JPH0594940A - Measurement of dimension of integrated circuit and defect inspecting system - Google Patents

Abstract

PURPOSE:To correct the shift of a focal length due to the warpage of a wafer to be measured by a method wherein the amount of the shift of a focal point at the time of lithographing at the position of measurement of the wafer to be measured is read in from an aligner and the current or the voltage of an objective lens, which corresponds to the amount of the sift of the focal point, is added to the amount of the shift of the focal point. CONSTITUTION:A focal length correction amount calculating part 9 of a measuring system obtains the amount of change of a focal length in each chip at the time of lithographing from an aligner 200 for correcting the shift of the focal length due to the warpage of the wafer and is previously provided with a focal length lithography table 10. Moreover, a change in the focal lengths at some places of the stage is read in for correcting the inclination of the stage on the side of a measuring device 100 and a table 11 for stage inclination correction use is formed in advance. A current or a voltage of an objective lens is changed using these two tables 10 and 11 and the inclination of the stage and the shift of the focal length due to the warpage of the wafer are corrected. Thereby, a reduction in a measuring time, the reproducibility of measurement and the improvement of a measurement accuracy can be contrived as the whole measuring system.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention irradiates a pattern such as a metal, semiconductor, or insulator in an integrated circuit such as VLSI with a charged beam to detect reflected electrons or secondary electrons and measure the pattern size or pattern. The present invention relates to a pattern measuring and inspecting apparatus for inspecting defects, and to a dimension inspecting system for an integrated circuit and a defect inspecting system for improving the setting accuracy of focusing of an electron optical system.
In particular, the present invention relates to a technique for achieving a fully automated measurement / inspection with high accuracy and high throughput.

[0002]

2. Description of the Related Art Along with the miniaturization of the pattern size of LSI and the like, a dimension measuring device using a charged beam is used. In this type of device, the stage is moved so that the measurement pattern is located directly below the beam, and the initial setting of the objective lens,
Focusing is performed by fine adjustment, the charged beam is scanned, the secondary electron signal waveform in the direction perpendicular to the measurement pattern is obtained, and the dimension of the measured pattern is measured. Various methods for measuring dimensions from the secondary electron signal waveform include setting the appropriate slice level in the secondary electron signal, binarizing it, and measuring the pattern dimensions from the rising and falling intervals, as well as edge measurement. .A method of approximating each of the baselines with a straight line and measuring the pattern size from the interval of the intersections (Japanese Patent Laid-Open No. 61-
No. 80011) are used.

In a series of operations for measuring such pattern dimensions, the stage is moved to the position of the pattern to be measured for focusing, and then measurement and inspection are carried out. Here, when the focus shifts, the slope of the secondary electron signal waveform changes so that it becomes gentle, so when setting the slice level,
When approximation is performed with a straight line, a measurement error occurs in any case. FIG. 4 is an example showing changes in measured values due to defocus. The relationship between the pattern width of the same pattern measured at the same slice level and the amount of deviation of the focal length is shown. When the signal is in focus, the signal rises and falls sharply. Although the dimension measured at the interval is almost unaffected even if the slice level changes, the rising and falling slopes of the waveform become gentle as the focus shifts, and the dimension (measurement value) is measured finely. In this way, since the defocus causes an error in the length measurement value, it is necessary to improve the setting accuracy.

In general, the focal length changes depending on the inclination of the stage and the warp of the wafer. Therefore, focusing is necessary after changing the measurement point (moving the stage).
As a method of automatic focusing in a scanning electron microscope (hereinafter referred to as SEM) using a charged beam or an exposure apparatus, after setting the focal length to a certain initial value, the focal length is gradually changed stepwise. A method of determining the focal length by the least-squares method so that the inclination of the secondary electron signal becomes the steepest (see Japanese Patent Laid-Open No. 60-54152) is used. In such an automatic focusing method, if the difference between the initial setting value and the actual focal length is large, the focusing cannot be performed, or the number of steps for changing the focal length increases, and thus it takes a long time to perform the focusing. there were.

Ordinary focusing is performed in two stages of initial setting of the objective lens and fine adjustment of focus, and this fine adjustment part is called focusing. As described above, a method is used in which the focal length is changed stepwise, the secondary electron signal of the measured pattern is taken in, and the focal length is determined so that the inclination becomes the steepest. The correction of the focal length is performed by adjusting the current (voltage in the case of an electrostatic lens) when an electromagnetic lens is used as the objective lens. The method of initial setting of the objective lens is disclosed in, for example, Japanese Patent Laid-Open No. 63-202835. less than,
The outline will be described.

