present invention will be described with reference to FIGS. 13, 14 and 15. In the second embodiment, a plurality of marks M provided on the reticle R are arranged in parallel at a constant pitch in a grid pattern, that is, a mark having a periodic structure for forming so-called line and space. FIG. 13 is a cross section in the scanning line direction of the reticle R provided with the grid-like marks, and the image signal S _{v in} the rear focus state, the in-focus state, and the front focus state,
The light intensity distribution C _w on the wafer W corresponding to the scanning line when the image signal S _v is obtained is shown. FIG. 13 (a) shows the rear pin state as shown in FIG. When the light reflected by the wafer W again passes between the lattice-shaped marks and passes through the transmission portion between the marks, the edge periphery of the mark rises as shown in the image signal S _v. , Falling and drooling. FIG. 13B shows the in-focus state, in which the image projected on the wafer W has the highest contrast and the superimposed image also has the highest contrast. Fig. 13 (C)
Represents the front focus state, and both the image signal S _v and the intensity distribution C _w are similar to the characteristics in the rear focus state. In the above three states, in the transmissive portion between the marks, the waveform of the peak portion a of the image signal S _v and the waveform of the peak portion a ′ of the intensity distribution C _w are similar. However, since the mark of the reticle R is projected on the wafer W and the reflection image thereof is observed, the peak portion a of the image signal S _v is different from that of the waveform of the peak portion a ′ of the intensity distribution C _w . The dullness (blurring degree) of the waveform is doubled. On the other hand, FIG. 14 shows a case where the amounts of the front focus and the rear focus are largely deviated from the focused state, and the intensity distribution C _w becomes almost uniform, and the intensity distribution C _w passes through the transparent portion between the marks of the reticle R. The light is considered to illuminate the wafer W uniformly. However, its intensity distribution C
The size of _w is smaller than the size of the peak portion in the case of Fig. 13 (b). For this reason, the image signal S _v has both a rising edge and an abrupt edge, and shows a characteristic like an in-focus state.
The size of the peak portion is smaller than that in the true focused state.

Therefore, when the rising edge and the falling edge of the image signal S _v are detected in the same manner as in the first embodiment, the characteristics shown in FIG. 15 are obtained. The horizontal axis of FIG. 15 represents the position of the wafer W in the Z direction, and the vertical axis represents the value PB (contrast value). Focus is located Z _O, position Z value PB in accordance _O away from decreases, further increases defocus the value PB
Will increase again. If the surface of the wafer W is aligned with the position Z _O at which this value PB shows an extreme value, a focused state can be obtained.
The waveform shown in the figure may be further differentiated (second derivative) to obtain the extreme value.

As described above, in the present embodiment, since a plurality of light-shielding marks are arranged in the scanning direction, a plurality of rising edges and falling edges occur in the image signal during one horizontal scanning. Therefore, the contrast can be detected by repeating a plurality of rising sections or falling sections in the image signal, so that not only the error due to noise in the image signal can be reduced, but also the accuracy can be improved by the repetition. There is also an advantage. Further, when such a grid-like mark is used, the image signal has a continuous repetitive waveform, so by detecting by Fourier transform to what degree the higher harmonic components included in this waveform exist, You can also find out the in-focus state.

In each of the above embodiments, the photoelectric detection device is a television camera, but the same effect can be obtained by using a solid-state image sensor in which a plurality of light receiving elements are arranged one-dimensionally or two-dimensionally.

Further, in each of the above-mentioned embodiments, the mark M is made light-absorbing, but depending on the optical configuration of the projection lens 6, a light-reflecting mark formed of a gold-side thin film such as chrome can also obtain the same effect. . The optical configuration of this type of projection lens 6 is roughly classified into two types. There are two types, one is a telecentric optical system on both the reticle R side and the wafer W side, and one is a non-telecentric optical system on the reticle R side and a telecentric optical system on the wafer side.
In the former case, the principal ray of the luminous flux that illuminates the mark M of the reticle R and the principal ray of the observation optical system are perpendicular to the reticle R (parallel to the optical axis of the projection lens), so that the mark M is the same as in the above embodiment. Is made light-absorbing, or the reticle R is illuminated with polarized light, and a member (for example, a quarter-wave plate) that changes the polarization state is inserted in the optical path between the reticle R and the wafer W, Unless an observation optical system having a polarization characteristic that cuts the reflected light from the mark M and transmits the reflected light from the wafer W is transmitted, the reflected light from the mark M is transmitted to the observation optical system 5 and the television camera 12. Upon incidence, the optical image M ′ of the mark m becomes whitish, and with the reflected light from the wafer W,
A whitish image is formed on the entire light receiving surface, which makes it difficult to process the image signal. On the other hand, in the latter case, since the reticle R side is a non-telecentric system, if the mark M is provided at a position where the optical axis of the projection lens does not pass, for example, if it is provided around the reticle R, the principal ray of the illumination light flux of the mark M and the observation optical system 5 The principal ray of is coincident with the line connecting the mark M and the center of the pupil of the projection lens and is inclined with respect to the optical axis of the projection lens, that is, both principal rays are not perpendicular to the pattern surface of the reticle R. , It will be tilted at an angle. Therefore, even if the mark M is light-reflecting, the reflected light of the light that illuminates the mark M does not return to the observation optical system 5 and the television camera 12 but travels in the other direction. The reflected light is transmitted and illuminated, and the light image M ′ of the mark M is captured as a dark shadow. As described above, when the reticle R side is a non-telecentric system, even if the mark M is reflective and light-shielding, the image signal processing can be performed in the same manner as in each of the above embodiments.

