JP2607652B2 - Irradiation condition determination method for measurement device using charged beam and evaluation pattern used for it - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention measures a line width of a pattern of a semiconductor or insulator such as a VLSI by irradiating a charged beam and detecting a reflected electron or a secondary electron. The present invention relates to an irradiation condition determination method for efficiently determining irradiation conditions with good measurement reproducibility in a length measuring device or an inspection device.

[Prior art] In order to manufacture VLSI (Very Large Scale Integrated Circuit) with high yield, it is important to measure device pattern dimensions such as channel length with high accuracy and reflect it in the processing conditions of manufacturing equipment. There are many factors affecting the processing dimensions of the base film, such as (1) base film material, (2) resist film thickness, (3) exposure intensity, (4) development time, and (5) reactive gas species. Here, the process of determining the processing conditions such as (1) to (5) in order to obtain the optimum processing shape is called a manufacturing process development, and is a pattern dimension measurement as an apparatus for evaluating the processing shape and the pattern size. The device is used. Once the processing conditions are determined and the above process is introduced into the production line, management standards are set to maintain and improve the yield, and the stability of each processing condition is monitored by evaluation of the processing shape. A pattern size measuring device is used. in this way,
Pattern dimension measuring devices are used for various purposes such as determination of processing conditions and monitoring of processing stability, and their importance is increasing with miniaturization of dimensions.

On the other hand, in a dimension measuring device using such a charged beam, the charged beam is scanned, and a reflected electron or a secondary electron is detected in synchronization with the scanning signal, whereby a secondary beam in a direction perpendicular to the measurement pattern is detected. Obtain an electronic signal waveform and measure the pattern dimensions. As a method of obtaining a pattern size from the secondary electron signal waveform, a method of setting an appropriate slice level in the secondary electron signal waveform, binarizing the binarized signal, and measuring the pattern size from the interval is widely used. In addition, there has been proposed a device (Japanese Patent Laid-Open No. 61-8001 "Dimension measuring device") for approximating each of the edge and the base line with a straight line and obtaining a pattern size from the distance between the intersections. However, when performing such measurements, if the irradiation conditions, such as acceleration voltage and beam current, and the analysis conditions for signal waveforms are not optimized, charge-up occurs, contrast cannot be obtained, and so on. And the required measurement accuracy cannot be satisfied.

For example, FIG. 6 (a), (b), FIG. 7 (a),
FIGS. 8 (b), 8 (a) and 8 (b) are diagrams for explaining the effect of charge-up. FIGS. 6 and 7 show examples of changes in the secondary electron signal waveform due to the acceleration voltage. FIG. 8 shows the correspondence with the cross-sectional shape of the pattern. 6 and 7, (a) is the waveform at the time of the first measurement, (b) is 10
It is a waveform at the time of the second measurement. 6 and 7 differ in the acceleration voltage. In FIG. 6, there is almost no influence of the charge-up, and the changes in the signal waveform of the pattern portion P and the signal waveform of the background are small. However, in FIG. 7, the change in the ground level, which is the signal of the non-pattern portion B, depending on the location largely falls rightward. On the other hand, the left-right asymmetry of the signal waveform of the pattern section P is also large, and the waveform rises to the right in the tenth waveform. This change in waveform causes a decrease in measurement accuracy and reproducibility. For example, when the same pattern of an insulating sample is repeatedly measured, as shown in FIG. 9, the measured values include the results of (1) variation due to noise, (2) variation due to sample, and (3) variation due to charge-up. can get. Among them, (1) and (3) are error factors on the measuring device side, and particularly, fluctuation due to charge-up is a large error factor. Therefore, in order to perform measurement with good reproducibility, it is necessary to optimize irradiation conditions, and this optimization of irradiation conditions can be treated in the same way as securing reproducibility by minimizing charge-up.

