JPS61133843A - Surface inspector - Google Patents

Abstract

PURPOSE:To eliminate variations in the judging of defects, by making the beam spot oval while the scanning speed is kept constant throughout the inspection processes to ensure a large feeding pitch in the spiral scanning. CONSTITUTION:Light beam from a laser 1 is expanded in the beam spot diameter radially with respect to a sample 6 (semiconductor wafer) to be inspected with an oval beam expander 3 and focused with a condenser 4 on the surface of the sample in a small oval shape. Scattered light from a defect on the sample 6 is made incident into a detector through an optical fiber 14. A spindle 7 having the sample 6 sucked securely thereon is turned with a DC motor 9 and a timing belt at the speed corresponding to the radial position of scanning. A moving table 11 for radial scanning is moved with the DC motor 9 at a variable speed corresponding to the radial position.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention provides a circular object with a high radial scanning speed so that even if the diameter of the object to be inspected is large, the inspection can be completed within a relatively short time. The present invention relates to a surface inspection device.

[Problems to be Solved by the Invention] Conventionally, in order to detect defects such as foreign matter, dirt, and scratches on mirror-finished surfaces such as semiconductor wafers and aluminum disks, laser beams have been used to scan the surface of circular objects to be inspected. A surface inspection device that scans and inspects the surface of a circular object in a dix spiral according to the principle that if scattered light is detected, it is assumed that a defect exists in the irradiated area. It was used.

However, in the past, a normal optical system was used to irradiate the inspection object with a laser beam spot, so the spot shape at the irradiated surface position of the inspection object was, for example, a circle with a diameter of about 40 μm. . When performing circular scanning with a laser beam, the scanning feed pitch is limited to at most half the beam spot diameter (for example, 25 to 30 μm), as shown in Figure 2, when considering inspection reproducibility and reliability. It is. Therefore, conventionally laser beam control was used.

It took a long time to complete the spiral scan.

In addition, conventionally, when performing spiral scanning, scanning was performed with a constant sample rotation speed, so as the diameter of the sample to be inspected becomes larger, the difference in scanning speed between the outside and the inside becomes large. It had become. Even for scattered light caused by surface defects of the same degree, if the scanning speed is fast and the incident time to the scattered light detection means is short,
The frequency becomes high (and the response of the signal processing system is delayed, making it impossible to obtain a normal peak value.For this reason, with the conventional method, as the diameter of the sample becomes larger, the determination of the degree of defect may vary depending on the location. A problem has arisen in that it becomes more noticeable.

[Problem that the invention seeks to solve]

The present invention solves the problems that occurred in the conventional inspection method described above, such as not being able to use a large feed pitch in spiral scanning, and with a constant rotational angular velocity, there is a large difference in scanning speed depending on the scanning position, causing variations in defect determination. In order to eliminate the problem of dots, detect a large feed pitch in the spiral scan, and eliminate variations in defect determination, we have developed a surface inspection device that has a nearly constant (linear) scanning speed, which is a composite of the scanning speeds in the circumferential and radial directions. The purpose is to provide.

[Means for solving problems]

In order to achieve the above object, in the present invention, the feed pitch of the helical scan is made thicker (the reason why this feed pitch cannot be adopted is that there is a restriction that this feed pitch is limited to approximately half the laser beam diameter. In the present invention, the diameter of the laser beam in the radial direction of the object to be inspected is expanded using a special irradiation optical system, so that a nearly elliptical laser beam spot is obtained at the laser beam irradiation position, and the feed pitch is increased. (Also, in the conventional method of rotating the inspection target sample at a constant angular velocity, the difference in the scanning speed between the inside and outside of the sample causes variations in the determination of the degree of defects between the inside and outside of the large-diameter inspection target.) Therefore, in the device of the present invention, the rotational angular velocity is slowed at the outer periphery and faster at the inner periphery, and the scanning speed that is the combination of the circumferential scanning speed and the radial scanning speed is used for all inspections. It was decided to use a scanning drive system that would remain approximately constant throughout the process.

