JP2009085615A - Shape evaluation method of diffraction grating - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a shape evaluation method of a diffraction grating capable of evaluating with high accuracy the shape of the diffraction grating that has a phase shift part. <P>SOLUTION: This shape evaluation method of the diffraction grating having the phase shift part includes an interference fringe detection process for detecting an interference fringe, corresponding to the irregularities of the diffraction grating, and a shape evaluation process for evaluating the shape of the diffraction grating, based on the interference fringe detected in the interference fringe detection process. In the shape evaluation method of the diffraction grating, the interference fringe, corresponding to irregularities of the diffraction grating, is detected, and the shape of the diffraction grating is evaluated based on the interference fringe. Since the interference fringe changes on the phase shift part, in the shape evaluation method of the diffraction grating, the shape of the diffraction grating having the phase shift part can be evaluated with high accuracy. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for evaluating the shape of a diffraction grating having a phase shift portion.

  When producing a diffraction grating for a distributed feedback semiconductor laser, first, an etching mask pattern of the diffraction grating is formed on a semiconductor substrate by interference exposure or electron beam exposure, and etching is performed using this mask to obtain a semiconductor. Unevenness was formed on the substrate to form a diffraction grating. For example, as a semiconductor laser used for optical communication, the period of the diffraction grating is about 200 nm to 250 nm.

  When the diffraction grating is formed by the two-beam interference exposure method, the pattern itself is a method that uses coherence, so the dimensional accuracy of the period of the diffraction grating to be formed is very high. It becomes. In addition, since a diffraction grating having a uniform period (uniform diffraction grating) is formed on the entire surface of the wafer, if the same light source as the light source used for exposure is used to irradiate the diffraction grating, the light is diffracted and The period of the diffraction grating can be easily obtained from the diffraction angle.

  In a semiconductor laser with a uniform diffraction grating, the reflectance of the coating film on the chip exit end face is made asymmetrical to a single wavelength, but it is very difficult to control the wavelength stability, and the diffraction grating at the end face is difficult to control. There was a problem that the phase greatly fluctuated.

  Therefore, research is being conducted on a technique for realizing high wavelength stability by introducing a phase shift portion having a half period (1/2 period) of the period of the diffraction grating into the diffraction grating.

  Since it is difficult to form the diffraction grating with the phase shift portion introduced by the interference exposure method, the diffraction grating is generally formed by the electron beam exposure method. In this electron beam exposure method, since the diffraction grating pattern is drawn one by one, the phase shift portion can be easily formed.

  In this electron beam exposure method, since diffraction gratings are drawn independently of each other, a technique for accurately evaluating a drawing pattern is required.

  In the electron beam exposure method, a diffraction grating is drawn only in a region where electron beam drawing is required for the purpose of shortening the long drawing time. For example, in the buried semiconductor laser, the ratio of the region where the diffraction grating needs to be formed is very small, and is only a region having a width of about 1.5 μm corresponding to the active layer width. Also in this case, a technique for accurately evaluating the drawing pattern is required.

  The period measurement of the diffraction grating formed by the electron beam exposure method described above is very difficult by the method of measuring the diffraction angle used in the two-beam interference exposure method because the light intensity of the diffracted light is small. In addition, in the method of measuring the diffraction angle and measuring the period of the diffraction grating, it is very difficult to inspect the shape of the phase shift portion.

  Therefore, a scanning probe microscope such as a scanning secondary electron microscope (SEM) or an atomic force microscope is usually used for shape evaluation of a diffraction grating having a phase shift portion.

  There are the following two prior art documents related to this application.

(Patent Document 1: JP-A-11-26878)
In this invention, the semi-transparent mirror that branches the light emitted from the light source, the light that is branched by the semi-transparent mirror and reflected by the reflecting mirror (reference light), and the other branched light are measured. Interference with the light (measurement light) incident on the power diffraction grating, reflected by the diffraction grating, and returned to the semi-transparent mirror, and the phase phase of the interference contrast changes in the phase shift section. From this shift amount, the phase shift amount Apparatus and method for measuring In addition, a photoresist is applied on a substrate on which a diffraction grating having a phase shift to be measured is formed, and a uniform diffraction grating pattern is formed on top of each other, and a phase shift amount is measured from a moire fringe obtained from a two-layer diffraction grating. It is a method to do.
In such interference fringe analysis using the reference light, the contrast is affected by the phase difference between the reference light and the measurement light, and therefore the interval between the interference fringes is affected by the unevenness of the sample surface. In order to reduce this influence, if the wavelength of irradiation light is lengthened, the positional accuracy of measurement will deteriorate.

