JP2003017399A - Aligner and alignment method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aligner and an alignment method which can achieve higher-precision alignment by compensating the measurement precision of a base line quantity as an aligner equipped with an off-axis type alignment system and its alignment method. SOLUTION: By the alignment method of the aligner which indirectly positions a reticle and a wafer by using a reference mark fixed on a wafer stage, the base line quantity showing the relative position relation between the reticle and wafer is measured and the measured value of the base line quantity is used to predict a base line quantity for next exposure processing, thereby positioning the reticle and wafer according to the predicted value of the base line quantity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure apparatus used in a semiconductor device manufacturing process and the like, and particularly to an alignment method therefor, and in particular, it is possible to accurately control a baseline amount in an off-axis alignment system. The present invention relates to an exposure apparatus and an alignment method.

[0002]

2. Description of the Related Art In an exposure apparatus equipped with an off-axis alignment system, a reference mark plate is fixed on a wafer stage that holds a semiconductor wafer and moves two-dimensionally by a step-and-repeat method. The reference mark plate is used to control the distance (baseline amount) between the alignment optical system and the projection optical system, and the alignment between the reticle and the semiconductor wafer is indirectly performed using this baseline amount as a reference. Therefore, in an exposure apparatus equipped with an off-axis type alignment system, it is extremely important to accurately control the baseline amount in order to improve the alignment accuracy.

On the other hand, if the exposure process is continuously performed, the temperature near the wafer stage changes. This temperature change causes a drift variation in the baseline amount, which in turn deteriorates the alignment accuracy. Therefore, JP-A-63-224
Japanese Unexamined Patent Publication No. 326 and Japanese Unexamined Patent Publication No. 6-97032 disclose an alignment method in which the drift variation of the baseline amount is taken into consideration.

Japanese Unexamined Patent Publication No. 63-224326 proposes a method of measuring the baseline amount every time a wafer is exchanged or every fixed number of wafers. According to this method, the offset amount due to the drift variation of the baseline amount can be reset every time a predetermined number of processes are performed, so that the influence of the drift variation on the alignment accuracy can be reduced.

Further, Japanese Patent Laid-Open No. 6-97032 discloses that
A method of changing the measurement interval of the baseline amount according to the tendency of the drift variation of the baseline amount is disclosed.
According to this method, it is possible to optimize the number of times the baseline amount is measured, and thus it is possible to reduce the decrease in throughput due to the baseline amount measurement.

[0006]

In the above-described conventional alignment method, the measured value of the baseline amount is used as it is as the reference amount when performing the alignment. For this reason,
It is premised that the measurement error of the baseline amount is sufficiently small with respect to the required alignment accuracy. The required alignment accuracy is said to be 1/3 or less of the pattern rule. For example, when a fine pattern rule of 130 nm or less is applied, the alignment accuracy is at most 40.
It is required to be suppressed to about nm, and the offset needs to be as close to 0 as possible. However, according to the recent measurement results by the inventors of the present application, the measurement error of the baseline amount is about 10 nm, which is larger than the fluctuation amount of the baseline amount due to the drift, and the offset is 10 nm or less in the current baseline amount measurement accuracy. It was very difficult to control

Further, the fluctuation of the baseline amount due to the drift directly leads to the increase of the variation between lots and the variation within the lot. For example, in the case of performing offset management within a lot using a preceding wafer, the amount of baseline variation between the preceding wafer and the processing time of the main lot causes an extra offset, and it is intended that the original offset amount is reduced. The offset will remain. Therefore, the conventional alignment method described above cannot eliminate the offset based on the difference in processing time between the preceding wafer and the main lot.

An object of the present invention is to provide an exposure apparatus and an alignment method capable of compensating the measurement accuracy of the baseline amount and performing alignment with higher accuracy.

[0009]

The above object is to provide an alignment method for an exposure apparatus which indirectly aligns a reticle and a wafer by using a reference mark fixed on a wafer stage. A baseline amount that serves as a reference representing a relative positional relationship between the reticle and the wafer is measured, and the measured baseline amount is used to predict the baseline amount for the next exposure process. Then, the alignment method is achieved by aligning the reticle and the wafer based on the predicted value of the baseline amount.

Further, the above object is an exposure apparatus which indirectly aligns a reticle and a wafer by using a reference mark fixed on a wafer stage, and a relative distance between the reticle and the wafer is determined. A storage device that stores a drift function that describes a change over time of a baseline amount that serves as a reference, an arithmetic device that predicts the baseline amount when performing an exposure process next based on the drift function, It is also achieved by an exposure apparatus characterized in that it has an alignment means for aligning the reticle and the wafer on the basis of the predicted value of the baseline amount.

