JP5471951B2 - Etching monitoring device - Google Patents

Description

  The present invention relates to an etching monitoring apparatus using Fourier Domain Optical Coherent Tomography (FD-OCT).

  Optical coherent tomography (OCT) is a technique for analyzing the three-dimensional structure of an object to be measured using a light interference phenomenon. This method was originally developed for medical use, but in recent years it has also been applied to semiconductor manufacturing, and a portion etched when etching a substrate to be processed (hereinafter referred to as an “etched portion”). It is used as an etching monitoring device that measures the hole depth (see, for example, Patent Document 1).

  There are several OCT methods. Among them, the FD-OCT method is suitable for an etching monitoring apparatus. In an etching monitoring apparatus using the FD-OCT method, light is irradiated onto a substrate to be processed, and the reflected light on the surface of the layer to be etched (hereinafter referred to as “etched layer”) and the etched material in the etched layer. The reflected light at the bottom of the part is caused to interfere, the interference light is dispersed by a spectroscope, and each wavelength is detected simultaneously by a line sensor or the like. And the hole depth of a to-be-etched part is calculated by Fourier-transforming the detection signal of each wavelength. Thereby, the etching depth can be measured in real time.

Japanese Patent Laid-Open No. 2-71517

  In recent years, in order to reduce the size of electronic devices, it has been performed to shorten the wiring distance between substrates by connecting substrates stacked in the thickness direction with through electrodes. In such a three-dimensional mounting using a through electrode, it is necessary to provide a through hole in the substrate. However, in this etching process, the ratio of the etched portion in the etched layer is generally small, while the groove width or hole of the etched portion is small. The ratio of the depth to the diameter (aspect ratio) is large. Therefore, unless the light irradiation position (that is, the measurement position) is appropriately adjusted according to the etched portion, the light is not reflected from the etched portion with sufficient intensity, and the S / N ratio is lowered.

  Conventionally, adjustment of the measurement position has been performed by photographing the layer to be etched and analyzing the position of the portion to be etched from the density of the obtained image. However, an inexpensive camera with low sensitivity and resolution has insufficient positioning accuracy, and there is a problem that the cost increases when an expensive camera with high sensitivity and resolution is used.

  The problem to be solved by the present invention is to provide an etching monitoring apparatus capable of appropriately adjusting a measurement position regardless of image processing.

The present invention made to solve the above problems
A light source that generates coherent light having a predetermined wavelength width, an introduction optical system that guides the coherent light onto the substrate to be processed with a spot diameter larger than the diameter or width of the etched portion, and the substrate to be processed In an etching monitoring apparatus, comprising: an interference optical system that causes the reflected coherent light to interfere; and a detection unit that detects, for each wavelength, the interference light obtained by the interference optical system, of the coherent light on the substrate to be processed. An irradiation position moving means for moving the irradiation position;
Interference intensity acquisition means for acquiring the intensity of the interference component included in the detection signal of the light detection unit for each of the irradiation positions scanned on the substrate to be processed at intervals equal to or less than the spot diameter;
A measurement position determining means for searching for a position on the substrate to be processed where the intensity of the interference component is maximum and moving the irradiation position so as to measure the etching depth at the maximum position;
It is characterized by having.

  The emission wavelength width of the coherent light is preferably 10 nm or more at the full width at half maximum, and the center wavelength is preferably 900 nm or less. An example of a light source that generates coherent light having such a wavelength width is a super luminescent diode (SLD).

  The etching monitoring apparatus according to the present invention further includes an auxiliary optical system for photographing the surface of the substrate to be processed, and a rough image for determining an initial irradiation position on the substrate to be processed by analyzing an image photographed by the auxiliary optical system. Position determining means. This makes it possible to make the initial irradiation position close to the etched portion, so that the position (measurement position) where the intensity of the interference component becomes maximum can be quickly determined in the subsequent processing.

  Since the etching monitoring apparatus according to the present invention can provide an optimum measurement position to the etched portion, it is possible to increase the intensity of the interference component related to the etched portion. As a result, the S / N ratio can be improved in the subsequent measurement of the etching depth. As a result, the etching depth can be accurately measured even for grooves and holes having a high aspect ratio. In addition, since it is not necessary to use a highly sensitive camera or a highly accurate optical lens in order to determine the measurement position, the cost of the apparatus can be reduced.