As a requirement for such optimization of irradiation conditions, in the manufacturing process development stage, there are many sample types before the selection of process technology, and it is necessary to frequently determine irradiation conditions.
Simple determination of irradiation conditions is required. Also, in the production line monitoring stage, the number of sample types is small and the frequency of sample condition determination is low since the process technology has been selected. However, it is necessary to control the dimensional drift during the line operation for a large number of samples, and it is necessary to improve the measurement accuracy and automation, and the required measurement accuracy is high. Has become. Conventionally, such irradiation conditions have been optimized by trial and error depending on the intuition of an experienced person.

[Problems to be Solved by the Invention] However, in the pattern dimension measuring apparatus using a charged beam in the above-described conventional technique, optimization of irradiation conditions is performed by trial and error depending on the intuition of an experienced person. When a user tries to measure a pattern of a new material, it is necessary to change the conditions at random and search for good irradiation conditions, which requires a lot of labor and requires easy deciding of irradiation conditions. Could not respond to. Also, when trying to determine irradiation conditions while observing secondary electron waveforms or images from a scanning electron microscope (SEM), the effects of charge-up under the previous irradiation conditions remain even if the irradiation conditions are changed. However, there has been a problem that accurate conditions cannot be set.

The present invention has been devised to solve the above problems, and uses a charged beam for efficiently determining the optimum irradiation conditions when measuring a new sample having no measurement experience. An object of the present invention is to provide a method for determining irradiation conditions in a measuring apparatus and an evaluation pattern used for the method.

[Means for Solving the Problems] In order to achieve the above object, a configuration of an irradiation condition determining method in a measuring apparatus using a charged beam according to the present invention includes the steps of: Repeat the measurement of the same pattern of the sample while repeating the procedure to determine the optimum acceleration voltage that minimizes the change over time of the measured dimensions, and then perform continuous measurement of the same pattern of the sample while changing the beam current at the optimum acceleration voltage. Perform the procedure to calculate the optimal beam current that minimizes measurement reproducibility, or in a measurement device using a charged beam,
While changing the beam current, measure the reproducibility by continuous measurement of the same pattern of the sample, perform the procedure of determining the optimum beam current that minimizes the reproducibility, and then change the acceleration voltage with the beam current. The method is characterized in that a procedure of repeatedly measuring the same pattern of the sample and calculating an optimum acceleration voltage that minimizes a change with time of the measured dimension is performed.

In addition, the configuration of the evaluation pattern used in the irradiation condition determination method in the measuring apparatus using the charged beam of the present invention for achieving the above object is to determine the irradiation condition in which the pattern of the sample is arranged at a fixed position of the sample. It is characterized in that it is a pattern exclusively for evaluation.

[Operation] When the same portion of the pattern of the sample is repeatedly measured, a result is obtained in which the measured dimension value is mainly affected by fluctuation due to charge-up and fluctuation due to noise. In the present invention, the fluctuation due to the charge-up at that time shows acceleration voltage dependency,
Focusing on the fact that the fluctuation due to noise indicates the beam current dependency, in the procedure for determining the optimum acceleration voltage, measuring the time-dependent change in the measured dimensions due to charge-up to optimize the acceleration voltage, while determining the optimum beam current In the above, the variation of the measurement dimension due to noise, that is, measurement reproducibility is measured to optimize the beam current. As described above, in the determination of the irradiation condition of the present invention, the time-dependent change of the dimension due to charge-up and the variation of the dimension due to noise are separately measured, and the acceleration voltage and the beam current are determined one by one in order to optimize them. Efficiently. In addition, the pattern of the sample used for the above-described measurement is prepared in advance at a predetermined position on the sample as a pattern dedicated to optimization evaluation, thereby making the measurement procedure the same and making the procedure more efficient.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of a measuring apparatus using a charged beam to which the present invention is applied. The present embodiment is a measuring device that uses an electron beam as a charged beam, scans the sample, irradiates the sample, and measures the pattern size of the sample from the signal waveform of secondary electrons or reflected electrons emitted from the sample. The configuration of the measuring apparatus of the present embodiment is as follows. 1 is an electron beam generating source, 2 is an accelerating unit for accelerating a charged beam EB generated by the electron beam generating source 1, 3 is a beam adjusting unit for adjusting a beam current, and 4 is a predetermined range of a sample 5 scanned and irradiated. A beam deflector, 6 is a signal detector for detecting secondary electrons SE (or reflected electrons) emitted from the sample 5, and 7 is a main part of the present embodiment. Is output to the acceleration unit 2. An accelerating voltage setting unit 8 is also a main part of the present embodiment, and a beam current setting unit that determines an optimum beam current and outputs the beam current to the beam adjusting unit 3, and 9 corresponds to a pattern size of the sample 5 to be measured. A beam scanning unit 10 for providing a scanning signal to the beam deflector 4 at a preset magnification, and a beam scanning unit 10 in the direction perpendicular to the pattern from the secondary electron signal detected by the signal detecting unit 6 in synchronization with the scanning signal of the beam scanning unit 9. A measuring unit 11 for obtaining a secondary electron signal waveform and measuring a pattern dimension, 11 instructs a magnification to a beam scanning unit 9, and instructs an accelerating voltage setting unit 7 and a beam current setting unit 8 to determine an irradiation condition according to a procedure. A control controller for providing an initial value or a measured value from the measuring unit 10 when necessary, 12
Is an initial value input unit for inputting the above magnification and initial value.

