JP5009560B2 - Apparatus for measuring the shape of a thin object to be measured - Google Patents

Description

  The present invention relates to a technique for measuring the shape of a thin sample such as a semiconductor substrate (wafer), specifically, a thickness distribution and a warped shape.

  A thin sample such as a semiconductor substrate (wafer) is required to be as uniform in thickness as possible and free from warping.

[Prior art 1]
As a method for simultaneously measuring the thickness distribution and the warp shape of such a flaky sample (object to be measured), for example, a non-contact displacement meter 2 such as an interferometer, a laser displacement meter, a capacitance displacement meter or the like. A method is disclosed in which a table is placed opposite and placed on the front and back of a sample, and these are scanned to measure the height (displacement) distribution of the sample surface, and the thickness distribution and warpage shape are simultaneously obtained from the measured values. (See Patent Document 1). In this method, a sample is held horizontally, two displacement meters are fixed to a frame, and this frame is moved horizontally to scan.

  Here, in general, in a thin sample such as a semiconductor substrate (wafer), the amount of warpage is much larger than the variation in thickness. For example, in a silicon wafer, the magnitude of warpage is on the order of 10 to 100 μm, whereas the variation in thickness is on the order of 100 nm to 1 μm on the entire surface of the wafer. For this reason, the resolution or reproducibility required for measurement of warpage is about 0.1 μm (= 100 nm), whereas the resolution or reproducibility of thickness is about 10 nm. When scanning the surface of the sample having the warp of the order as described above with the displacement meter of the prior art 1, it is necessary to install the displacement meter several hundred μm away from the sample surface for safety. For this reason, the displacement meter is required to have a measurement range of several hundred μm and a resolution or reproducibility of 10 nm, and a dynamic range of several hundred μm / 10 nm = tens of thousands. Moreover, in recent years, further flatness is required for wafers used in the manufacture of LSIs, and there is a demand for wafers with thickness variations reduced to about 1 nm. On the other hand, the size of the warp has not changed greatly since the size of the wafer has increased from 200 mm to 300 mm. For this reason, the dynamic range of the displacement meter is required to be larger, and it is difficult to measure the thickness with high accuracy in view of restrictions on the capacity of signal processing.

  That is, if the distance between the two displacement meters is narrowed too close to the sample in order to increase the measurement accuracy, the displacement meter may come into contact with the sample if the sample warps greatly. For this reason, the distance between the two displacement meters must be increased to some extent, and it has been difficult to simultaneously measure the thickness of the sample and the size of the warp with high accuracy.

[Prior art 2]
On the other hand, as a device for measuring the surface shape of a sample (subject), a capacitance sensor is movably attached to a fixed frame via a displacement mechanism such as a linear motor, and the capacitance sensor is operated by operating the displacement mechanism. An apparatus is disclosed that can improve measurement accuracy by performing measurement while moving the sample (subject) from a surface within a certain distance (see Patent Document 2).

  According to this apparatus, the displacement mechanism can be brought close to the sample without bringing it into contact with the sample, and the surface shape (that is, the warped shape) of the sample can be measured with high accuracy. However, since this apparatus uses one displacement meter, the back surface shape cannot be measured at the same time, and the thickness of the sample cannot be measured.

JP 2003-75147 A JP 2001-221607 A

Accordingly, an object of the present invention is to provide a shape measuring apparatus capable of simultaneously measuring the thickness distribution and warpage shape of a flaky object to be measured with high accuracy.

The invention according to claim 1 is an apparatus for measuring the shape of a thin object to be measured by scanning it from one end to the other end with a non-contact type objective displacement sensor, and sandwiching the front and back of the object to be measured. A pair of non-contact type object displacement sensors fixed to the frame and opposed to each other with the measurement object interposed therebetween, and the frame in the thickness direction of the measurement object A PZT element as a frame moving means that is movable to the frame, a frame displacement detection means for detecting a movement amount of the frame, and a movement amount of one of the pair of objective displacement sensors as the PZT element. A sensor position control means for adjusting the position of the object to be measured to maintain a substantially constant gap with the object to be measured, a movement amount detected by the frame movement amount detection means, and the pair of objective displacement sensors. Shape measuring means, comprising: shape calculating means for calculating the thickness of the object to be measured and the degree of warping based on the distance from the objective sensor to the surface and the back surface of the object to be measured Device.

The invention according to claim 2 is the shape measuring apparatus according to claim 1, wherein the objective displacement sensor is a capacitance sensor or a laser interferometer.

