JPH04124807A - Measuring method of measurement mark for base line measurement - Google Patents

Abstract

PURPOSE:To enable measuring a measurement mark with a high accuracy even after repetitive use of a reversible photosensitive body by measuring and storing the contrast information of a transfer part before the transfer of the measurement mark and by subtracting said contrast information from a measurement mark information measured after the transfer to calculate the quantity of positional correction. CONSTITUTION:In an exposure device having an alignment system by off-axis, when a measurement mark is transferred to an object having a reversible photosensitive material by an exposure system and the measurement mark is measured by an off-axis system for the purpose of calculating the quantities of positional correction of the off-axis system and exposure system, the contrast information of a transfer part is measured and stored by the off-axis system before the transfer of said measurement mark and said contrast information obtained before the transfer is subtracted from the measurement mark information measured after the transfer for the purpose of calculating the quantity of positional correction. For example, step 1 for carrying a dummy wafer to a measuring position, step 2 for measuring the measurement mark (after-image) before exposure, step 3 for storing the measurement mark information obtained before the exposure and step 7 for subtracting the measurement nark information obtained after the exposure are added to the title method.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is directed to a method for measuring the distance between the position reference of the off-axis system and the position reference of the exposure system, that is, the height line in an exposure apparatus having an off-axis alignment system. ,
The present invention relates to a method for measuring measurement marks recorded on an object including a reversible photosensitive material, such as a dummy wafer for an exposure device.

[Prior Art] An exposure apparatus for manufacturing semiconductor devices such as ICs and LSIs is required to have two basic performances: resolution performance and overlay performance.

The resolution performance required for future exposure equipment is 0.5
The exposure method that can achieve this is in the vicinity of μm.
Examples include exposure equipment using an excimer laser as the light source, proximity type aligners using X-rays as the exposure source, and direct writing methods using electron beams (EB).However, in terms of productivity, excimer lasers are preferable. The exposure method is considered the most promising.

On the other hand, overlay accuracy (alignment accuracy) is required to be 1/3 to 115 of the minimum line width for printing, and achieving this accuracy is generally equivalent to or better than achieving resolution performance. It is accompanied by difficulties. For alignment, TT makes the alignment light the same wavelength as the exposure wavelength.
There are two possible off-axis methods in which the alignment light can have a wavelength different from the exposure wavelength by separating the on-axis alignment system and the alignment optical system from the projection lens. However, when a TTL on-axis alignment system is adopted in an exposure apparatus using an excimer laser, the signal light from the wafer is attenuated at the 81 end in a process such as absorption resist.

Further, alignment light of other wavelengths can be used in a TTL on-axis system, but in this case, a correction optical system is required, resulting in a long optical path length and a complicated alignment system.

On the other hand, in an exposure apparatus having an off-axis system, the wafer alignment optical system can be freely designed without being subject to any restrictions of the projection lens, and this degree of freedom can enhance process adaptability.

However, with this method, it is not possible to observe the reticle and wafer simultaneously; the reticle is aligned to a predetermined standard using a reticle alignment microscope, and the wafer is aligned using a wafer alignment microscope (hereinafter referred to as "wafer microscope"). Alignment is performed to the reference inside the microscope. For this reason, there is a factor of error between the reticle and the sea urchin. Furthermore, after wafer alignment, in order to overlap the wafer pattern with the projected image of the reticle, a predetermined distance l is set.
I! (This is referred to as the "reference length" or "baseline.") The wafer must be moved. Therefore, this method results in an increased system error factor.

In an apparatus having an alignment method that includes such system errors, it is necessary to try to keep these error factors stable and at the same time periodically check and correct the amount.

To measure this amount of correction, we use a tummy wafer that is sensitive to the light that irradiates the reticle (exposure light) and has a reversible photosensitive material that can be written and erased, measure the baseline, and make corrections. The amount is being calculated.

Conventionally, baseline measurement has been performed using the method shown in the flowchart of FIG. 6.

Here, FIG. 6 will be briefly explained.

First, the dummy wafer transported from the transport unit is transported to a predetermined position where the measurement mark for measuring the baseline is exposed, and the measurement mark on the reticle is exposed (
write). Next, the exposed dummy wafer is moved under an off-axis wafer microscope, and the amount of error relative to a predetermined standard is measured. The center of gravity position was calculated from the measured measurement mark information, and the baseline correction amount was calculated.

[Problems to be Solved by the Invention] Incidentally, the dummy wafer used for baseline measurement is regenerated with written measurement marks erased, and used again. In this case, it is generally known that as the number of times the dummy wafer is played increases, the contrast decreases on the writing side 23 and increases on the erasing side 24, as shown in FIG. .