The schematic block diagram which shows one Example of the etching monitoring apparatus which concerns on this invention. Schematic which shows the incident spot light irradiated to the to-be-processed substrate before starting etching, and the reflected light from this to-be-processed substrate. The flowchart which shows the procedure of the calculation method of the measurement position which concerns on this invention. Schematic which shows the incident spot light irradiated to the to-be-processed substrate of the initial stage of etching, and the reflected light from this to-be-processed substrate. The graph which shows the detection signal (input signal) of the light intensity for every wavelength acquired by irradiating incident spot light to the initial irradiation position of the to-be-processed substrate in the early stage of etching. The graph which shows the detection signal (signal at the time of non-irradiation) of the light intensity for every wavelength acquired when the light is not generated from the SLD light source. The graph which shows the detection signal (reference signal) of the light intensity for every wavelength acquired by irradiating incident spot light with respect to a wafer without an unevenness | corrugation. The graph which shows the normalization signal for every wavelength calculated with respect to the initial irradiation position from the input signal, the non-irradiation signal, and the reference signal. The graph which expanded only the attention range of the normalization signal in an initial irradiation position. Schematic which shows the relationship between each center point of the irradiation position scanned on a to-be-processed substrate, and the weight calculated in each rectangular area. Schematic which shows the calculation result of the last irradiation position (measurement position) in the rectangular area | region which has the largest weight. The graph which shows the input signal for every wavelength acquired with respect to the last irradiation position. The graph which expanded only the attention range of the normalization signal in the last irradiation position.

  An embodiment of an etching monitoring apparatus according to the present invention is shown in FIG. In this embodiment, an SLD light source having a center wavelength of 835 nm and a full width at half maximum of 40 nm is used as the light source 10 that generates coherent light having a high light intensity and a wavelength width necessary for detecting the hole depth. The light generated from the SLD light source 10 is irradiated onto the substrate S to be processed placed in the etching chamber 15 through the measurement window 14 by the introduction optical system including the optical fiber 11, the collimator lens 12, the objective lens 13, and the like. Is done. Further, the irradiation position on the substrate S to be processed is adjusted by moving the introduction optical system by the micro-moving stage 16 placed on the top plate of the etching chamber 15.

  As shown in FIG. 2, the substrate to be processed S has a wafer 31 as an etching layer, a resist layer 32 made of an etching protection resist applied thereon, and the wafer 31 exposed without applying a resist. To-be-etched part 33. In the present embodiment, the light generated from the light source 10 is transmitted by the introduction optical system so that the spot diameter R of the incident spot light 40 on the substrate to be processed S is sufficiently larger than the hole diameter W of the etched portion 33. It is led on the substrate S to be processed. As a result, reflected light 41 from the surface of the wafer 31, reflected light 42 from the surface of the resist layer 32, and reflected light 43 from the etched portion 33 are generated on the substrate S to be processed.

  These reflected lights are scattered in various directions, and the light reflected in the direction of the measurement window 14 traces the objective lens 13 and the collimator lens 12 in the reverse direction and enters the optical fiber 11, in which the optical path difference is detected. Different reflected lights interfere with each other. That is, the introduction optical system described above also functions as an interference optical system. The interference light obtained by this interference optical system is dispersed for each wavelength by the spectroscope 17 and then detected in parallel by the CCD line sensor 18 for each wavelength. These detection signals are sent to the control / data processing unit 19 through predetermined signal processing, and are used for calculation of a measurement position by a method described later and calculation of a hole depth at the measurement position.

  The etching monitoring apparatus according to the present embodiment includes an auxiliary optical system including an observation camera 20 and an auxiliary light source 21 for determining an initial irradiation position on the substrate S to be processed. This auxiliary optical system has no direct relationship with the calculation of the measurement position and the etching depth, and is not an essential component, but it is desirable to prepare for quickly calculating the measurement position. It was described in.

  The light generated from the auxiliary light source 21 is introduced onto the substrate to be processed S before starting etching (FIG. 2) through a predetermined optical path, and the reflected light is photographed by the observation camera 20. This image data is sent to the control / data processing unit 19, and the coarse position determination unit 22 detects the difference in density between the resist layer 32 and the etched portion 33, and calculates the position of the etched portion 33 (although it is coarse). Is done. The irradiation position control unit 23 controls the operation of the minute movement stage 16 so that the incident spot light 40 is irradiated to the position calculated by the coarse position determination unit 22. Thereby, the initial irradiation position of the incident spot light 40 can be adjusted to the vicinity of the etched portion 33.