[0011]

DETAILED DESCRIPTION OF THE INVENTION An exposure apparatus and an alignment method according to an embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a schematic diagram showing an exposure apparatus according to the present embodiment, FIG. 2 is a flowchart showing an alignment method of the exposure apparatus according to the present embodiment, and FIG. 3 is a graph showing a variation of a baseline amount and a drift function accompanying the exposure processing. .

First, the exposure apparatus according to the present embodiment will be explained with reference to FIG.

A semiconductor wafer 12 to be exposed is placed on the wafer stage 10. A reference mark plate 14 on which alignment marks 16 are formed is fixed near the semiconductor wafer 12 on the wafer stage 10. A wafer mark (not shown) for alignment is formed in each shot area of the semiconductor wafer 12.

A reticle stage 20 is provided above the wafer stage 10. Reticle stage 20
A reticle 22 on which a predetermined pattern to be printed on the semiconductor wafer 12 is drawn is arranged on the top. A pair of reticle marks 24 a and 24 b for alignment are formed outside the pattern area on the lower surface of the reticle 22. Above the reticle marks 24a and 24b, TTR (through the.
Reticle) reticle alignment system 28a, 28
b is arranged.

A light source system (not shown) is provided on the reticle 22.
A condenser lens 30 is provided to collect the exposure light emitted from the reticle and illuminate the reticle uniformly. A projection optical system 32 is provided between the reticle 22 and the semiconductor wafer 12 to project the exposure light passing through the reticle 22 onto a predetermined region of the semiconductor wafer 12 at a predetermined magnification (for example, 1/5).

An off-axis type wafer alignment system 34 is arranged outside the projection optical system 32. The optical axis of the wafer alignment system 34 is parallel to the optical axis of the projection optical system 32 on the wafer stage. An index plate 36 having index marks is fixed inside the wafer alignment system 34. Index plate 3
The formation surface of the index mark 6 is conjugate with the surface of the reference mark plate 14.

A main control system 40 is connected to the wafer stage 10 via a wafer stage drive system 42, and the wafer stage 10 can be driven based on a signal from the main control system 40. Similarly, a main control system 40 is connected to the reticle stage 20 via a reticle stage drive system 44, and the reticle stage 20 can be driven based on a signal from the main control system 40.

The reticle alignment system 28 and the wafer alignment system 34 are also connected to the main control system 40, respectively, and the wafer stage 10 and the reticle stage 20 can be controlled based on the positional information output from these alignment systems 28 and 34. It is like this.

The main control system 40 also stores a measurement result of the past baseline amount BL and a memory for predicting the baseline amount BL for the next exposure from the past transition of the baseline amount BL. -The arithmetic unit 46 is connected.

As described above, the exposure apparatus according to the present embodiment stores the measurement result of the past baseline amount BL and predicts the baseline amount BL for the next exposure from the past transition of the baseline amount BL. The main feature is to have a memory / arithmetic unit 46 that operates. By configuring the exposure apparatus in this way, for example, when performing offset management in a lot using a preceding wafer, even if the baseline amount fluctuates between the processing time of the preceding wafer and the main body lot, You can accurately predict fluctuations,
The offset due to the difference in processing time between the preceding wafer and the main lot can be reduced. Further, since the next baseline amount is predicted from the past series of measured values of the baseline amount, the accuracy can be compensated even if the baseline amount measurement accuracy is not sufficient.

Next, the exposure apparatus according to the present embodiment will be described in detail along the procedure of the alignment method for the exposure apparatus according to the present embodiment.

In the alignment method according to the present embodiment, as shown in FIG. 2, the baseline amount BL of the wafer alignment system 34 is measured (steps S11 to S13), and the measured baseline amount BL and the storage / calculation device 46 are stored. A drift function is calculated from the stored series of baseline amounts BL (step S14), and the baseline amount BL for the next exposure is predicted from the drift function (step S14).
15) Align the wafer based on the predicted baseline amount BL and perform exposure processing (step S16).
~ S17) is the main feature. Hereinafter, each of these steps will be described in detail.

First, the wafer stage 10 is positioned (step S11). First, the wafer stage 10 is driven to move the mark 16 on the reference mark plate 14 to a position directly below the wafer alignment system 34. Then, the image of the mark 16 and the wafer alignment system 3
The position shift amount from the index mark 36 in 4 and the coordinates of the wafer stage 10 at that time are read. This allows
The coordinates (X1, Y1) of the wafer stage 10 when the mark 16 is on the optical axis of the wafer alignment system 34 are obtained.