Next, an embodiment of the irradiation condition determining method of the present invention in the measuring apparatus having the above configuration will be described. FIG. 2 is a flowchart showing one embodiment of the method, and the procedure is as follows.

The parameters involved in the charge-up are acceleration voltage, beam current, and magnification. Of these, the magnification must be set according to the size of the pattern to be measured.

Rough irradiation conditions (V _MIN , V _MAX , I _P0 ) are determined from SEM (scanning electron microscope) image observation or secondary electron signal waveform observation, and these are input as initial values.

In the approximate range (V _{MIN to} V _MAX ) obtained in
Fixed to I _PO, while changing the acceleration voltage V _P at an acceleration voltage setting unit, to measure the time course of the dimensions when the same portion was measured continuously, acceleration voltage this dimension change over time is minimized
Calculates and sets the V _M. Set (accelerating voltage determination flow) accelerating voltage V _M, continuous measurement while changing the beam current in the beam current setting unit calculates the beam current I _PM reproducibility (3 [sigma]) is minimized. The above-mentioned σ indicates the standard deviation of the measured value, and in this embodiment, the reproducibility evaluation value is multiplied by three. However, it is needless to say that n may be n (= 1, 2,...).

If the reproducibility (3σ) is equal to or less than the allowable value (OK), the irradiation condition is satisfied, and the process is terminated.

If the reproducibility (3σ) is equal to or larger than the allowable value (NG) and there is no dimensional change with time, the beam current _IP is increased and the beam current determination process is retried.

Reproducibility (3 [sigma]) is the allowable value or more, and if the dimensional change over time, to lower the beam current I _P, to retry the determination processing of the beam current (or ~ is beam current determination flow) beam current I _P it or when lowered it not, sets the beam current I _PM, by reducing the increment of the acceleration voltage changes to retry the following acceleration voltage determination flow. If the reproducibility (3σ) is equal to or more than the allowable value (NG) even after this retry,
Even if the reproducibility (3σ) is equal to or more than the allowable value assuming that the effect of the retry is not obtained, the processing is terminated.

 The operation of the embodiment configured as described above will be described.

(1) Initial value setting of irradiation conditions A search center value for determining the optimum values of the acceleration voltage and the beam current is given. Without measurement, it is determined by observing the SEM screen or observing the waveform of the two-electron signal. For example, in the secondary electron signal waveform shown in the description of the prior art, it is preferable to select the value in FIG.

(2) Determination of Acceleration Voltage Charge-up occurs because the secondary electron emission ratio δ of the material to be measured deviates from 1. Moreover, in the LSI, since a pattern of another material such as a resist is formed on the base film, it is theoretically difficult for both to simultaneously satisfy δ = 1. The parameters involved in such charge-up are acceleration voltage, beam current, and magnification, but charge-up has the following acceleration voltage dependence. Therefore, in the present embodiment, the acceleration voltage is used as a parameter to optimize so that the change over time of the measured dimension due to charge-up is minimized.