[Calculation method of measurement position]
Hereinafter, the calculation method of the measurement position according to the present invention will be described with reference to the flowchart of FIG. In the present embodiment, as described above, the irradiation position of the incident spot light 40 is moved by the minute movement stage 16. The movement accuracy of the minute movement stage 16 is determined by the spot diameter R on the substrate S to be processed. It is necessary to make it sufficiently smaller than the hole diameter W of the etched portion. The accuracy of the fine moving stage 16 used in this embodiment is 0.2 μm for the spot diameter R of the incident spot light 40 of 110 μm and the hole diameter W of the etched portion of several μm.

First, the incident spot light 40 is irradiated to the initial irradiation position of the substrate S to be processed in the initial stage of etching (FIG. 4) (step A1). In the substrate S to be processed of FIG. 4, the etched portion 33 is etched to some extent.
i) reflected light 41 from the surface of the wafer 31 and reflected light 42 from the surface of the resist layer 32;
ii) reflected light 41 from the surface of the wafer 31 and reflected light 43 from the etched portion 33;
iii) reflected light 42 from the surface of the resist layer 32 and reflected light 43 from the etched portion 33;
An optical path difference occurs between each of the two, and the reflected lights interfere with each other by passing through the interference optical system. The interference light thus obtained is dispersed by the spectroscope 17 and detected for each wavelength by the CCD line sensor 18 (step A2). This detection signal is converted into a digital signal and sent to the control / data processing unit 19 as an input signal (FIG. 5).

ここで、無照射時信号とは、光源１０から光を発生していないときにCCDラインセンサ１８から取得される検出信号（暗信号）のことであり（図６）、参照信号とは、凹凸のない（レジスト層３２や被エッチング部３３が設けられていない）ウエハ３１に対して入射スポット光を照射した際にCCDラインセンサ１８から取得される検出信号のことである（図７）。また、αは
α=参照信号面積／入力信号面積
であり、参照信号面積は参照信号とベースライン（波長に対して信号強度が0で一定の直線）とが為す面積、入力信号面積は入力信号とベースラインとが為す面積である。これら無照射時信号と参照信号のデータは、予備実験等により予め取得され、記憶部２４に格納されている。この式(1)によって、図８に示すような正規化信号のデータが得られる。 The interference intensity acquisition unit 25 in the control / data processing unit 19 first performs the following normalization for each wavelength of the input signal (step A3).

Here, the non-irradiation signal is a detection signal (dark signal) acquired from the CCD line sensor 18 when no light is generated from the light source 10 (FIG. 6). This is a detection signal acquired from the CCD line sensor 18 when the wafer 31 is irradiated with incident spot light (no resist layer 32 or etched portion 33 is provided) (FIG. 7). Α is α = reference signal area / input signal area, the reference signal area is the area formed by the reference signal and the baseline (a straight line with a signal intensity of 0 with respect to the wavelength), and the input signal area is the input signal area. And the baseline. The non-irradiation signal and the reference signal data are acquired in advance by a preliminary experiment or the like and stored in the storage unit 24. By this expression (1), data of a normalized signal as shown in FIG. 8 is obtained.

  The graph of FIG. 8 is obtained by normalization according to equation (1) based on the data of FIGS. The normalized signal obtained in this way includes interference components and noise due to the optical path differences of i), ii), and iii). The ratio of this noise to the normalized signal increases as the intensity of the reference signal approaches zero. For this reason, a predetermined wavelength range in which noise is relatively small (for example, 810 nm to 850 nm in the example of FIG. 7) is set as a target range, and only normalized signal data in this target range is used (FIG. 9).

  The normalized signal calculated by Equation (1) indicates that the interference component is not included in the input signal when the intensity is constant at 0 with respect to the wavelength. That is, on the contrary, the larger the amplitude from the baseline or the area of the region made by the baseline and the normalized signal (hereinafter referred to as “normalized signal area”), the greater the interference component contained in the input signal. It turns out that the intensity | strength of becomes large. The interference intensity acquisition unit 25 of the present embodiment acquires the intensity of the interference component included in the input signal by calculating the normalized signal area in the attention range of the normalized signal calculated by the equation (1) (step) A4).

  The interference component necessary for calculating the etching depth is due to the optical path difference of the above ii) and iii) related to the etched portion 33. The light intensity in the incident spot light 40 on the plane parallel to the wafer 31 (FIG. 4B) is the strongest at the central portion and becomes weaker as the distance from the central portion increases. Therefore, in order to increase the intensity of the interference component due to ii) and iii) included in the input signal, it is necessary to adjust the irradiation position so that the central portion of the incident spot light 40 overlaps the etched portion 33. Note that the interference component due to the optical path difference of i) does not change greatly depending on the irradiation position of the incident spot light 40 because the resist layer 32 covers most of the surface of the wafer 31.

