CN103091993A - Test mark and measuring method used in lithography machine lens thermal effect measuring - Google Patents

Abstract

A test mark, the test mark is a square, the center is a light-transmitting square hole, and from the center to the outside is a square frame with a certain width; from the light-transmitting square hole to the outermost box, the light-transmitting area and the non- The light-transmitting regions alternate; from the outermost light-transmitting square frame to the light-transmitting square hole, the area of the light-transmitting regions gradually increases. A method for measuring the thermal effect of a lithography machine lens using the test mark, the test mark is located on a mask, the test mark is irradiated by a light beam and forms a test mark image on a workpiece stage through a projection objective lens, and a light intensity sensor is used to detect The light intensity at the test mark image place includes the steps of: pre-calibrating the relationship between the light intensity and the focal plane drift of the projection objective lens; performing thermal effect measurement; determining the focal plane drift data at each moment in the thermal effect measurement , fitting and calculating the parameters of the double-exponential model of lens thermal effect.

Description

Test mark and measurement method for measuring thermal effect of lithography machine lens

technical field

The invention relates to the field of lens thermal effect measurement, in particular to lens thermal effect measurement of projection objective lens of photolithography machine.

Background technique

Lens thermal effect refers to the phenomenon that the lens is deformed by heat, which leads to the change of imaging quality. In the design of projection lithography machines, the thermal effect of the lens caused by long-term mass production cannot be ignored. The vertical thermal effect will cause changes in image quality such as magnification and distortion; while the axial thermal effect will cause image quality such as focal plane and field curvature to occur. These changes will affect key indicators such as line width consistency CDU and overlay accuracy.

Therefore, the software system of the lithography machine needs to be designed with a mechanism for correcting thermal effects. This mechanism needs to measure and calibrate the relationship between image quality and heating time in advance. This relationship is generally described by the following double-exponential model:

Heating process:

。 Cooling process:

.

、

分别代表加热过程、冷却过程像质的变化，

、

分别代表加热过程、冷却过程的比例因子，、

分别代表加热过程、冷却过程的时间常数，

、、

、

为待拟合的参数。 in

,

represent the change of image quality in the heating process and cooling process, respectively,

,

represent the scaling factors of the heating process and cooling process, respectively, ,

represent the time constants of the heating process and the cooling process, respectively,

,,

,

is the parameter to be fitted.

In the prior art, there are two methods for measuring the thermal effect of a lens: exposure measurement and aerial image sensor measurement. The exposure measurement method is to expose a specific mark on the silicon wafer at different times during the process of exposing the light source to heat the lens until the lens is thermally saturated. The image quality at each moment, and then fit the parameters of the double exponential model. The disadvantage of this method is that it cannot measure the thermal effect in the cooling stage, and the measurement period is long, and it is easily affected by process factors. US Patent No. 5,998,071 discloses a method for measuring the thermal effect of a lens by exposure. The aerial image sensor measurement method is: during the heating or cooling process of the lens, the aerial image sensor is used to measure the mark on the mask surface at the moment when the image quality needs to be sampled, and the image quality is calculated, and then the coefficient of the double exponential model is fitted. The disadvantages of this method are: multiple sets of data need to be collected, the measurement speed is slow, and the cost of configuring a set of aerial image sensors is high.

Both of the above two methods have the disadvantages of long measurement process and need to interrupt the heating process or cooling process for a long time, and the calculation accuracy of the double exponential model is low.

Contents of the invention

The technical problem to be solved by the invention is that the lens heating or cooling process needs to be interrupted in the measurement process of the thermal effect of the lens, and the measurement time is long. In order to solve the above technical problems, the present invention provides a test mark, which is characterized in that the test mark is a square; the center of the test mark is a light-transmitting square hole, and from the center to the outside is a square frame with a certain width; From the transparent square hole to the outermost square frame, the transparent area and the non-transparent area alternate; from the outermost transparent square frame to the transparent square hole, the area of the transparent area gradually increases.

Further, the area of the light-transmitting square hole is no less than a quarter of the area of the test mark.

Further, the size of the outermost transparent frame is close to the resolution of the photolithography machine.

Further, from the outermost light-transmitting square frame to the light-transmitting square hole, the area of the light-transmitting region increases proportionally.

Further, a plurality of the test marks are distributed on the mask in a matrix array for measuring thermal effects of field curvature and astigmatism.

A method for measuring the thermal effect of a lithography machine lens using the test mark, the test mark is located on a mask, the test mark is irradiated by a light beam and forms a test mark image on a workpiece stage through a projection objective lens, and a light intensity sensor is used to detect The light intensity at the test mark image includes the following steps: pre-calibrating the relationship between the light intensity and the focal plane drift of the projection objective lens; performing thermal effect measurement, including during the heating or cooling process of the lens, when necessary At the moment when the focal plane data is collected, the light intensity sensor samples the light intensity until the measurement ends; determine the focal plane drift data at each moment in the thermal effect measurement, and fit and calculate the parameters of the lens thermal effect double-exponential model .

