JPH0384403A - Device for measuring shape of plane - Google Patents

Abstract

PURPOSE:To accurately measure the shape of the plane of a sample whose reflectance is changed by comparing a value measured by a measuring optical system with the value of a memory, storing a new value in the memory when change is found between the values and outputting a signal showing the shape of the plane. CONSTITUTION:Reflected light from the surface of the sample 10 to be measured is received by the eight-division photodetector of a measuring optical system output circuit 20 and the output value of the photodetector is stored in the memory 30. Next, a comparison computing element 40 compares the output of the photodetector which is successively measured in terms of time division with the value of the memory and judges that it is the plane having the same reflectance that the values coincide with each other and that it is an area where the reflectance is changed that the change is found between the values. In the case of the area where the reflectance is changed, the new value is stored in the memory 30 and calculation is performed by using the value of the memory, then the signal showing the shape of the plane is outputted. Thus, the shape of the plane is accurately measured even in the sample whose reflectance is changed.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention is applicable to submicron planar shapes (surface irregularities).
This invention relates to improving measurement accuracy in a planar shape measuring device that measures by using an optical focus error method.

<Prior Art> Conventionally, there has been a planar shape measuring device that measures the submicron planar shape of a measurement sample using an optical focus error type. Focus error methods include a critical angle method and an astigmatism method.

FIG. 6(a) is a diagram showing the basic configuration of the critical angle method, and FIG. 6(b) is a diagram showing its illumination force characteristics. The light reflected on the object surface passes through the objective lens l and enters the critical angle prism 2,
After being reflected by the end face of the prism, it enters the photoelectric detectors 3a and 3b. The two divided photoelectric detectors are arranged with the optical axis as a boundary, and the differential amplifier 4 is designed to determine the difference (Vout) between the outputs of the photoelectric detection elements.

In the figure, point B is the focal point, and at that time the same amount of light returns to the photoelectric detectors 3a and 3b, and the output Vout becomes zero. Point A and 0 are positions shifted from the focal point, and at that time, if the angle is smaller than the critical angle of prism 2, the light will not return to the photoelectric detector, and Vout will be zero as shown at points A and C shown in the same figure (b). FIG. 7(a) is a diagram showing the principle configuration of the astigmatism method, and FIG. 7(b) is a diagram showing its output characteristics. By installing the cylindrical lens 5 after the objective lens 1 on the reflected optical path of the measurement sample, astigmatism occurs at points A (in-focus) and C (out-of-focus). As shown in the same figure (c), photoelectric detectors a, l), c, and d, which are divided into four parts, are placed near the reflected light convergence point, and when the reflected light is received, the photoelectric detectors shown in the same figure (b) It becomes a light spot shape like this.

(c+b)-(a+d) is calculated from the output of each photoelectric detector, and the focus state can be determined from the value.

The focus error has characteristics as shown in FIG. 7 (2).

<Problems to be Solved by the Invention> However, the critical angle method has the following problems. As shown in Figure 8, when the reflectance of the target sample with a uniform surface at the focal point is partially different (low reflectance and high reflectance), when the optical system moves, the high reflectance surface a and On the low reflectance surface C, the amount of light returning to the photoelectric detectors 3a and 3b is the same, and the output vout of the differential amplifier 4 is zero, but near the boundary where the reflectance changes, even though they are on the same plane. The amount of light returned is not the same regardless of the output vo
ut does not become zero.

That is, there is a problem in that focus errors occur due to changes in reflectance, making it impossible to accurately measure the planar shape. Note that the same applies to the astigmatism method.

An object of the present invention has been made in view of the above points, and is to provide a planar shape measuring device that can accurately measure the planar shape even of a sample whose reflectance changes.

Means for Solving the Problem> In order to achieve this objective, the present invention irradiates the surface of the sample to be measured with light and places the reflected light at a position shifted from the focal point position of the converging lens. a measurement optical system output circuit that receives the signal from the multi-divided photoelectric detector, converts it into an electrical signal by each photoelectric converter, and outputs it; a memory in which the output signal of the measurement optical system output circuit is stored; and the measurement optical system. Comparison that compares the new measured value given by the system output circuit with the previous value stored in the memory, and if there is a change, stores the new value in the memory and outputs a signal indicating the planar shape at the same time. It is characterized by being equipped with a computing unit.

Effect> In the present invention, the reflected light from the sample to be measured is received by a multi-divided photoelectric detector, and the photoelectric detector is placed at a position slightly shifted from the focal position.

The comparison calculator compares and observes the previous measured value stored in memory and the current value output from the multi-divided charge detector.
Depending on the state of change, it is determined whether the surface has the same reflectance or a reflectance boundary area, and a new value is written into the memory and a signal indicating the planar shape is output.