  Next, the irradiation position is scanned by the minute movement stage 16 and the irradiation position control unit 23 (steps A5 and A6), and the processing of steps A1 to A4 is similarly performed on each irradiation position. The scanning of the irradiation position is performed by moving up / down / left / right from the initial irradiation position by 1 / (2n) of the spot diameter R of the incident spot light 40 (110 μm in this embodiment) measured in advance (FIG. 10).

  When the center point 44 of the initial irradiation position is sufficiently close to the etched portion 33 and the etched portion 33 is included in the spot diameter of the incident spot light 40 from the beginning, the irradiation position is scanned from the initial irradiation position. Just move it up, down, left and right (n-1) times. The presence / absence of the etched portion 33 within the spot diameter of the incident spot light 40 can be determined based on whether the normalized signal area is equal to or larger than a predetermined threshold value. On the other hand, if the etched portion 33 is not included in the spot diameter of the incident spot light 40 at the initial irradiation position, scanning is initially performed with n reduced (for example, n = 1), and the normalized signal area If the value becomes equal to or greater than a predetermined threshold value, n may be increased (for example, n ≧ 2) around the irradiation position at that time, and scanning may be performed (n−1) times vertically and horizontally.

  When scanning of the irradiation position is completed, the measurement position determining unit 26 searches for an irradiation position on the substrate S to be processed (hereinafter referred to as “maximum irradiation position”) that maximizes the normalized signal area (step A5 and A7). The search for the maximum irradiation position will be described with reference to FIGS. In the example of FIG. 10, n = 4, and the irradiation position is moved three times in the vertical and horizontal directions.

First, let the center point of each irradiation position be _{1a- 7g,} and let the normalized signal area obtained with respect to each irradiation position be D1a-D7g. Next, a minimum rectangular area whose vertex is any one of the points 1a to 7g is created, and each minimum rectangular area is weighted by the sum of the normalized signal areas at the four vertices. For example, in the case of a rectangular area composed of points 3c, 3d, 4d, and 4c, the weight is given as follows.
D _3c4d = D _3c + D _3d + D _4d + D _4c … (2)
After weighting all the minimum rectangular areas as shown in Expression (2), a rectangular area having the maximum weight is searched. Here, it is assumed that the minimum rectangular area having the maximum weight is an area having vertices at the points 5e, 5f, 6f, and 6e (that is, the maximum weight is D _5e6f ).

すなわち、4つの辺の内分点はそれぞれ、各辺の両端の2つの頂点における正規化信号面積の逆比の位置になる。このようにして得られた4つの内分点に対して、向かい合う辺の内分点同士を線分で繋ぎ、2つの線分の交点４６を正規化信号面積が最大となる点として、以降のエッチング深さの測定に用いる測定位置の中心点とする。 In the minimum rectangular area having the maximum weight, for the side consisting of points 5e and 5f, the side consisting of points 5f and 6f, the side consisting of points 6f and 6e, and the side consisting of points 6e and 5e The internal dividing points 45a, 45b, 45c and 45d on these sides are given as follows.

That is, the internal dividing points of the four sides are respectively the positions of the inverse ratio of the normalized signal area at the two vertices at both ends of each side. With respect to the four internal dividing points obtained in this way, the internal dividing points of opposite sides are connected by line segments, and the intersection 46 of the two line segments is defined as the point where the normalized signal area is maximized. The center point of the measurement position used for the measurement of the etching depth.

  The measurement position data calculated by the measurement position determination unit 26 is sent to the irradiation position control unit 23. The irradiation position control unit 23 controls the minute movement stage 16 based on this data, and moves the irradiation position to the measurement position. Let Thereafter, etching is started, and the change in etching depth is calculated and measured in real time by the depth calculator 27.

  12 and 13 respectively show a graph of the input signal obtained when the irradiation position is moved to the measurement position, and a graph in which only the attention range is enlarged to the normalized signal for the input signal. As can be seen by comparing FIGS. 9 and 13, in the etching monitoring apparatus according to the present embodiment, the interference component due to the intensity of the reflected light from the etched portion 33 and the optical path difference of the above ii) and iii) by the above series of operations. The strength of can be increased. Therefore, the S / N ratio can be improved in the subsequent measurement of the etching depth.