Wherein, the pre-calibration includes the following steps: Step 1, upload the mask, move the workpiece table to the nominal focal plane position; Step 2, axially move the The workpiece table, the light intensity sensor continuously samples the light intensity; Step 3, recording the first group of light intensity and the first group of axial positions of the workpiece table to obtain the first measured focal plane; Step 4 , using the first measured focal plane as the nominal focal plane of step one, repeating the steps one to three, recording the second group of dominant light intensity and the second group of axial positions of the workpiece table; step five, The first measured focal plane is subtracted from the second group of axial positions to obtain the second group of relationships between the light intensity and focal plane drift.

Further, the method for determining the focal plane drift data at each moment in the thermal effect measurement is obtained by using cubic spline interpolation through the relationship between the pre-calibrated light intensity and the focal plane drift of the projection objective lens.

Further, the size of the test mark is close to the photosensitive diameter of the light intensity sensor.

Further, the area of the mask except the test mark is a light-transmitting area.

Further, there are a plurality of test marks, and the plurality of test marks are distributed in a matrix array on the mask.

The invention has the advantages of no process exposure, high measurement speed, little impact on the lens heating and cooling process, low cost, and convenient further expansion for measurement and calibration of field curvature and image heat dissipation effect.

Description of drawings

The advantages and spirit of the present invention can be further understood through the following detailed description of the invention and the accompanying drawings.

FIG. 1 is a schematic structural diagram of the optical system of a projection lithography machine;

Fig. 2 is a schematic structural view of a test mark used for measuring the thermal effect of a lithography machine lens according to the present invention;

Fig. 3 is a curve diagram of the relationship between the light intensity measured by the light intensity sensor and the position of the workpiece table in the method for measuring the thermal effect of the lens of the lithography machine of the present invention;

Fig. 4 is a curve diagram of the relationship between light intensity and focal plane drift measured by the light intensity sensor in the method for measuring the thermal effect of the lens of the lithography machine of the present invention;

FIG. 5 is a schematic diagram of a mark array composed of test marks used for measuring the thermal effect of a lithography machine lens according to the present invention.

Detailed ways

Specific embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings.

Referring to FIG. 1 , the optical system of a projection lithography machine includes: a mask 1 , a projection objective lens 2 , and a workpiece table 3 in sequence along the optical axis. The mask has a test mark 4 for measuring the lens thermal effect of the projection objective lens 2 , and a light intensity sensor 5 is placed on the workpiece table 3 to detect the light intensity of the image formed by the test mark 4 .

This embodiment is mainly described for the measurement of the thermal effect of the axial focal plane of the lens.

FIG. 2 shows the specific structure of the test marker 4 of the present invention. In this embodiment, the mark is specifically a centrosymmetric graphic mark with alternate translucent areas and non-transparent areas. The center of the mark is a translucent square hole 6 , and from the center to the outside there are square frames with a certain width. The white of this mark is the light-transmitting area, and the black is the non-light-transmitting area. The size of the whole mark is similar to the photosensitive diameter of the light intensity sensor 5, and the size of the light-transmitting area gradually increases from the outside to the inside. The preferred situation is to increase proportionally, and the specific ratio can be adjusted according to the actual effect. The area of the light-transmitting square hole 6 is not less than 1/4 of the area of the entire test mark 4, and the size of the outermost light-transmitting area 7 is consistent with the limit resolution of the photolithography machine.

After the photolithography machine is integrated, the relationship between the light intensity and the focal plane drift of the projection objective lens 2 is pre-calibrated according to the following process:

Step 1, upload the mask 1 with the test mark 4, move the workpiece table 3 to the focal plane position calibrated by the process exposure test, set it as the nominal focal plane position, and place the light intensity sensor 5 at the imaging position of the test mark 4;

Step 2, move the workpiece table 3 axially within the approximate focal depth range near the nominal focal plane position, and the light intensity sensor 5 continuously samples the image formed by the test mark 4;

Step 3, during the sampling process, record the light intensity signal of the light intensity sensor 5 as the first group of light intensity, record the axial position of the workpiece table 3 as the first group of axial position, obtain the light intensity signal and the workpiece table Axial position relationship curve;

The relationship curve between the obtained light intensity signal and the axial position of the workpiece table is shown in Figure 3. The extreme point of the curve, that is, the axial position corresponding to the maximum light intensity is the focal plane, which is recorded as the first measured focal plane. When defocusing, the light intensity value measured by the light intensity sensor will decrease. The special design of the test mark shown in Figure 2 can improve the sensitivity of the light intensity sensor measurement value to defocusing, thereby improving the test accuracy.

Step 4, taking the first measured focal plane determined in step 3 as the nominal focal plane, repeating steps 2 to 3 to obtain a second set of light intensity and a second set of axial positions;

Step 5: After subtracting the first measured focal plane from the second group of axial positions recorded in step 4, the relationship between the light intensity measured by the light intensity sensor and the focal plane drift is obtained, as shown in FIG. 4 .

