JP5321815B2 - Nondestructive single crystal substrate warpage measuring method and measuring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To measure warping without destructing a package, on a plurality of respective monocrystal substrates stored in the package by being mutually laminated. <P>SOLUTION: A method includes: a step of irradiating the package with an X-ray; a step of selectively detecting only a diffraction X-ray from a desired monocrystal substrate from a plurality of diffraction X-rays diffracted by the plurality of monocrystal substrates stored in the package, by adjusting a light receiving slit and by adjusting a position in the height direction of the package; a step of measuring a rocking curve of the diffraction X-ray from the desired monocrystal substrate; and a step of moving the package by a specific distance in the predetermined direction. The step of measuring the rocking curve and the step of moving the package by the specific distance in the predetermined direction, are repeatedly performed. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a measuring method and a measuring apparatus for measuring a warp of a single crystal substrate, and more particularly to a measuring method and a measuring apparatus for measuring a warp of a single crystal substrate mounted in a package without destroying the package.

  In general, many of the physical properties and properties of a crystal material vary depending on the stress and the lattice strain that accompanies it. In addition, the stress and lattice strain of a crystal material change depending on the growth conditions and processing conditions of the same material, or the stress exerted by other materials in contact with the crystal material. Therefore, it is indispensable for the development of high-performance and highly reliable materials and products to evaluate the stress and lattice strain of the crystal material and to optimize the processing and growth conditions according to the application. In particular, when an electronic product is manufactured using a semiconductor single crystal, the single crystal is processed into a thin plate-like wafer, and elements such as a diode, a transistor, and a capacitor are formed on one surface. Since the process for forming these elements includes steps such as oxidation, film formation, impurity introduction, etching, and heat treatment, stress is generated in the semiconductor wafer and appears as a macroscopic warpage. Since the warpage of the wafer changes the physical properties and properties of the semiconductor single crystal, it can affect the performance and reliability of the device. Therefore, it is necessary to evaluate the warpage of the semiconductor wafer.

  Further, in recent LSI packaging technology, in consideration of a demand for space saving, a technology has been studied in which a semiconductor wafer is thinned after an LSI diffusion process and then divided into chips and then packaged. The thinned semiconductor chip is easily affected by the stress of the packaging material, and the warped state of the semiconductor chip after packaging varies depending on the packaging material and process. Therefore, it is necessary to measure the warpage of the semiconductor chip after packaging in order to control the performance of the semiconductor product and improve the reliability.

  Conventionally, as a technique for nondestructively evaluating the shape of a crystal sample enclosed in a package, the crystal sample is placed on the xy plane, irradiated with X-rays parallel to the xz plane, and diffracted parallel to the xz plane. A method for detecting a line is known (for example, JP 2008-203212 A).

JP 2008-203212 A

  In recent semiconductor chip mounting technology, multiple semiconductor chips with the same function (mainly memory chips) are stacked and packaged to greatly increase the information processing capability and information storage capability of the product. There is a multi-chip packaging (MCP) technology as a technology that can be improved without any problems. There is also a technology called system in package (SiP) in which a plurality of semiconductor chips having different types of functions are stacked in one package and the package has a new function.

  In these mounting techniques represented by MCP and SiP, each semiconductor chip is thinned to 100 μm or less. For this reason, the semiconductor chip is more susceptible to the stress of the package material than in the conventional package technology. Therefore, when semiconductor chips are stacked like MCP and SiP, it is desirable that the warpage can be measured for each of the stacked semiconductor chips.

  The present invention has been made in consideration of the above problems, and an object of the present invention is to provide a method for non-destructively and directly measuring warpage of a packaged single crystal substrate, particularly a semiconductor chip, and an apparatus for realizing the method. .

  In order to solve the above problems, a method for measuring a warpage of a single crystal substrate according to the first aspect of the present invention destroys the warpage of each of a plurality of single crystal substrates stacked in one another and accommodated in a package. A method for measuring the warpage of a single crystal substrate, wherein the step of irradiating the package with X-rays, adjusting the light receiving slit and adjusting the position in the height direction of the package, A step of selectively detecting only a diffracted X-ray from a desired single crystal substrate from a plurality of diffracted X-rays diffracted by the crystal substrate; and a rocking curve of the diffracted X-ray from the desired single crystal substrate. Measuring, moving the package in a predetermined direction by a predetermined distance, measuring the rocking curve, and moving the package in a predetermined direction by a predetermined distance. And performing repeatedly the steps to.

  A single crystal substrate warpage measuring apparatus according to a second aspect of the present invention is a single crystal substrate that measures each warp of a plurality of single crystal substrates stacked on each other and accommodated in a package without destroying the package. A warp measuring apparatus for irradiating the package with X-rays; detecting means for detecting diffracted X-rays from the package; and diffracted X-rays from the package from a desired single crystal substrate. A light receiving slit for selectively allowing only diffracted X-rays to pass; means for adjusting a height position of the package so that diffracted X-rays from the desired single crystal substrate pass through the light receiving slit; Means for rotating the package about a predetermined rotation axis to measure a rocking curve of diffracted X-rays from the single crystal substrate, and moving the package in a predetermined direction by a predetermined distance Characterized in that it comprises means for the.

  According to the present invention, it is possible to reliably measure the warp of a desired single crystal substrate among a plurality of single crystal substrates stacked and sealed in a package without destroying the package. This makes it possible to optimize mounting materials and processes based on the measurement results, and promote the development of high-performance and highly reliable products.