  In this embodiment, an SLD light source having a light emission wavelength full width at half maximum of 40 nm is used as the light source 10. However, the required value of the full width at half maximum depends on the depth of the etched portion 33 to be measured and the center wavelength of the light source 10. Come different. The full width at half maximum preferably includes about one wavelength of interference waves generated by the reflected light from the surface of the wafer 31 and the etched portion 33, and at least 10 nm is necessary.

  The search for the maximum irradiation position in step A7 is, for example, three-dimensional data in which the center point of each irradiation position is the x axis and the y axis, and the size of the normalized signal area obtained for each irradiation position is the z axis. It can also be performed by interpolating by two-dimensional spline interpolation or the like and searching for the (x, y) coordinate where the z-axis is maximum in the interpolated data.

  As described above, the etching monitoring apparatus according to the present invention has been described using the embodiments. However, changes, modifications, or additions may be made as appropriate within the scope of the present invention.

  For example, in the above-described embodiment, the control / data processing unit 19 that controls each part of the apparatus and processes data acquired from the CCD line sensor 18 is described as being integrated with the etching monitoring apparatus. The processing unit 19 may be realized by an external computer connected to the etching monitoring apparatus via a network or the like.

DESCRIPTION OF SYMBOLS 10 ... Light source 11 ... Optical fiber 12 ... Collimator lens 13 ... Objective lens 14 ... Measurement window 15 ... Etching chamber 16 ... Micro movement stage 17 ... Spectroscope 18 ... CCD line sensor 19 ... Control / data processing part 20 ... Observation camera 21 ... Auxiliary light source 22 ... Rough position determination unit 23 ... Irradiation position control unit 24 ... Storage unit 25 ... Interference intensity acquisition unit 26 ... Measurement position determination unit 27 ... Depth calculation unit 31 ... Wafer (etched layer)
32 ... resist layer 33 ... etched portion 40 ... incident spot light 41 ... reflected light from wafer surface 42 ... reflected light from resist layer surface 43 ... reflected light 44 from etched portion ... center point 45a of initial irradiation position, 45b, 45c, 45d ... internal dividing point 46 ... center point 1a-7g of maximum irradiation position (measurement position) ... point (center point of irradiation position)
S: Substrate to be processed

Claims (6)

A light source that generates coherent light having a predetermined wavelength width, an introduction optical system that guides the coherent light onto the substrate to be processed with a spot diameter larger than the diameter or width of the etched portion, and the substrate to be processed In an etching monitoring apparatus, comprising: an interference optical system that causes the reflected coherent light to interfere; and a detection unit that detects, for each wavelength, the interference light obtained by the interference optical system, of the coherent light on the substrate to be processed. An irradiation position moving means for moving the irradiation position;
Interference intensity acquisition means for acquiring the intensity of the interference component included in the detection signal of the light detection unit for each of the irradiation positions scanned on the substrate to be processed at intervals equal to or less than the spot diameter;
A measurement position determining means for searching for a position on the substrate to be processed where the intensity of the interference component is maximum and moving the irradiation position so as to measure the etching depth at the maximum position;
An etching monitoring apparatus comprising:   The etching monitoring apparatus according to claim 1, wherein the full width at half maximum of the emission wavelength of the coherent light is 10 nm or more.   The etching monitoring apparatus according to claim 1, wherein the light source is a superluminescent diode.   Furthermore, an auxiliary optical system for photographing the surface of the substrate to be processed, and a rough position calculating means for analyzing an image photographed by the auxiliary optical system and determining an initial irradiation position on the substrate to be processed are provided. The etching monitoring apparatus according to any one of claims 1 to 3. The interference intensity acquisition means is

(However, the input signal is the detection signal obtained for the substrate to be etched, the non-irradiation signal is the detection signal obtained when no light is emitted from the light source, and the reference signal has no irregularities. The detection signal obtained for the substrate to be processed, α is the ratio of the area of the reference signal from the base line to the area of the input signal from the base line)). 5. The intensity of an interference component is calculated by calculating an area of a region formed by a normalized signal and a straight line having a signal intensity of 0 with respect to a wavelength. Etching monitoring device described in 1. The measurement position determining means performs weighting by the sum of the intensity of the interference component corresponding to the vertex in the minimum rectangular region having the vertex of the center point of the irradiation position, and in the minimum rectangular region where the weight is maximum The inner dividing point of each side is obtained by the inverse ratio of the intensity of the interference component at the two vertices at both ends of each side, the inner dividing points of the opposite sides are connected by a line segment, and the intersection of the two line segments is The etching monitoring apparatus according to claim 1, wherein the etching monitoring apparatus is a center point of an irradiation position where the intensity of the interference component is maximized.

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