According to the present invention, since the pair of displaceable optical sensors which faces can be scanned in a state of being as close as possible to the front and back of the object, equipment that can simultaneously measure the thickness distribution and camber profile of the workpiece with high precision Can be realized.

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

Embodiment 1
FIG. 1 shows a schematic configuration of a shape measuring apparatus according to an embodiment of the present invention. An object to be measured S such as a wafer is supported horizontally via a plurality of support points 12 on a stage 7 that can move in the horizontal direction. Reference numerals 1 and 1 ′ denote fixed frames, which are arranged so as to sandwich the front and back of the object S to be measured. Reference numerals 2 and 2 ′ are electrostatic capacitance sensors as a pair of non-contact type objective displacement sensors, and are arranged to face each other with the object S to be measured interposed therebetween. Reference numerals 3 and 3 ′ denote PZT elements as sensor moving means, which are fixed to the frames 1 and 1 ′, respectively, and the capacitance sensors 2 and 2 ′ are respectively measured with respect to the frames 1 and 1 ′. It is supported so as to be movable in the thickness direction of S (that is, in the vertical direction). Reference numerals 4 and 4 ′ denote capacitance sensors as sensor movement amount detection means, which detect the movement amounts of the objective displacement sensors 2 and 2 ′ with respect to the frames 1 and 1 ′, respectively. Reference numeral 5 denotes a sensor position control unit serving as a sensor position control means, which adjusts the amount of movement of one (upper part in this example) of the capacitance sensor 2 and holds the gap with the measured object S substantially constant. The distance between the pair of capacitance sensors 2 and 2 ′ is moved by moving the other (lower in this example) capacitance sensor 2 ′ by the same amount as the movement of one (upper) capacitance sensor 2. Is kept constant. Reference numeral 6 denotes a shape calculation unit as a shape calculation means, which is detected by the movement amount of the capacitance sensors 2 and 2 ′ detected by the capacitance sensors 4 and 4 ′ and the pair of capacitance sensors 2 and 2 ′. The thickness of the measurement object S and the degree of warpage are calculated based on the distances from the respective capacitance sensors 2 and 2 ′ to the front and back surfaces of the measurement object S.

  Next, a method for measuring the shape of the object S to be measured using the shape measuring apparatus of this example will be described. First, the distance between the capacitance sensors 2 and 2 ′ is as close as possible to the measurement object S in a range not contacting the measurement object S, and thus varies depending on the diameter of the capacitance sensors 2 and 2 ′. For example, the thickness of the object to be measured S is about +100 to 2000 μm, more preferably the thickness of the object to be measured +100 to about 200 μm. Then, the stage 7 is set so as to be positioned at an initial position, that is, between a pair of electrostatic capacitance sensors 2 and 2 ′ with one end of the object S to be measured facing each other. At this time, the height positions of the capacitance sensors 2 and 2 ′ are determined in advance so that the gaps between the capacitance sensors 2 and 2 ′ and the object S to be measured are substantially the same. Capacitance sensors 2 and 2 'are set. In addition, a signal of the distance to the surface of the measurement object S detected by the capacitance sensor 2 is always sent to the sensor position controller 5.

  When the horizontal movement of the stage 7 is started to scan the measurement object S, if the measurement object S is warped, the size of the gap between the upper capacitance sensor 2 and the surface of the measurement object S is large. Changes.

  When detecting the change in the size of the gap, the sensor position control unit 5 changes the input voltage of the upper PZT element 3 so as to be displaced (expanded / contracted) by the same amount as this change. As a result, the upper PZT element 3 is displaced (stretched), and the relative position in the vertical direction of the upper capacitive sensor 2 with respect to the frame 1 is also displaced. The reason why the PZT element 3 is used as the sensor moving means is that the settling time for positioning can be shortened. However, since the displacement characteristic of the PZT element 3 with respect to the input voltage has hysteresis and temperature drift, the actual displacement amount of the PZT element 3 cannot be determined from the input voltage alone, and the actual displacement amount of the capacitance sensor 2 can be determined. It cannot be determined accurately. For this reason, the actual displacement amount of the upper capacitance sensor 2 is measured by the capacitance sensor 4.

  Next, the sensor position control unit 5 changes the input voltage of the lower PZT element 3 ′ so that the lower capacitance sensor 2 ′ is displaced by the same amount as the actual displacement amount of the upper capacitance sensor 2. Change. Similar to the upper part, the actual displacement of the lower capacitive sensor 2 'is measured by the capacitive sensor 4'.