For this reason, in the position measurement according to the conventional example described above, there is no problem with the dummy wafer used for the first time, but there is an inconvenience that the position measurement error of the measurement mark increases as the number of reproductions increases.

This is because as the number of reproductions increases, even if the previously written measurement mark is erased, it cannot be completely erased and remains on the dummy wafer as an afterimage. Then, when using this dummy wafer to measure the correction amount again,
The dummy wafer is pre-aligned by the transport section and transported to the write exposure position, but since the position repeatability of the pre-alignment section is not very good, it is unlikely that measurement marks will be written exactly at the same position as the previous one. Conceivable. In other words, a measurement mark is formed on the dummy wafer in the vicinity of the previously written measurement mark or overlapping the previous measurement mark to some extent. If the position of the measurement mark formed in this way is measured using the conventional method, the information of the afterimage measurement mark will also be taken in, which will cause an error in the center of gravity position, which will disturb the correction amount and increase the height of the measurement mark. A problem arises in that accurate baseline measurement cannot be performed.

The phenomenon in which an error occurs in the position of the center of gravity will be explained in detail with reference to FIGS. 7, 4, and 5.

FIG. 7 is an explanatory diagram showing the state of the dummy wafer when it is exposed for the first time and when it is re-exposed after erasing.
On the left side (-1 side) of the figure, the state of the dummy wafer at that time,
The contrast versus dummy wafer position is shown on the right side (-2 side).

In FIG. 7, 12 is a substrate, 13 is a reversible photosensitive material, 16 is the first exposure light, 17 is an exposed area on a dummy wafer, and 18 is a state where the lid is not completely erased. Shows the afterimage after erasing. Reference numeral 19 indicates the second exposure light, and 20 indicates the exposure area exposed the second time.

14.15 indicates the coordinate position of the graph shown on the right side of the board, 14 is the position of the origin (0), and 15 is the distance a from the origin 14.
Distant position (a) is shown. (a-1), (b-1), (
c-1), (d-1) and (e-1) schematically represent the state when the dummy wafer is exposed and erased repeatedly. (a-1) shows an unexposed substrate coated with a reversible photosensitive material, and (b-1) shows a state after the first exposure. Next, (c-1) is the state after the first erasing operation, (d-1) is the state after the second exposure, and (e-1) is the state after the second erasing operation. Indicates what has been done. Also, (a-2) shown on the right side
(e-2) is a graph showing the contrast in each state on the vertical axis and the position on the vertical axis. In this graph, the graph of contrast due to the first exposure V (b-2)
When calculating the center of gravity for , it becomes the center position of the exposure light, but the contrast graph (
If the center of gravity position is calculated for d-2), it can be easily considered that the center of gravity is closer to the origin than the original exposure center position due to the influence of afterimages. This is clearly shown in FIGS. 4 and 5.

Figure 4 is a graph assuming that there is no afterimage.Actual exposure light is exposed at the position shown in this figure, and the true center of gravity position of the X coordinate is 8a indicated by 22 in Figure 4. It is a point. On the other hand, FIG. 345 is a graph of contrast when a substrate with an afterimage is exposed to light, and is a graph similar to (d-2) in FIG. 7. Here, when determining the center of gravity of the X coordinate of the graph in FIG. 5, the center of gravity is at position 21 of 7.58 from the equation Σ5iXi/ΣSi, resulting in an error of 0.58 from the true position.

The present invention has been made in view of the above-mentioned problems in the conventional example, and even when a reversible photoreceptor, such as a dummy wafer, is recycled and used multiple times, it is possible to measure measurement marks for baseline measurement with high precision. The purpose of this invention is to provide a method for measuring measurement marks that can be performed.

[Means for Solving the Problems] In order to achieve the above object, in the present invention, before writing measurement marks on a reversible photoreceptor, for example, a dummy wafer,
- Take in the afterimage information of the dummy wafer before writing using the Evaporation microscope, and then write measurement marks and take in the measurement mark information on the dummy wafer as before. Thereafter, the afterimage information is subtracted from the captured measurement mark information on the dummy wafer, the center of gravity position is determined based on the information, and the baseline correction amount is calculated. As a result, it is possible to calculate a correction amount without introducing afterimage information, that is, without any error.

According to the method of the present invention, even if there are factors that cause errors other than afterimages on the dummy wafer (for example, uneven coating of dust-sensitive paint, etc.), these factors are also compensated for, so that accurate correction amounts can be obtained. can be calculated.

[Example] Fig. 1 is a schematic configuration diagram of an off-axis alignment type exposure apparatus according to an embodiment of the present invention, and Fig. 2 is a plan view of a portion including a wafer stage and a wafer transport system in Fig. 1. .