FIG. 3 shows the result of repeatedly measuring the pattern dimensions of a plurality of chips using the acceleration voltage as a parameter, and determining the amount of change over time in the average measured dimension. Every time measurement is repeated, the measured size tends to decrease slightly at 600 V, but tends to increase at other accelerating voltages due to charge-up. FIG. 4 shows the measurement results of the acceleration voltage dependence of the time-dependent change in the dimensions based on the dimensional change amounts of the first and tenth measurements.
It can be seen that there is no change with time around 575V and 620V. In this way, it is possible to determine the optimum acceleration voltage with which the measured dimension does not change with time. In the case of measuring the time-dependent change of the measured size as shown in FIG. 3 or FIG. 4, if the number of the measurement chips is small, the deviation due to the time-dependent change in the size may not be able to be measured due to the change in the size due to noise. For this reason, it is desirable to measure a plurality of chip positions or a plurality of similar pattern positions and measure how the average value changes. At this time, there may be a difference in dimension depending on the chip or the measurement position, and the problem is not the absolute value of the dimension but the amount of change with time of the dimension.

(3) Determination of Beam Current FIG. 5 shows the result of repeatedly measuring the pattern dimensions using the beam current as a parameter and obtaining the measurement reproducibility. This is a case where a different pattern was measured. The result is
In general, when the beam current is large, the S / N ratio is large and the fluctuation due to noise is reduced, but the dimensional change due to charge-up is large, and when the beam current is small, the opposite result is shown. For this reason, there is a certain optimum value, and the measurement reproducibility deteriorates at any of the lower beam current and the higher beam current. In the pattern measurement in the figure, about 4 pA is optimal. In the pattern measurement, the beam current dependency of reproducibility is small because the dimension does not change with time as compared with the pattern, but the optimum beam current (8 pA) is obtained in the same manner as in the pattern measurement. In this way, an optimal beam current can be determined.

(4) Check of allowable value of measurement reproducibility The measurement reproducibility under the irradiation conditions obtained by the procedure up to (3) is quantified (3σ) to evaluate whether the irradiation conditions are optimal. If the measurement reproducibility (3σ) is equal to or less than the allowable value, the irradiation condition is optimized and the process ends. This tolerance is
Depends on purpose. The measurement reproducibility may be somewhat poor, but if the irradiation conditions are found earlier, this tolerance is increased. When measurement accuracy is required, such as for line monitoring, this allowable value is set small.

(5) Redetermination In the above (4), when the measurement reproducibility (3σ) is not within the allowable value, the determination of the acceleration voltage and the beam current is retried. In this case, there are a case where the reproducibility is poor due to a change over time in dimensions due to charge-up, and a case where there is no change over time in the dimensions due to noise. In the latter case, the beam current is increased to increase the S / N ratio. In the former, the beam current is reduced to reduce the effect of charge-up. If the beam current is close to the lower limit, the determination of the acceleration voltage is re-determined with the initial condition and step size reduced.

According to the above-described processing, the present embodiment can set the irradiation conditions of the acceleration voltage and the beam current to optimal values.