Next, the reticle 22 is positioned (step S12). First, the reticle 22 is placed on the reticle stage 20. Next, the wafer stage 10 is driven to sequentially move the marks 16 on the reference mark plate 14 to positions near the conjugate positions of the reticle marks 24a and 24b, respectively, and the image of the marks 16 and the reticle marks 24a and 24b.
The amount of positional deviation from b and the wafer stage 1 at that time
Read the 0 coordinate. Thus, the wafer stage 10 when the mark 16 is at the center of the reticle mark 24a and the reticle mark 24b, that is, at the center of the reticle.
The coordinates (X2, Y2) of are calculated.

Next, the coordinates (X1,
Based on Y1) and the coordinates (X2, Y2), the baseline amount BL of the wafer alignment system 34 is obtained (step S13). The baseline amount BL of the wafer alignment system 34 is defined as an interval between the optical axis of the wafer alignment system 34 on the wafer stage 10 and the projection point of the center of the reticle 22 by the projection optical system 32 (see FIG. 1). ). That is, the baseline amount BL can be calculated as the interval between the coordinates (X1, Y1) and the coordinates (X2, Y2).

The baseline amount BL thus obtained is stored / stored in association with temporal parameters such as the time from the start of the exposure process and the number of processed wafers.
It is stored in the arithmetic unit 46. The recording with the temporal parameter is to accurately grasp the temporal change of the baseline amount BL due to the drift.

Next, a series of baseline amounts BL stored in the storage / calculation unit 46 are referred to, and a drift function is calculated based on the values of these baseline amounts (step S14). Here, the drift function referred to in the specification of the present application is a function that describes the change over time in the baseline amount due to drift, and based on the values of a plurality of baseline amounts stored in association with temporal parameters, For example, it can be calculated using the least squares method. By using the drift function obtained in this way, the tendency of the change in the baseline amount BL due to the drift can be known and
Subsequent drift trends can be predicted.

Next, in consideration of the time difference from the time when the baseline amount BL is measured to the time when the next exposure is performed, the baseline amount BL which will be reached when the next exposure is performed by the drift function. Is predicted (step S15). The baseline amount BL thus obtained is used as a reference value when performing alignment.

If the data of the baseline amount BL related to the next exposure process is not stored in the storage / calculation unit 46, such as when the preceding wafer is processed, the measured value of the baseline amount BL. Is used as it is as a reference amount when performing alignment. Alternatively, the baseline amount BL may be repeatedly measured a plurality of times, and the average value thereof may be used as the reference amount. As a result, the measurement accuracy can be compensated for the initial baseline amount BL.

Next, based on the thus calculated baseline amount BL, each shot area of the semiconductor wafer 12 is positioned within the exposure area of the projection optical system (step S1).
6). First, the semiconductor wafer 12 is placed on the wafer stage 10. Next, the coordinates (X3, Y3) of the wafer mark in each shot area on the semiconductor wafer 12 are
Each is read by the wafer alignment system 34. In this way, the position information of each shot area on the semiconductor wafer 12 is acquired.

Next, each shot area on the semiconductor wafer 12 is exposed (step S17). First, the coordinates (X3,
Based on Y3), the distance XP between the center of the shot area and the wafer mark in the X direction and the distance YP in the Y direction are obtained.
Then, the wafer stage 10 is moved in the X direction (X3-B
Lx-XP) and (Y3-BLy-YP) in the Y direction. Note that BLx and BLy are the X-direction component and the Y-direction component of the baseline amount BL, respectively. As a result, the center of the shot area designated by the wafer mark and the projection point of the center of the reticle can be matched. Next, the wafer stage 10 is fixed in a state where the center of the shot area and the projection point of the center of the reticle match each other, and a predetermined exposure process is performed. Similarly, exposure processing is performed on other shot areas.

Next, the semiconductor wafer 12 is exchanged, and the series of exposure processes described above is repeated. At this time, if necessary, the baseline amount BL is measured again, the baseline amount is added to the existing baseline amount data, the fitting is performed again, and the drift function is calculated again. By doing so, the accuracy of the drift function can be increased, and the drift of the baseline amount can be captured and corrected with high accuracy.

Thus, the exposure of the required number of wafers is completed.

FIG. 3 shows the baseline amount B associated with the exposure process.
It is a graph which shows an example of the variation of L. 3A shows the baseline fluctuation in the X direction, and FIG. 3B shows the baseline fluctuation in the Y direction. Also, in the figure, ◆ and ●
The mark shows the actual measured value of the baseline amount BL, and the solid line shows the drift function approximated by a linear function.