In the present invention, as described above, the mark M may be either light reflective or light absorptive, and if the projection lens is a non-telecentric system on the reticle R side, it is not necessary to consider the reflection and absorptivity of the mark M. In the case of a telecentric system, a completely similar effect can be obtained only by illuminating the mark M with polarized light and giving polarization characteristics to the projection lens and the observation optical system.

(Effect of the Invention) As described above, according to the present invention, the absolute position of the in-focus point is detected by detecting the contrast of the image-forming light flux that reciprocates between the first substrate and the second substrate through the projection optical system. Then, since the oblique incident light type focus detection system is calibrated with respect to the detected focus point, even if the optical characteristics of the projection optical system fluctuate (focus shift), it is possible to follow it. When the detection reference position of the oblique incident light type focus detection system is corrected and the mask pattern is exposed on the photosensitive substrate, even if the photosensitive substrate is focused by relying only on the oblique incident light type focus detection system, A precise pattern image is projected. Further, since a movable optical element such as a parallel plate glass is provided in the optical path of the oblique-incident light type focus detection system to shift the detection reference position, the detection signal is compared with a case where the detection signal is simply electrically offset. There is also an advantage that the characteristics of the signal, for example, the symmetry with respect to the zero point of the S-curve signal, is not impaired. Further, as shown in the embodiment, if the pattern of the first substrate is projected onto the second substrate and the contrast of the image obtained by superimposing the reflected image and the pattern of the first substrate is detected, the image contrast is improved. The image-forming light flux to be determined reciprocates in the projection optical system. For example, as compared with the case where the image cosidence due to the image-forming light beam from the pattern provided on the second substrate is only detected through the projection optical system. The detection sensitivity can be increased about twice.

[Brief description of drawings]

1 is a schematic configuration diagram of a projection optical device according to a first embodiment of the present invention, FIG. 2 is a schematic configuration diagram of an indirect focus detection device, FIG. 3 is a block diagram of a control system, FIG. 4 shows a processing circuit for processing the image signal and detecting the contrast of the image.
41 is a circuit block diagram, FIG. 5 is a circuit block diagram for calibrating parallel plate glass, and FIG. 6 is a flowchart diagram for the operation of matching the surface of the wafer with the image plane of the projection lens. FIG. 7 is a flow chart diagram for calibration of an indirect focus detection device,
FIG. 8 is a time chart diagram in the circuit block of FIG. 4, FIG. 9 is a diagram showing an example of a superimposed image, FIG. 10 is a waveform diagram showing an image signal and its differential signal, and FIG. 11 is a contrast diagram. A characteristic diagram showing the relationship with the focal position, FIG. 12 is a characteristic diagram showing the state of the detection signal of the phase-locked detection circuit, FIG. 13,
FIG. 14 is a waveform diagram showing the state of an image signal according to the second embodiment of the present invention, and FIG. 15 is a characteristic diagram showing the relationship between the contrast and the focus position in the second embodiment. [Explanation of symbols for main parts] 5 ... Observation optical system, 6 ... Projection lens 11 ... Z stage, 12 ... TV camera 13 ... Projector, 14 ... Receiver 24 ... Parallel flat glass , 40 …… Camera control unit 41 …… Signal processing circuit

Claims (4)

[Claims] 1. A projection optical system for projecting a pattern formed on a first substrate onto a predetermined image forming surface, and a surface of a second substrate disposed on the side of the predetermined image forming surface is substantially the same as the predetermined image forming surface. While holding the second substrate so as to be parallel to each other, holding means that is movable in the optical axis direction of the projection optical system, and obliquely projects illumination light from the outside of the projection optical system onto the surface of the second substrate. Then
By detecting a change in the light receiving position of the reflected light from the surface, a skew for detecting the amount of deviation of the second substrate in the optical axis direction with respect to a reference position preset so as to substantially match the predetermined image plane. In a projection optical device comprising incident light type focus detection means and first control means for controlling movement of the holding means in the optical axis direction based on the detected shift amount, in the optical path of the focus detection means. The second position so that the reference position is displaced in the optical axis direction of the projection optical system.
A movable optical element that relatively shifts a light receiving position of reflected light from the substrate; and a change in the contrast of an image-forming light flux that reciprocates between the first substrate and the second substrate through the projection optical system, the holding Focusing point detecting means for detecting a focusing point between the surface of the second substrate and the predetermined image plane by moving the means in the optical axis direction for detection; and the second substrate at the detected focusing point. A second control means for controlling the drive of the movable optical element so that the amount of deviation to be detected by the oblique incident light type focus detection means becomes a constant value when the surface of the A projection optical apparatus for calibrating a positional relationship between the reference position and the predetermined image plane in the optical axis direction. 2. The movable optical element is a parallel plate glass which is tiltably arranged in the optical path of the oblique-incidence light type focus detection means, and the second control means controls the surface of the second substrate. The drive unit for changing the inclination of the parallel flat plate glass so that the amount of deviation detected by the focus detection unit when being positioned at the in-focus point is substantially zero. The device according to paragraph. 3. The first substrate is a mask having a pattern such as a semiconductor element, and the holding means holds a photosensitive substrate to be exposed with the pattern of the mask projected by the projection optical system, The reference plate provided at substantially the same height as the surface of the photosensitive substrate is held as the second substrate.
The device according to paragraph. 4. The in-focus point detecting means detects, as the in-focus point, a position at which a contrast change has an extreme value under an exposure illumination light beam for exposing the mask pattern onto the photosensitive substrate. 4. The apparatus according to claim 3, wherein the illumination light of the oblique incident light type focus detection means is set to a wavelength that is non-photosensitive to the photosensitive substrate.