  In order to make the displacement of the upper and lower electrostatic capacitance sensors 2 and 2 ′ as similar as possible, the upper and lower PZT elements 3 and 3 ′ are selected to have the same shape and the same characteristics, The fixing method and the load applied to the PZT elements 3 and 3 ′ are the same, and it is desirable that the dynamic characteristics of the upper and lower PZT elements 3 and 3 ′ are the same as much as possible.

  In addition, when a thin object to be measured S such as a wafer is placed sideways, deflection due to the weight of the object to be measured S is generated in addition to warping. The amount of deflection is determined in advance by the support position of the stage 7 and the object to be measured. The influence can be removed by calculating from the diameter, thickness, crystal orientation, elastic modulus, etc. of the object S.

  Next, the shape calculation unit 6 calculates the thickness of the workpiece S and the degree of warpage.

  The amount of warpage of the object S to be measured is determined in advance with respect to a predetermined height position (for example, the height position of the upper capacitive sensor 2 at the start of measurement) and the distance to the surface of the object S to be measured. It can be obtained from the difference from the distance to the reference plane (for example, the height position of the end of the object S to be measured). In addition, when a thin object to be measured S such as a wafer is placed sideways, in addition to warping, deflection due to the weight of the object to be measured S is generated. The influence can be removed by calculating from the diameter, thickness, crystal orientation, elastic modulus, etc. of the object S.

Further, the thickness t of the measurement object S can be obtained as follows with reference to FIG. That is, if the gaps between the capacitance sensors 2 and 2 ′ and the object S to be measured are a and b, respectively, and the distance between the upper and lower capacitance sensors 2 and 2 ′ is c,
t = c−a−b Formula (1)
There is a relationship. Here, the distance c between the upper and lower capacitance sensors 2 and 2 'cannot be directly measured during scanning of the object S to be measured, but since this distance is always kept constant, it is known in advance. It may have been measured with a calibration strip having a thickness of t _0. If the gaps between the capacitance sensors 2 and 2 ′ and the calibration piece are a ₀ and b ₀ , respectively, c is
c = t ₀ + a ₀ + b ₀ Formula (2)
It is obtained by. Therefore, the thickness t of the measurement object S can be obtained by using the value of c in the equation (1). The values of a and b may be obtained individually based on the outputs from the electrostatic capacitance sensors 2 and 2 ′. However, the sensor position control unit 5 electrically adds the outputs to obtain the value of a + b. Can also be obtained directly.

  In this way, the warp shape and thickness distribution of the measurement object S can be measured by repeating the measurement and calculation while scanning the measurement object S to the other end.

  FIG. 3 is a schematic diagram illustrating the concept of the measurement, in which the horizontal axis indicates the horizontal position and the vertical axis indicates the height position. In the figure, in order from the top, the locus of the height position of the upper capacitance sensor 2, the surface shape of the object S to be measured, the back surface shape of the object S to be measured, and the height position of the upper capacitance sensor 2 ′. Show the trajectory. As described above, the gap a between the electrostatic capacitance sensor 2 and the surface of the object S to be measured is controlled to be constant. However, the upper static is controlled because the object S is scanned at a predetermined speed. The height position of the capacitance sensor 2 cannot completely follow the surface shape of the object S to be measured. That is, it can follow a rough surface shape, but cannot follow the surface shape of a portion having a high spatial frequency and a fine change, and a control error remains in this portion. In addition, the height position of the lower capacitance sensor 2 ′ changes in conjunction with the height position of the upper capacitance sensor 2, so that a certain offset from the locus of the upper capacitance sensor 2 exists. The locus is shifted downward.

  4 shows the outputs of the upper and lower capacitance sensors 2, 2 ′ obtained during the measurement of FIG. 3 (that is, the distance a from the upper capacitance sensor 2 to the surface of the object S to be measured and the lower capacitance). The distance b) from the capacitance sensor 2 to the back surface of the object S to be measured and the distribution of the thickness t calculated by the above equation (1) based on these outputs a and b are shown. The outputs a and b of the upper and lower electrostatic capacitance sensors 2 and 2 'are both substantially free from warping, and the surface shape of the portion having a fine change is faithfully reproduced. The true thickness of the measurement object S is reproduced with high accuracy.

[Embodiment 2]
FIG. 5 shows an outline of a shape measuring apparatus according to another embodiment of the present invention. Unlike the first embodiment, this example fixes a pair of non-contact type capacitive sensors 12, 12 'as a non-contact type objective displacement sensor to face each other and fixes the frame 11 as a frame moving means. It is fixed to another fixed frame 18 via the PZT element 13 so as to be movable up and down. Further, in order to detect the amount of movement of the frame 11 with respect to the fixed frame 18, a capacitance sensor 14 as a frame displacement detecting means is provided on the fixed frame 18 so as to face the bottom surface of the frame 11.