When an electromagnetic lens is used as the objective lens, if the current value applied to the electromagnetic lens is the same, the focal length is almost inversely proportional to the square root of the beam acceleration voltage (V). Therefore, when the accelerating voltage for measurement is changed, it is necessary to change the current value applied to the objective lens substantially in proportion to the square root of the accelerating voltage. In order to further improve the accuracy, it is possible to initialize with high accuracy by approximating the current value (o | hv) applied to the objective lens by a linear expression of the square root of the acceleration voltage. This term corrects the deviation of the objective lens current due to the change of the acceleration voltage.

O | hv = p × V ^0.5 + q p = (o | ₁ −o | ₂ ) / (V ₁ ^0.5 −V ₂ ^0.5 ), q = o | ₁ −pV ₁ ^0.5 (1) p and q are , Is a constant determined from the objective lens currents o | ₁ and o | _{2 at} certain reference acceleration voltages V ₁ and V ₂ . When an electrostatic lens is used as the objective lens, it is possible to approximate the voltage applied to the objective lens by a linear expression of the acceleration voltage and correct it.

O | hv = uV + v u = (o | ₁ −o | ₂ ) / (V ₁ −V ₂ ), v = o | ₁ −pV ₁ (1 ′) u, v is a reference acceleration voltage V ₁ the objective lens current at -V ₂ o | _1, o | is a constant determined from the _2.

On the other hand, the focal length also changes due to the movement of the position, which requires correction. This is due to the warp of the wafer and the inclination of the stage. The component due to the tilt of the stage is unique to the apparatus and can be corrected in advance. That is, with reference to a certain position of the stage, the deviation of the focal length at the grid point (Xi, Yj) which divides the stage into a grid shape having a side length W is tabulated as Δij. The correction amount o | stg of the objective lens current at the point (X, Y) between the lattice point (Xi, yj) and the lattice point (Xi + 1, Yj + 1) is: o | stg = a {(Δi + 1j-Δij) (X -Xi) + ([Delta] ij + 1- [Delta] ij) (Y-Yj)} / W (2). Where a is the focal length of 1 μm
It is a proportional constant determined from the objective lens current required to change it, and is a function of the acceleration voltage V. However, since the deviation amount is small, this functional form can be given in proportion to the square root of the acceleration voltage (objective lens Proportional to the acceleration voltage when using an electrostatic lens). However, if the adhesion between the wafer and the stage is poor at the time of loading the wafer, the inclination on the wafer surface will be different, so this equation cannot be used as it is.

On the other hand, a wafer such as a VLSI that is being manufactured may be warped. Due to the warp of the wafer, the focal length changes at the center and the end of the wafer. This value varies from wafer to wafer and is usually 50 μm or less. However, an error in the measurement value due to defocus occurs from the deviation of the focal length of about ± 3 to 5 μm (see FIG. 4), so the measurement position If it changes, focusing is necessary.

[0011]

However, the conventional method of initial setting of the objective lens cannot correct the warp of the wafer, so that the range of change of the focal length must be large when the focus is finely adjusted. I had to do it. For this reason, in order to perform focusing with high accuracy, it is necessary to increase the number of repetitions in order to improve the S / N, or to make the focal length finer, and it takes time. Therefore, it is possible to improve the accuracy and shorten the time by making the initial setting of the objective lens (initial setting of the focal length) as accurate as possible by including the wafer warp and reducing the change in the focal length during the fine adjustment.

An object of the present invention is to perform accurate dimension measurement in pattern dimension measurement and defect inspection using a charged beam, even if there is a change in focal length due to wafer warpage or the like. An object of the present invention is to provide an integrated circuit dimensional measurement and defect inspection system that can be performed.

[0013]

A dimension measurement and defect inspection system of an integrated circuit according to the present invention reads a defocus amount at the time of writing at a measurement position of a measurement wafer from an exposure device and corresponds to the defocus amount. By applying a current or a voltage to the objective lens, the deviation of the focal length due to the warp of the wafer at the measured pattern position is corrected, and focusing is performed.

Further, at the time of wafer alignment, the focal length at each mark position is stored, the tilt of the wafer is calculated from the focal length data at the plurality of mark positions, and the change amount due to the defocus due to the warp of the wafer is added to this value. Is set by adding to the current or voltage of the objective lens to perform focusing.