(Patent Document 2: Japanese Patent Laid-Open No. 2005-10003)
In the present invention, when a SEM observation is performed on a material to be deformed in advance, a grid pattern having different secondary electron generation efficiency is formed. After the material is deformed, the scanning interval and the grid pattern of the SEM are The amount of moire fringe deformation caused by interference is measured.
This is for observing microscopic deformation with a scanning electron microscope, and is effective only for comparison before and after deformation.
Japanese Patent Laid-Open No. 11-26878 JP 2005-10003 A

  However, the above-described method for evaluating the shape of the phase shift of the diffraction grating has the following problems. That is, the scanning probe microscope is suitable for high-magnification observation (for example, observation of the local structure around the phase shift portion), but simultaneously observes the structure of the phase shift portion and the periodic structure including the phase shift portion, It was difficult to accurately evaluate the shape.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a diffraction grating shape evaluation method capable of performing highly accurate shape evaluation of a diffraction grating having a phase shift unit. .

  The diffraction grating shape evaluation method according to the present invention is a diffraction grating shape evaluation method having a phase shift unit, and includes an interference fringe detection step for detecting interference fringes corresponding to the irregularities of the diffraction grating, and an interference fringe detection step. And a shape evaluation step for evaluating the shape of the diffraction grating based on the detected interference fringes.

  In this diffraction grating shape evaluation method, interference fringes corresponding to the irregularities of the diffraction grating are detected, and the diffraction grating shape evaluation is performed based on the interference fringes. Since this interference fringe changes in the phase shift part, in this diffraction grating shape evaluation method, the shape evaluation of the diffraction grating having the phase shift part can be performed with high accuracy.

  Moreover, the aspect which detects an interference fringe with a scanning electron microscope in the case of an interference fringe detection process may be sufficient. In this case, the shape can be evaluated over a wide range using the interference fringes, and if necessary, high-magnification local observation can be performed with the same apparatus.

  Furthermore, in the interference fringe detection step, the scanning direction of the scanning electron microscope and the arrangement direction of the diffraction gratings may be inclined. In this case, since the change of the interference fringes in the phase shift portion becomes remarkable, the shape evaluation of the diffraction grating can be performed more easily.

  ADVANTAGE OF THE INVENTION According to this invention, the shape evaluation method of the diffraction grating which can perform the shape evaluation of the diffraction grating which has a phase shift part with high precision is provided.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, embodiments that are considered to be the best in carrying out the invention will be described in detail with reference to the accompanying drawings. In addition, the same code | symbol is attached | subjected about the same or equivalent element, and the description is abbreviate | omitted when description overlaps.

  The inventors observed the diffraction grating of the distributed feedback semiconductor laser using a scanning secondary electron microscope (SEM) at a plurality of different magnifications, and obtained the results shown in FIG.

  That is, at a high magnification (3000 times, 5000 times and 10,000 times) at which the diffraction grating can be directly observed, a grayscale image corresponding to the period of the unevenness of the diffraction grating is detected, and a lower magnification (500 times). , 900 times, 1000 times, 1100 times, and 1200 times), grayscale images different from the period of the unevenness of the diffraction grating were detected. The grayscale image detected at a low magnification has a period longer than the period of unevenness of the diffraction grating, and exhibits a stripe shape.

  As a result of intensive studies, the inventors have found that the striped gray image detected at a low magnification is an interference fringe generated as a result of interference between the period of the diffraction grating and the scanning period of the SEM (FIG. 2). reference). Here, FIG. 2 is a diagram showing the interference fringes in a diffraction grating having a uniform period.