  The sensor position control unit 15 as sensor position control means adjusts the amount of movement of the frame 11 to keep the gap between the upper and lower capacitance sensors 12, 12 ′ and the measured object S substantially constant. is there. Further, the shape calculation unit 16 as the shape calculation means includes the movement amount of the frame 11 detected by the capacitance sensor 14 and the capacitance sensors 12, 12 detected by the pair of capacitance sensors 12, 12 ′. 'The thickness and the degree of warpage of the device under test S are calculated based on the distances from the respective surfaces to the front and back surfaces of the device under test S.

  With the above configuration, since the distance between the pair of capacitance sensors 12 and 12 ′ is fixed, the number of PZT elements and capacitance sensors can be saved as compared with the first embodiment.

  For example, as shown in FIG. 5, the frame 11 has a U-shaped cross section, and has rigidity so that an error does not occur due to variation in the distance between the pair of capacitance sensors 12 and 12 ′ attached to the frame 11. It may be made of a material such as stainless steel.

[Modification]
In the said Example 1, 2, although the electrostatic capacitance sensor was illustrated as an objective displacement sensor, a laser interference length meter can also be used.

  In the first embodiment, the capacitance sensor is exemplified as the sensor movement amount detection unit. However, a laser displacement meter, a linear scale, or the like may be used.

  In the second embodiment, the capacitance sensor is exemplified as the frame movement amount detection unit. However, as with the sensor movement amount detection unit, a laser displacement meter, a linear scale, or the like may be used.

  In the first and second embodiments, an example is shown in which the object to be measured is placed horizontally. However, there is no particular restriction on how to place the object to be measured, and for example, the object can be placed vertically.

  In the first and second embodiments, the control for making the gap between the upper capacitance sensor and the object to be measured constant is illustrated, but the control for making the gap between the lower capacitance sensor and the object to be measured S constant. Is of course possible.

  In the first and second embodiments, the example in which the measurement object is scanned by prohibiting the horizontal movement of the frame and moving the stage horizontally is shown. However, the measurement object is placed on a fixed stage. It is also possible to scan by moving the frame horizontally.

  In the first embodiment, an example in which the frame is composed of two fixed frames has been shown. However, it is also possible to construct a single frame. For example, a U-shaped frame similar to the frame in the second embodiment may be used. It can be configured with one piece.

  In the second embodiment, a U-shaped frame is illustrated, but a frame having a U-shaped cross section can also be used.

It is a longitudinal cross-sectional view which shows schematic structure of the shape measuring apparatus which concerns on Embodiment 1. FIG. It is a fragmentary longitudinal cross-section explaining the method of calculating | requiring the thickness of to-be-measured object. It is a graph which shows the concept of the measurement which concerns on this invention. It is a graph which shows distribution of the thickness of a to-be-measured object. It is a longitudinal cross-sectional view which shows schematic structure of the shape measuring apparatus which concerns on Embodiment 2. FIG.

Explanation of symbols

1, 1 ', 11: Frame 2, 2', 12, 12 ': Objective displacement sensor (capacitance sensor)
3, 3 ': Sensor moving means (PZT element)
4, 4 ': Sensor movement amount detection means (capacitance sensor)
5, 15: Sensor position control means (sensor position control unit)
6, 16: Shape calculation means (shape calculation unit)
7: Stage 13: Frame moving means (PZT element)
14: Frame displacement detection means (capacitance sensor)
18: Fixed frame S: Object to be measured (wafer)

Claims (2)

A device for measuring the shape of a thin object to be measured by scanning it from one end to the other end with a non-contact type objective displacement sensor,
A frame arranged so as to sandwich the front and back of the object to be measured;
A pair of non-contact type objective displacement sensors fixed to the frame and arranged to face each other with the object to be measured interposed therebetween;
A PZT element as a frame moving means that enables the frame to move in the thickness direction of the object to be measured;
Frame displacement detection means for detecting the amount of movement of the frame;
Sensor position control means for adjusting the amount of movement of one of the pair of objective displacement sensors by displacing the PZT element so as to keep the gap between the object to be measured substantially constant;
The thickness and warpage of the object to be measured based on the amount of movement detected by the frame movement amount detection means and the distance from the object sensor detected by the pair of object displacement sensors to the front and back surfaces of the object to be measured. Shape calculating means for calculating the degree of
A shape measuring apparatus comprising: The shape measuring apparatus according to claim 1, wherein the objective displacement sensor is a capacitance sensor or a laser interferometer.

Images