After the pre-calibration is completed, the thermal effect measurement is carried out. During the heating or cooling process of the lens, when the focal plane data collection is required, the light intensity sensor is used to measure the light intensity value at the marked imaging position and record it until the end of the measurement.

According to the relationship between the aforementioned pre-calibrated light intensity and focal plane drift, cubic spline interpolation is used, that is, a smooth curve through a series of shape value points, and the process of mathematically obtaining the curve function group by solving the three-curved matrix equations is determined. The focal plane drift data recorded at each time during the measurement process are then fitted to calculate the parameters required by the thermal effect double exponential model, namely the time factor and the proportional constant. The double exponential model of the thermal effect of the lens is

及

and

，

,

分别代表加热过程、冷却过程像质的变化，

、

分别代表加热过程、冷却过程的比例因子，

、

分别代表加热过程、冷却过程的时间常数，拟合的参数为比例因子和时间常数。 in ,

represent the change of image quality in the heating process and cooling process, respectively,

,

represent the scaling factors of the heating process and cooling process, respectively,

,

Represent the time constants of the heating process and the cooling process, respectively, and the fitting parameters are the scaling factor and the time constant.

The area of the mask involved in the method except the test mark can be set as a light-transmitting area as far as possible, so as to facilitate the heating process of the lens.

Referring to Fig. 5, this embodiment can be further expanded. According to the size of the imaging field of view, multi-point test marks are arranged on the mask, distributed in a matrix array, so that the test marks cover the entire field of view. What is shown in Fig. 5 is a 5 x5 marker array. Based on the same principle as above, according to the focal plane drift of each test mark in the field of view, the thermal effect of field curvature and astigmatism can be measured and calculated.

What is described in this specification is only preferred specific embodiments of the present invention, and the above embodiments are only used to illustrate the technical solutions of the present invention rather than limit the present invention. All technical solutions obtained by those skilled in the art through logical analysis, reasoning or limited experiments according to the concept of the present invention shall fall within the scope of the present invention.

Claims (11)

1. A test marker, characterized in that, The test is marked as a square; The center of the test mark is a light-transmitting square hole, and from the center to the outside is a square frame with a certain width; From the light-transmitting square hole to the outermost frame, light-transmitting areas and non-light-transmitting areas alternate; From the outermost light-transmitting square frame to the light-transmitting square hole, the area of the light-transmitting region gradually increases. 2 . The test mark according to claim 1 , wherein the area of the light-transmitting square hole is no less than a quarter of the area of the test mark. 3 . 3 . The test mark according to claim 1 , wherein the size of the outermost light-transmitting square frame is close to the resolution of a photolithography machine. 4 . 4 . The test mark according to claim 1 , wherein, from the outermost light-transmitting square frame to the light-transmitting square hole, the area of the light-transmitting area increases proportionally. 5 . The test mark according to claim 1 , wherein a plurality of test marks are distributed on the mask in a matrix array for measuring thermal effects of field curvature and astigmatism. 6 . 6. A photolithography machine lens thermal effect measurement method using the test mark of claim 1, the test mark is located on the mask, and the test mark is formed on the workpiece table through the projection objective lens by light beam irradiation, and the light The intensity sensor is used to detect the light intensity at the image of the test mark, comprising the following steps: Pre-calibrating the relationship between the light intensity and the focal plane drift of the projection objective lens; Performing thermal effect measurement, including sampling the light intensity by the light intensity sensor at the moment when focal plane data collection is required during the heating or cooling process of the lens until the measurement ends; Determine the focal plane drift data at each moment in the thermal effect measurement, and fit and calculate the parameters of the double-exponential model of the thermal effect of the lens. 7. measuring method according to claim 6, is characterized in that, described pre-calibration comprises the following steps: Step 1, uploading the mask, and moving the workpiece stage to the position of the nominal focal plane; Step 2, moving the workpiece table axially within a focal depth range near the nominal focal plane position, and the light intensity sensor continuously samples the light intensity; Step 3, recording the light intensity of the first group and the first group of axial positions of the workpiece table to obtain the first measured focal plane; Step 4, using the first measured focal plane as the nominal focal plane in step 1, repeating the steps 1 to 3, recording the second group of light intensities and the second group of axial positions of the workpiece table; Step 5, subtracting the first measured focal plane from the second group of axial positions to obtain the second group of relationships between the light intensity and focal plane drift. 8. The test method according to claim 6, wherein the method for determining the focal plane drift data at each moment in the thermal effect measurement is to combine the pre-marked light intensity with the focal plane of the projection objective lens. The relation of surface drift is obtained by using cubic spline interpolation. 9. The measuring method according to claim 6, characterized in that, the size of the test mark is close to the photosensitive diameter of the light intensity sensor. 10 . The measuring method according to claim 6 , wherein the area of the mask except the test mark is a light-transmitting area. 11 . 11. The measuring method according to claim 6, characterized in that there are a plurality of test marks, and the plurality of test marks are distributed in a matrix array on the mask.

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