〔Example〕

FIG. 1 shows an embodiment of the present invention, in which 1 is a He-Ne laser, 2 is a mirror, 3 is an elliptical beam expander, 4 is a condenser lens, 5 is a pinhole, and 6 is a sample (semiconductor wafer).
, 7 is a spindle, 8 is a rotary encoder for detecting rotation angle, 9 is a DC motor for rotation, 10 is a table on which these rotation related members 7, 8, 9 are mounted, 11 is a table for radial movement, 12 is a DC motor, 13 is a rotary encoder for position detection, and 14 is an optical fiber.

The optical fiber 14 is the head of a scattered light detection means connected to a photomultiplier tube (not shown). In this figure, it appears that 3 (1 m) optical fibers are arranged around the irradiation point of the laser beam that is incident at right angles to the surface of the sample 6, but in reality, 12 optical fibers (14) are placed around the irradiation point. are arranged, and the scattered light is reliably detected no matter what direction it is scattered in. The beam emitted from the He-Ne laser 1 with a continuous output of 5 mW has a circular cross section; The beam spot diameter is expanded in the radial direction of the specimen to be inspected by an elliptical beam expander 3 that is integrated with a combination of cylindrical lenses, and the elliptical beam spot is further focused onto the specimen surface by a condensing lens 4 made of a biconvex lens. The light is focused into an extremely small ellipse (major axis approximately 184 μm, minor axis approximately 39 μm).This elliptical beam shape is condensed onto the sample surface so that the major axis is focused in the radial direction of the sample and the minor axis is focused in the circumferential direction of the sample. Determine the position of the beam expander 3.In addition, in this optical system, select the condenser lens 4 so that the major axis and minor axis are aberrated (narrowed down) on the sample surface.The laser beam spot on the sample surface has a radius Since the Gaussian beam extends long in the direction, the base of the Gaussian beam widens, and therefore, when scanning in the radial direction, scattered light from the edge of the sample (for example, a semiconductor wafer) enters the receiver as stray light, causing incorrect processing of the edge. By inserting a pinhole between the condenser lens and the sample, stray light from the outer periphery (if there is no pinhole, scattered light from the edge illuminates the condenser lens, is reflected by the lens surface, and is detected by the optical fiber. The sample (for example, a semiconductor wafer) 6 is fixed by suction, and the spindle 7 is rotated by a DC motor 9 and a timing belt at a speed corresponding to the radial position of the scan. .The rotation angle is detected by rotary encoder 8 (1024 pulses/rotation).
Used for defect sampling pulse. The radial movement table 11 is for performing radial scanning, and is moved by a DC motor 12 at a variable speed corresponding to the radial position. Radial position detection is done by rotary encoder 13
(1000 pulses/rotation) and is used for data acquisition timing. Note that the rotation speed when scanning the inside of the sample can be set to high (and the rotation speed near the outer periphery can be slowed down).
This is carried out in a well-known manner using the MPU and following the procedures stored in the ROM in advance.

In this way, the feed pitch of the helical scan is approximately 140μ.
m (average), high-speed scanning is possible,
It now takes about 115 minutes compared to the conventional inspection time.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to increase the radial feed pitch, reducing the time required to scan the spiral shape of the inspection target, and furthermore, the composite line scanning speed is almost constant, resulting in high resolution and reliability. It is possible to perform surface inspection with high quality.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, and FIG. 2 is a diagram showing the relationship between the laser beam spot diameter and the helical scanning feed pitch. 1.-He-Ne laser, 2- Vera-13.
- Elliptical beam expander, 4 - Condenser lens, 5 - Pinhole, 6 - Sample (semiconductor wafer), 7 - Spindle, 8 - Rotary encoder for rotation angle detection, 9 10--Table, 11--Table for radial movement, 12--DC motor, 13--Rotary encoder for position detection, 14--Optical fiber.

Claims (1)

[Claims] A method in which the surface of a circular object is irradiated and scanned with a laser beam so that the irradiation trajectory forms a spiral, and it is assumed that defects such as scratches, dirt, and surface foreign matter exist at the irradiation position where scattered light is detected. In surface inspection equipment, by using an irradiation optical system in which the laser beam spot at the irradiation position extends beyond its original size in the radial direction of the object to be inspected and converges into an approximately elliptical shape, the radial spacing of the helical irradiation trajectory can be reduced. , and the surface scanning velocity which is the composite of the circumferential scanning velocity and the radial scanning velocity of the beam spot is
A surface inspection device characterized by using a scanning drive system that is always substantially constant throughout the entire scanning process.