In FIG. 1, the light beam from the illumination system 25 is transmitted to the platen 26.
The reticle 27 as the first object held by the reticle is irradiated. Then, a pattern such as an electronic circuit formed on the surface of the reticle 27 is projected and transferred onto the surface of the wafer 29 as a second object using the projection lens 28.

The reticle 27 and the wafer 29 can be exchanged by means of transport means (not shown). A reticle reference mark 30 is arranged around the lower part of the reticle 27 with a slight distance from the reticle 27 for correctly arranging the reticle 27 with respect to the coordinate system of the apparatus. A reticle microscope fi31 for aligning the reticle 27 with respect to the reticle reference mark 30 is provided at a position facing the reticle 27 therebetween.

Note that the reticle microscope 31 and the reticle reference mark 30 are provided, for example, at two locations symmetrically about the center of the reticle.

Reticle alignment is performed by using a reticle microscope 31 to read the relative positional error between the reticle set mark provided on the reticle 27 surface and the reticle reference mark 30, and then moving the reticle 2 to the reticle stage 32, which is movable in the XYθ directions.
7 and platen 26 in a direction in which the relative position error approaches τ. Then, when the relative position error at this time falls within a predetermined tolerance range, the process ends. Note that 33 is an image pickup tube, a CCD, or the like.

A wafer microscope 34 is arranged near the projection lens 28. The wafer 29 is held by vacuum suction on a wafer holding table 35, and the wafer holding table 35 is held on a θ2 stage 36 which is movable in the rotation direction and the vertical direction.
The Z stage 36 is configured to be movable in the XY force directions.

Note that the θ direction here indicates the rotation direction around the Z axis.

At the end of the XY stage 37, an optical mirror 38 for detecting position coordinates in the Y direction and a laser interferometer (hereinafter referred to as "interferometer") 39 for making a light beam incident on the optical mirror 38 are arranged. has been done. Similarly, an optical mirror 40 (not shown) and an interferometer 4 (not shown) for detecting position coordinates in the X direction
1 is placed. And these two interferometers 41
39 to position the XY stage 37 and wafer 2.
The XY position coordinates of 9 are being read.

The above data processing such as reticle alignment, wafer alignment, and stage position information is performed on a routine basis. Further, sequential operations and the like are performed by the control device 42. Creation of jobs, commands to the apparatus, settings of parameters, etc. are performed on the display 43 through console input using the keyboard 44.

′! In FIG. J2, 45 is the optical axis of the projection lens 28, and for convenience, it will be explained below by aligning it with the origin 0 of the XY stage 37.

The Y-axis and the X-axis are optical mirrors 40 and 38, respectively.
is represented by the directions of mirror surfaces 46 and 47. In this embodiment, the optical axis 48 (point P) of the wafer microscope 34 is placed on the Y axis (X=0, Y=-1) for convenience.

Optical axis 45 of projection lens 28 and optical axis 4 of wafer microscope 34
The distance 1111 from 8 is the so-called reference length ([1aseli
ne). The stroke of the XY stage 37 is the maximum diameter for the cross in the X direction, and (
D+1) or a value close to it, thereby allowing the wafer microscope 34 to observe the entire surface of the wafer 29 and expose the entire wafer 29.

The projectable area of the projection lens 28 is a circle 4 on the wafer 29 surface.
Since the reticle 27 is generally rectangular, the effective area is a rectangular area 50. This area 50 is printed on the reticle 2 on the wafer 29 surface in one exposure.
This is an area where patterns on seven sides are projected and transferred.

The XY stage 37 is depicted so that the center of the wafer 29 mounted thereon coincides with the optical axis 45 of the projection lens 28, but the XY stage 37 is positioned at ±D/2 in the X direction and Y +D/2 in the direction. - Can move within the range of (D/2+1).

On the other hand, for a normal continuous processing routine, wafers are transferred from a wafer cassette 51 containing unexposed wafers onto a pre-alignment stage 53 by a conveying means such as a supply belt 52. Furthermore, after approximately positioning the wafer on the pre-alignment stage 53 based on the outer shape of the wafer,
The wafer is transferred to the wafer delivery position Q by the supply hand 54.
Place the wafer on the wafer holding table 35 at the point.

On the other hand, the wafer that is on the XY stage 37 and has already been exposed is placed on a collection belt 56 by a collection hand 55 and stored in a cassette 57 on the collection side.

Dummy wafer 5 used for calibration during this time
8 is stored on the standby stage 59. The dummy wafer 58 is configured to have a reversible material such as a magneto-optical recording material or a photochromic material on its substrate.