<Example> The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of a planar shape measuring device according to the present invention. In FIG. 0, 10 is a target sample, 20 is a measurement optical system output circuit, 30 is a memory, and 40 is a comparison calculator. .

The measurement optical system output port '#I20 is equipped with a multi-divided photoelectric detector as will be described later, and the output value of the photoelectric detector is stored as is in the memory 30 at the initial stage of measurement.

The comparator 40 calculates the subsequent measured value (output f of the photoelectric detector)
R) with previous data stored in memory 30;
Only if there is a difference, the new measured value is stored.

FIG. 2 is a block diagram showing details of the measurement optical system output circuit 20. A light beam from a light source (not shown) such as a laser diode is focused on the sample 10 by an objective lens 21 after being reflected in the plane by a prism 22 .

The light reflected by the sample passes through the objective lens 21 and the prism 22.
The light passes through, is condensed by a converging lens 23, and enters a photodetector 24 located at a position slightly shifted from the focal point.

The photoelectric detector 24 is a photodiode sensor array divided into eight parts as shown in FIG. 3(A).

That is, the quadrilateral is first divided into four equal squares, and then further divided into circles centered at the center of the division. As a result, as shown in the figure, divided photoelectric detection elements a to d are formed on the outside and 1 to 4 on the inside.

In addition, when focusing, the output of the photoelectric detector (a + c + a) -
A photoelectric detector is installed at a position where (1+2+3+4) becomes zero, and when the focus shifts due to the planar shape (unevenness) of the sample, the signal changes to +- as shown in FIG.

The conversion port #t25 converts each output (current output) of the divided photoelectric detection element into an electric signal (voltage signal), and further converts this into a digital signal and outputs it.

The measurement operation in such a configuration will be explained next with reference to the operation flow shown in FIG.

■:: In the constant optical system output circuit 20, the photoelectric detector 24 receives the reflected light from the surface of the sample 10, and the conversion circuit 25 converts the output values of the detection elements a to d and 1 to 4 into the memory 30. Store in.

■::The output of the photoelectric detector (a+b
10c+d) Calculate (1+2+3+4). (2) Based on this calculation result, a focus error signal (planar shape signal) is output and held.

■:: If the process ends, the memory 30 is reset.

(2): Sampling is performed after an hour and the sum of the photoelectric detector outputs (a+b+c+d+1+2+3+4) is compared with the value in the memory. If they match (if there is no change), the reflectance surfaces are the same, and the calculation is performed as is.

A change in the sum total of the photoelectric detector output results in a region where the reflectance is changing.

■~■: If all of the photodetector outputs change, it means that the reflectance surface has been completely moved to a new reflectance surface, so new values are recorded in the memory 30. Since a change in only a part of the values means that the reflectance changes in the boundary region, the changed value is used to calculate a focus error signal using the value in the memory.

<Effects of the Invention> As explained in detail above, in the present invention, the focus error detection optical system is composed of a converging lens and an 8-divided photoelectric detector, and the signal from the optical system is converted into an electric signal.
Record in memory. By comparing the values of the signals sequentially measured in a time-division manner with the values of the signals in the memory, it becomes possible to determine whether the surface of the object to be measured is in a region where the reflectance is changing. As a result, in the case of a region where the reflectance is changing, correct values can be obtained even when measuring the planar shape (roughness) due to the change in reflectance by performing calculations using the values in the memory.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an embodiment of the planar shape measuring device according to the present invention, Fig. 2 is a block diagram showing details of the measurement optical system output circuit, and Fig. 3 is a block diagram showing details of the photoelectric detector. , Figure 4 is a diagram showing the output characteristics of the photoelectric detector, Figure 5 is a diagram showing the operation flow, Figure 6 is a diagram showing the principle of the critical angle method, and Figure 7 is a diagram of the principle of the astigmatism method. , FIG. 8 is a diagram for explaining the influence that a change in reflectance has on output. DESCRIPTION OF SYMBOLS 10... Sample to be measured, 20... Measurement optical system output circuit, 30... Memory, 40... Comparison calculator. Figure 3 Figure 4 Figure 5 Figure 6 (A) (B) Figure 7 [c+bl-(a0dl (d)

Claims (1)

[Scope of Claims] A measurement optical system output circuit that irradiates light onto the surface of a sample to be measured, receives the reflected light with a multi-divided photoelectric detector, converts it into an electrical signal by each photoelectric converter, and outputs it; A memory in which the output signal of the measurement optical system output circuit is stored, and a new measured value given from the measurement optical system output circuit is compared with the previous value stored in the memory, and if there is a change, the new measurement value is 1. A planar shape measuring device comprising: a comparator that stores a value in the memory and outputs a signal indicating a planar shape at the same time.