[0015]

In the present invention, the change in the focal length of each chip is read from the exposure device as data for drawing.
Therefore, even if the wafer is warped or tilted, the range of fine adjustment for focusing can be narrowed by obtaining the amount of change from the exposure apparatus and correcting it at the time of initial setting of the objective lens.

[0016]

FIG. 1 is a diagram showing an example of the configuration of a measuring system of the present invention. The measuring apparatus 100 includes an electron optical lens barrel (not shown) for irradiating a target sample with an electron beam, a stage (not shown) for moving the position of the sample, an SEM 1, a lens barrel control system 2, and a mirror. Tube control table 3, power supply interface 4, stage control system 5, signal processing measurement unit 6, secondary electron detection system 7, control computer 8, focal length correction amount calculation unit 9, focal length drawing table 10, stage tilt correction table Have 11. Reference numeral 200 is an exposure device. The lens barrel control system 2 refers to the lens barrel control table 3 to calculate how much current should be applied to each electromagnetic lens (voltage for an electrostatic lens) according to each irradiation condition, and to calculate the power interface. The current is passed through each lens through 4 to control the beam. In the stage control system 5,
In order to stop the stage at the target coordinates with high accuracy, feedback is given to the stage movement while reading the current position coordinates with a laser interferometer or a linear scale. The signal processing / measuring unit 6 obtains a signal image captured in synchronization with the scanning of the beam from the secondary electron detection system 7,
Detects target patterns, measures dimensions, detects defects, etc. The configurations 1 to 8 described above are also used in a normal SEM.

In the measuring system of the present invention, a focal length correction amount calculation unit 9 is added to improve the focusing accuracy. In the focal length correction amount calculation unit 9, in order to correct the deviation of the focal length due to the warp of the wafer, the focal length drawing table 10 is provided by previously obtaining the amount of change in the focal length of each chip at the time of drawing from the exposure apparatus 200. Make a table. Further, in order to correct the tilt of the stage on the measuring device 100 side, changes in the focal length at several points on the stage are read in advance and the stage tilt correction table 11 is created. By using the two tables 10 and 11, the current or voltage of the objective lens is changed to correct the deviation of the focal length due to the tilt of the stage and the warp of the wafer. The calculation method will be described later with reference to FIG. In the method of FIG. 3 in which the warp of the wafer is detected and corrected at the time of wafer alignment, the stage tilt correction table 11 in the drawing is necessary only for focusing when the mark for wafer alignment is detected, and is required at each measurement. Not needed when focusing on.

The pattern size in the wafer drawn by the exposure apparatus 200 is measured by the measuring apparatus 100. At this time, the exposure result data is transferred to the control computer 8 of the measuring apparatus 100 before the measurement. The data to be transferred contains information on the position dependency of the focal length. For example, in the case of the exposure apparatus 200, since batch exposure is performed on a chip-by-chip basis, the defocus amount at the time of drawing each chip is transferred as data. The exposure apparatus 200 and the measurement apparatus 100 do not have to be physically connected, and data may be transferred via a network. Of course, a device in which both are integrated may be used.

The control computer 8 of the measuring apparatus 100 registers the measurement conditions before the measurement. This may be designated each time, or a measurement condition created in advance may be called. These measurement conditions include the coordinates and shape of patterns used in wafer alignment, the position coordinates of patterns used to measure dimensions, and the beam irradiation conditions (acceleration voltage, beam current, etc.) used in these measurements. ing. less than,
A method for improving the accuracy of focal length correction (initial setting of the objective lens) by the measuring system of the present invention will be described.

FIG. 2 is a flow chart showing the initial setting procedure of the objective lens according to the present invention. In addition, (S1) ~ (S1
0) indicates each step. After the wafer alignment is completed (S1), the design coordinates (wafer coordinates) of the position to be measured are read from the measurement condition file (S2). The shift amount of the focal length at the measuring chip is obtained (S3). The exposure apparatus 200 receives the deviation amount of the focal length at the time of exposure of each chip (for example, the deviation amount with respect to the origin or a certain chip) as data, and uses the value. In the case of the EB exposure apparatus, since the inside of the chip is divided into a large number of figures for drawing, the average value of these (or the average value of the focal lengths at the time of detecting the chip mark) may be calculated and used. Read the irradiation conditions for measurement (acceleration voltage) (S4). The initial setting value o | of the objective lens is calculated (S5). Here, the initial setting value is the sum of three terms o | = o | hv + o | s
It is calculated by tg + o | wf (S6). The first term is a term that changes depending on the acceleration voltage and is given by the equation (1) described in the prior art. This term is at a certain location (for example, stage origin position)
This is the objective lens current corresponding to the focal length at. The second term is a term caused by the tilt of the stage, and corrects the focus shift caused by the movement of the stage. This term is given by the equation (2) described in the prior art. The third term is a term caused by the warp of the wafer and is corrected from the drawing data at the time of exposure. The amount of deviation of the focal length at the measuring tip is δ (μ
m), the correction amount of the objective lens current is o | wf = a
δ (where a is the same as a in equation (2), and is a proportional constant determined from the objective lens current required to change the focal length by 1 μm). The calculated value is set for the objective lens.