From the figure, by continuing the exposure process,
It can be seen that the baseline amount BL tends to increase.
However, the actual measured value of the baseline amount BL (◆
There are many measurement errors in (marks and ●), and there are many measurement points where accurate measurement values are not given. On the other hand, it is considered that the drift function (solid line) obtained by approximating these measured values by a linear function accurately represents the tendency of drift fluctuation of the baseline amount BL.

Therefore, by describing and predicting the baseline amount BL by a drift function as shown in the figure, for example, it becomes possible to suppress the influence of the measurement error of the baseline amount BL and perform highly accurate alignment.

As described above, according to the present embodiment, since the predicted value of the baseline amount BL calculated from the drift function is used at the time of alignment of the exposure apparatus, the shortage of the measurement accuracy of the baseline amount can be compensated for. The drift of the baseline amount can be captured and corrected with high accuracy.

The present invention is not limited to the above embodiment, and various modifications can be made.

For example, in the above embodiment, no particular reference is made to the timing of measuring the baseline amount BL, but the measurement of the baseline amount BL may be performed every reticle exchange, or every wafer exchange or constant. The measurement may be performed for each number of wafers, or the measurement interval may be changed according to the tendency of the drift of the baseline amount as described in JP-A-6-97032. It is desirable that the interval for measuring the baseline amount is appropriately set according to the variation amount of the baseline due to drift.

In the above embodiment, the drift function is approximated by a linear function, but it may be approximated by a higher-order function (for example, a quadratic function).

In the above embodiment, a plurality of drift functions may be prepared and the optimum drift function may be selected for each reticle, each layer or each device type.

In the above embodiment, the distance between the optical axis on the wafer stage of the wafer alignment system and the projection point of the projection optical system at the center of the reticle is used as the value of the baseline amount BL. The present invention can be applied even when the baseline amount defined by the method is adopted.

[0043]

As described above, according to the present invention, in the alignment method of the exposure apparatus for indirectly aligning the reticle and the wafer by using the reference mark fixed on the wafer stage, the reticle and the wafer are aligned. Measure the baseline amount that represents the relative positional relationship, use the measured baseline amount to predict the baseline amount for the next exposure process, and use the baseline amount to predict the baseline amount. Since the reticle and the wafer are aligned with each other by using the reticle, it is possible to prevent the alignment accuracy from deteriorating due to the time lag between the baseline amount measurement and the exposure.

Further, the drift function which describes the variation of the baseline amount over time is approximated by using the measured values of the baseline amount in the past, and the accuracy of the baseline amount is not sufficient. Also, the accuracy can be compensated and an accurate baseline amount can be calculated. Moreover, since the drift function is updated every time the baseline amount is measured, it is possible to accurately capture and correct the drift variation of the baseline amount.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a structure of an exposure apparatus according to an embodiment of the present invention.

FIG. 2 is a flowchart illustrating an alignment method according to an exemplary embodiment of the present invention.

FIG. 3 is a graph showing a variation in a baseline amount and a drift function associated with an exposure process.

[Explanation of symbols]

10 ... Wafer stage 12 ... Semiconductor wafer 14 ... Reference mark plate 16 ... Mark 20 ... Reticle stage 22 ... Reticle 24 ... Reticle mark 26 ... Mirror 28 ... Reticle alignment system 30 ... Condenser lens 32 ... Projection optical system 34 ... Wafer alignment system 36 ... Index plate 40 ... Main control system 42 ... Wafer stage drive system 44 ... Reticle stage drive system 46. Memory / calculation device

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Claims (5)

[Claims] 1. An alignment method for an exposure apparatus which indirectly aligns a reticle and a wafer by using a reference mark fixed on a wafer stage, wherein a relative mark between the reticle and the wafer is obtained by using the reference mark. A baseline amount that is a reference representing a physical positional relationship, and using the measured value of the baseline amount, predicts the baseline amount for the next exposure process, and predicts the baseline amount. An alignment method characterized in that the reticle and the wafer are aligned based on a value. 2. The alignment method according to claim 1, wherein the predicted value of the baseline amount is calculated based on a drift function that describes a temporal change of the baseline amount. . 3. The alignment method according to claim 2, wherein the drift function is updated by using the measurement value every time the baseline amount is measured. 4. The alignment method according to claim 2 or 3, wherein a plurality of the drift functions are prepared and the optimum drift function is used for each reticle, each layer or each device type. Alignment method. 5. An exposure apparatus for indirectly aligning a reticle and a wafer by using a reference mark fixed on a wafer stage, which is a reference for representing a relative distance between the reticle and the wafer. A storage device that stores a drift function that describes changes in the baseline amount over time; an arithmetic device that predicts the baseline amount when performing an exposure process next based on the drift function; An exposure apparatus comprising: an alignment unit that aligns the reticle and the wafer based on a predicted value.