Hereinafter, measurement patterns used in the irradiation condition determination method of the above embodiment will be described. When optimizing the irradiation conditions, when measuring the change over time / change in measurement reproducibility due to changes in the accelerating voltage and beam current, when measuring at the same position,
Since the influence of the charge-up under the previous irradiation condition remains, accurate condition setting cannot be performed. For this reason, it is necessary to change the measurement position along with changing the irradiation conditions. In general,
The measurement position is (a) another chip of the same design pattern, (b)
A similar pattern or the like in the same chip may be selected. In order to determine such measurement conditions efficiently, it is desirable to prepare a pattern dedicated to evaluation. In order to see the difference in the area ratio between the material of the base and the material of the upper layer, three patterns (for example, concave and convex) of the linear pattern of the base material, the linear pattern of the upper layer, and the two repetition patterns , Line & space), and one or more of these are arranged on a sample such as a wafer. It is more efficient to prepare the three types of individual patterns long. This is because, when the irradiation conditions are changed, continuous measurement is performed by shifting the location for each change of the conditions so as not to be affected by the charge-up under the previous irradiation conditions. The width of the pattern includes the pattern width of the object to be measured, and it is preferable to prepare a plurality of patterns having different dimensions. This is because the influence of the charge-up can be known because the magnification or the like changes depending on the pattern width of the measurement target, and the optimum irradiation condition may change depending on the pattern width. In addition to a linear pattern, it is preferable to prepare a square or round shape such as a through hole pattern. This is a through hole pattern
This is because the optimum irradiation condition may change because the secondary electron signal is less likely to go out than the linear pattern. These patterns themselves may be provided on chips mixed with other patterns, but the coordinates and chip arrangement are fixed. By configuring a dedicated evaluation pattern in this way, in the case of a new material, determination of measurement conditions is performed using this evaluation pattern in the same measurement procedure (the same measurement position) each time.
It is more efficient if only the search range is changed, so that it is more efficient.

In the above-described embodiment, the flowchart in which the acceleration voltage determination flow is earlier is described. However, even when the beam current determination flow is earlier, the irradiation condition can be optimized by a similar procedure. As described above, the present invention can be variously applied according to the gist and can take various embodiments.

[Effects of the Invention] As is apparent from the above description, the irradiation condition determination method in the measuring apparatus using the charged beam of the present invention separates and measures the time-dependent change in size due to charge-up and the change in size due to noise. Since the acceleration voltage and the beam current are determined one by one in order, the irradiation condition can be determined in a short time as compared with the case where the irradiation condition is randomly changed as in the related art. In addition, the evaluation pattern used for the irradiation condition determination method in the measurement apparatus using the charged beam of the present invention is the same in every time because the measurement procedure for determining the irradiation condition is the same even for a new sample. Can be done

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a measuring apparatus using a charged beam to which the present invention is applied, FIG. 2 is a flowchart showing an embodiment of the present invention, and FIGS. , FIG. 5 is an operation explanatory diagram in beam current determination, FIGS. 6 (a), (b), FIG. 7 (a),
FIGS. 8 (a) and 8 (a) and 8 (b) are diagrams for explaining the influence of charge-up on the conventional pattern dimension measurement, and FIG. 9 is an explanatory diagram of the essential pattern of the conventional pattern dimension measurement value. . DESCRIPTION OF SYMBOLS 1 ... Electron beam generation source, 2 ... Acceleration part, 3 ... Beam adjustment part, 5 ... Sample, 7 ... Acceleration voltage setting part, 8 ... Beam current setting part, 10 ... Measurement part, 11 ... ... Control controller, 12 ... Initial value input section.

Claims (3)

(57) [Claims] In a measuring apparatus using a charged beam, a procedure of repeatedly measuring the same pattern of a sample while changing an acceleration voltage to determine an optimal acceleration voltage that minimizes a change with time of a measured dimension is performed. Irradiation conditions in a measuring apparatus using a charged beam, wherein a procedure for calculating an optimum beam current that minimizes measurement reproducibility by continuous measurement of the same pattern of a sample while changing the beam current at the above optimum acceleration voltage is performed. Decision method. 2. A method for measuring the reproducibility of a continuous measurement of the same pattern of a sample while changing the beam current in a measuring apparatus using a charged beam, and determining an optimum beam current that minimizes the reproducibility of the measurement. And then repeating the measurement of the same pattern of the sample while changing the accelerating voltage with the optimal beam current to calculate an optimal accelerating voltage that minimizes the change with time of the measured dimension. A method for determining irradiation conditions in a measuring device using a beam. 3. A charged beam according to claim 1 or 2, wherein the pattern of the sample is a pattern dedicated to evaluation for determining irradiation conditions arranged at a predetermined position of the sample. An evaluation pattern used for a method for determining irradiation conditions in a measuring device.