Furthermore, the inventors have found that the interference fringes have the following relational expression when the diffraction grating period is Y1, the SEM scanning period is Y2, and the interference fringe period is Y3. It was.
Y1 = A × (sin (2π (x + x1) / Λ) +1)
Y2 = B × (sin (2π (x + x2) / D) +1)
Y3 = C × Y1 × Y2

  Here, A, B, and C are constants, x is a coordinate in a direction perpendicular to the diffraction grating stripes, x1 and x2 are constants, Λ is a period of the diffraction grating, and D is a scanning period. That is, when the SEM sampling interval is approximated by a trigonometric function, the interference fringe period Y3 is determined by the diffraction grating period Y1 and the scanning period Y2. Therefore, if the period of the diffraction grating changes, the period of interference fringes also changes accordingly. Since the scanning period varies depending on the observation magnification of the SEM, as shown in FIG. 1, the interference fringes are dependent on the observation magnification.

Y3 is
Y3 = C × (cos (2πx (1 / Λ−1 / D)) + 1)
And can be approximated.

  Therefore, the interference fringes change in the phase shift portion of the diffraction grating as shown in FIG. Specifically, if the shift amount of the phase shift unit is ½ period, the interference fringes are also shifted by ½ period in that portion (see FIG. 4).

Accordingly, the shape of the diffraction grating can be evaluated by the following procedure.
(Interference fringe detection process)

First, a distributed feedback semiconductor laser on which a diffraction grating is formed is set in the SEM, and the diffraction grating is observed at a low magnification (for example, 500 times). Then, as described above, a grayscale image of interference fringes is detected. FIG. 3 is an example of the detected gray image.
(Shape evaluation process)

  Next, the shape of the diffraction grating is actually evaluated using the interference fringes of the detected grayscale image. Here, examples of the shape evaluation of the diffraction grating include periodicity of the diffraction grating, presence / absence of the phase shift unit, shift amount of the phase shift unit, and the like.

  As described above, according to the diffraction grating shape evaluation method according to the embodiment of the present invention, the shape evaluation of the diffraction grating having the phase shift portion can be performed with high accuracy. In addition, when the interference fringes in the portion of the uniform periodic structure of the diffraction grating are observed, the presence or absence of fluctuations in the periodic structure can be easily detected over a wide range.

  As a result of further research, the inventors have found that when the alignment direction of the diffraction gratings is inclined with respect to the scanning direction of the SEM, the interference fringes are inclined as shown in FIG. It was found that the change of the stripes became remarkable. That is, since the contrast of the interference fringes is inverted on both sides of the phase shift unit, the shift amount of the phase shift unit can be measured with high accuracy using this. 5A is an image in which the arrangement direction of the diffraction gratings is inclined by 1 degree with respect to the scanning direction of the SEM, and FIG. 5B is an image of the diffraction gratings with respect to the scanning direction of the SEM. It is an image in which the arrangement direction is inclined by 5 degrees. As is clear from this figure, as the angle of inclination increases, the width of the interference fringes becomes narrower and the fringes come closer to each other.

  The present invention is not limited to the above embodiment, and various modifications are possible. For example, an apparatus for detecting interference fringes is not limited to an SEM, and may be a scanning ion microscope (SIM) or the like.

It is the figure which showed the SEM image which concerns on embodiment of this invention. It is the figure which showed the interference fringe in a uniform diffraction grating. It is the figure which showed the contrast SEM image of the interference fringe of a diffraction grating. It is the figure which showed the interference fringe in the diffraction grating which has a phase shift part. It is the figure which showed the contrast SEM image of the interference fringe of a diffraction grating.

Claims (3)

A method for evaluating the shape of a diffraction grating having a phase shift unit,
An interference fringe detection step for detecting interference fringes corresponding to the irregularities of the diffraction grating;
A shape evaluation method for a diffraction grating, comprising: a shape evaluation step for evaluating the shape of the diffraction grating based on the interference fringes detected in the interference fringe detection step.   The diffraction grating shape evaluation method according to claim 1, wherein the interference fringes are detected by a scanning electron microscope during the interference fringe detection step.   The diffraction grating shape evaluation method according to claim 2, wherein, in the interference fringe detection step, a scanning direction of the scanning electron microscope and an arrangement direction of the diffraction gratings are inclined.