FIG. 3 shows a flow diagram illustrating a baseline measurement operation performed in the apparatus of FIG. The flowchart in the same figure is the 6th
In the flowchart shown in the figure, Step 1 is to transport the dummy wafer to the measurement position, Step 2 is to measure measurement marks (afterimages) before exposure, Step 3 is to store measurement mark information before exposure, and measurement mark after exposure. A step 7 for subtracting information is added, and in step 8, the center of gravity position is calculated using the measurement mark information subtracted in step 7.

The present invention will now be explained along the flowchart of FIG. When a predetermined time input into the console by the operator or when a predetermined number of wafers have been processed, the normal wafer processing routine is temporarily stopped and the exposed wafer is held. After being removed from the table 35, the dummy wafer 58 is transferred to the wafer holding table 35 by the supply hand 54.

The dummy wafer 58 transferred to the wafer holding table 35 is suction-fixed to the wafer holding table 35, and is first placed under the wafer microscope 34.
(step 1). Next, in step 2, afterimage measurement on the dummy wafer 58 is performed, and in step 3, afterimage information before exposure is stored. Thereafter, the dummy wafer 58 is transferred to the exposure position 45 (step 4).
, a predetermined measurement mark on the reticle 27 is transferred onto the dummy wafer 58 (step 5). At this time, the measurement mark is written redundantly on the already existing afterimage information.

Thereafter, the measurement mark is measured again using the wafer microscope to obtain the measurement mark information written in duplicate (step 6). Here, the pre-exposure afterimage information obtained in step 3 is subtracted from the information obtained in step 6 (step 7). By performing this operation, the afterimage information before exposure disappears and only the information on the currently exposed measurement mark is left, and true measurement mark information is obtained. The centroid position of the measurement mark is calculated from this true measurement mark information (
Step 8): Calculate the baseline correction amount.

As described above, by taking the simple steps of memorizing the afterimage information on the dummy wafer before exposure and subtracting it from the overlapped measurement mark information after exposure, the most important baseline information in an off-axis alignment system can be stored. It is possible to remove errors in the amount of correction.

[Other Examples] The measurement method of the present invention is effective not only for dummy wafers but also for all photoreceptors coated with a reversible photosensitive material. For example, it can be applied when a photosensitive material is applied to a specific position on a stage and the baseline is measured using that specific position.

In the above, we have described the method of determining the center of gravity for position calculation, but this also applies when determining the position at the midpoint of the half width.
According to the present invention, if the measurement mark information before exposure is subtracted from the measurement mark information after exposure and the subtracted measurement mark information is used, overwritten measurement marks can be simplified when used multiple times. Therefore, the half width and midpoint can be easily selected.

[Effects of the invention] As explained above, even though the photoreceptor is reversible, there is an afterimage, so the measurement mark information before exposure is subtracted from the measurement mark information after exposure, and the position is calculated based on that information. By doing so, it is possible to calculate a baseline correction amount with no error in afterimage components, and to perform highly accurate baseline measurement. Furthermore, the number of times that a photoreceptor such as a dummy wafer is reused can be increased by two to three times compared to the conventional method, resulting in a substantial cost reduction effect. Furthermore, even if there is dust or the like adhering to the vicinity of the measurement mark exposure area, or even if there is unevenness in the application of the photosensitive material, the present invention can eliminate the effects of these dust, unevenness, etc. This also has the effect of making it possible to perform highly accurate measurements.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram of an off-axis alignment type exposure apparatus according to an embodiment of the present invention, FIG. 2 is a plan view of a portion including a wafer stage and a wafer transport system in FIG. 1, and FIG. , a flowchart showing the health line measurement operation executed in the apparatus shown in FIG. 1, FIG. An explanatory diagram showing the position of the center of gravity of the measurement mark to be measured. The iS diagram is a flowchart showing the conventional baseline measurement operation. Figure 7 shows the state of the dummy wafer when it is exposed for the first time and when it is re-exposed after erasing. FIG. 8 is a diagram showing the contrast with respect to the number of reproductions of a reversible photosensitive material. 25: Illumination system 27: Reticle 28 Two projection lenses 29: Wafer 30 Two reticle reference marks 34/Wafer microscope 42: Control device 58: Dummy wafer

Claims (1)

[Claims] (1) In an exposure apparatus that has an off-axis alignment system, the exposure system transfers a measurement mark onto an object containing a reversible photosensitive material, the off-axis system measures the measurement mark, and the off-axis alignment system transfers the measurement mark to the object. When calculating the positional correction amount with the system, before transferring the measurement mark, measure and store the contrast information of the transfer part with the off-axis system, and calculate the contrast before the transfer from the measurement mark information measured after transfer. A method for measuring measurement marks, characterized in that the positional correction amount is calculated by subtracting information.