After the above-described initial setting of the objective lens, fine adjustment is performed to perform focusing and measurement can be performed (S7). This initial setting method considers the influence of the acceleration voltage, stage position, and wafer warp on the initial value, so the amount of change in the objective lens (focal length) at the fine adjustment stage can be reduced, and the setting accuracy can be reduced. Since it can be made higher, the measurement reproducibility and measurement accuracy are also improved (S8) to (S10).

FIG. 3 is a flow chart showing the procedure of another embodiment of the present invention. In addition, (S11) to (S23) indicate each step. At the time of wafer alignment, the coordinates and focal length at each mark position are stored (S11), (S12), and the wafer inclination (focal length coordinate dependency) is calculated by the least squares method from the focal length data at these plural mark positions. (S13),
(S14) is stored (S15). In this case, the stage tilt correction table of FIG. 1 (o | s of the equation (2) of the method of FIG. 2)
(Correction by tg) 11 is necessary for initial setting of the objective lens at the time of focusing at the mark position. This table is unnecessary because the calculated wafer tilt is used for the initial setting of the objective lens at each measurement point. In this case, even if the contact between the wafer and the stage is not good, the tilt can be accurately corrected, so that the accuracy of the initial setting of the objective lens at the time of focusing can be further improved. (S16) ~ (S
Since (23) is the same as (S3) to (S10) in FIG. 2, its description is omitted.

During exposure, there is a resist on the entire surface of the wafer, and the pattern is formed by developing the resist. In this case, since the residual stress of the resist forming the pattern is small, the change in the warp due to the development does not matter.

[0024]

As described above, in the measuring system of the present invention, the change in the focal length of each chip is read from the exposure device as drawing data of the exposure device, and the warp of the wafer is corrected by the initial setting of the objective lens. It is carried out. Therefore, even if the wafer is warped, the initial setting of the objective lens is performed with high accuracy, so that the range of change of the focal length at the time of focusing can be narrowed, the setting accuracy can be improved, and the measurement time can be shortened.

Further, since the inclination of the wafer is obtained and corrected at the time of wafer alignment, the range of change of the focal length can be narrowed even when the adhesion between the wafer and the stage is poor, the setting accuracy is improved, and the measurement time is improved. Can be shortened.
Therefore, it is possible to shorten the measurement time and improve the measurement reproducibility and accuracy of the entire measurement system.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of an integrated circuit dimensional measurement and defect inspection system according to the present invention.

FIG. 2 is a flowchart showing a procedure of a focusing method according to the present invention.

FIG. 3 is a flowchart showing a procedure of another focusing method according to the present invention.

FIG. 4 is a diagram showing an influence of focusing accuracy on a measurement value in pattern measurement and inspection.

[Explanation of symbols]

 1 SEM 2 lens barrel control system 3 lens barrel control table 4 power supply interface 5 stage control system 6 signal processing measurement unit 7 secondary electron detection system 8 control computer 9 focal length correction amount calculation unit 10 focal length drawing table 11 stage tilt correction Table 100 Measuring device 200 Exposure device

Claims (2)

[Claims] 1. A lens barrel for irradiating a charged beam, a stage for moving a sample, means for detecting backscattered electrons or secondary electron signals, and at a measurement position of a measurement wafer processed by an exposure apparatus. The defocus amount at the time of writing is read from the exposure apparatus, and the current or voltage of the objective lens corresponding to the defocus amount is applied to correct the deviation of the focal length due to the warp of the wafer at the measured pattern position, Dimension measurement of integrated circuits characterized by performing focusing,
Defect inspection system. 2. The dimension measurement / defect inspection system according to claim 1, wherein during wafer alignment, focal lengths at respective mark positions are stored, and a wafer tilt is calculated from focal length data at the plurality of mark positions. An integrated circuit dimension measurement and defect inspection system is characterized in that this value is set by adding a variation due to defocus due to wafer warpage to the current or voltage of the objective lens to perform focusing.