It is a figure for demonstrating the principle of the curvature measuring method of this invention. It is a figure for demonstrating the principle of the curvature measuring method of this invention. It is a figure for demonstrating the principle of the curvature measuring method of this invention. It is a figure for demonstrating the principle of the curvature measuring method of this invention. It is a figure for demonstrating the principle of the curvature measuring method of this invention. It is a cross-sectional schematic diagram of an example of the sample used as the object of this invention. It is a schematic diagram which shows the positional relationship of incident X-rays, diffraction X-rays, and a sample. (A), (b), (c) and (d) are explanatory drawings for explaining a measurement procedure of warpage measurement according to the present invention. It is a graph which shows an example of the rocking curve obtained from MCP which mounted four Si chips. It is a graph which shows an example of the rocking curve obtained from the uppermost Si chip in MCP which mounted four Si chips. It is a figure for demonstrating the procedure which selectively detects the diffraction X-ray from one Si chip of MCP which mounted two Si chips. It is a figure for demonstrating the procedure performed following FIG. It is a figure for demonstrating the procedure performed following FIG. It is a graph which shows the change of (omega) _B with respect to the distance X from a chip | tip center, Comprising: (a), (b) and (c) are the thing of the 1st, 2nd, and 3rd Si chip from the top, respectively It is. 13 is a graph showing a change of Δω _B obtained from FIG. 12 with respect to the distance X from the center of the chip, wherein (a), (b), and (c) are the first, second, and third sheets from the top, respectively. For the Si chip of the eye. Si is a diagram for explaining the relationship between the chip warping and [Delta] [omega _B. Is a graph showing the relationship between the distance X and the warp of the chip center obtained from [Delta] [omega _B shown in FIG. 15.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a method for measuring warpage of a single crystal substrate and a measuring apparatus according to the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a diagram for explaining the principle of the warpage measuring method of the present invention.

  1 employs an X-ray diffraction method, and includes an X-ray source 11, an entrance slit 12, a sample stage 13, a light receiving slit 14, and a detector 15. Note that FIG. 1 is simplified for the sake of explanation, and does not completely match the apparatus used for actual measurement.

  An X-ray source 11 having an appropriate wavelength and output is used. A single crystal substrate 16 is held on the sample stage 13 as a sample.

  X-rays generated by the X-ray source 11 pass through the entrance slit 12 and enter the single crystal substrate 16. Here, the irradiation area 17 on which X-rays enter is area A.

When the rotation angle ω of the sample stage 13 measured from the direction of the wave vector 19 of the incident X-ray 18 is changed while the position and orientation of the detector 15 are fixed, the Bragg condition is satisfied, that is, the irradiation region 17 (region when taking a particular value alpha _B there is an angle alpha wave vectors 19 of the incident surface of a) X-ray 18, crystal diffraction X-ray is generated, the diffracted X-rays 20 are generated (intensity is strongest) . The diffracted X-ray 20 passes through the light receiving slit 14 and is detected by the detector 15. Here, ω when the diffracted X-ray 20 is detected is represented as ω _B. Further, the value of omega _B in the region A represents a omega _B (A). In the following, a graph representing the ω dependency of the intensity of diffracted X-rays is referred to as a rocking curve.

Next, as shown in FIG. 2, while maintaining ω at ω _B (A), the sample stage 13 is moved parallel to the X axis (in this case, moved in the positive direction). As a result, the irradiation area 17 changes from the area A to a different area B. At this time, if the single crystal substrate 16 is not warped, the diffracted X-rays 20 are detected as described above. This is because the angle α formed by the surface of the irradiation region 17 and the wave vector 19 of the incident X-ray 18 is equal to α _{B in} the region A and in the region B.

  On the other hand, when the single crystal substrate 16 is warped, the following occurs. Here, it is assumed that the single crystal substrate 16 is warped so as to protrude upward.

First, as in FIG. 1, a state in which the diffracted X-ray 20 is detected by the detector 15 is created. This is shown in FIG. At this time, the rotation angle ω of the sample stage 13 is ω _B (C). The angle α formed by the surface of the irradiation region 17 (region C) and the wave vector 19 of the incident X-ray 18 is α _B because the diffracted X-ray 20 is detected.

Next, while maintaining ω at ω _B (C), the sample stage 13 is moved parallel to the X axis (in this case, moved in the negative direction), and the irradiation region 17 is changed to a region D different from the region C. . The state is shown in FIG.

As can be understood from FIG. 4, when the single crystal substrate 16 is warped, the angle α formed by the surface of the irradiation region 17 and the wave vector 19 of the incident X-ray 18 is the case of the region C and the case of the region D. And different. That is, the α ≠ α _B. For this reason, the diffraction X-ray 20 cannot be detected by the detector 15 whose position is fixed.

To be able to detect by the detector 15 the diffracted X-rays from the area D, as the angle alpha of the wave vector 19 of the irradiation region 17 surface and the incident X-ray (area D) 18 is equal to alpha _B As shown in FIG. 5, the rotation angle ω of the sample stage may be changed. That is, the value of ω may be adjusted in a direction that cancels the change in α caused by the movement of the irradiation region 17 (measurement point). Quantitatively, when the angle formed by the surface of the region C and the surface of the region D is Δθ ≧ 0, the equation 1 may be used.

Thus, the irradiation region 17 becomes equal to the alpha is alpha _B when the region D, the diffraction X-ray 20 is detected.

  From the above description, it is understood that when the irradiation region 17 is moved when the single crystal substrate 16 is warped, ω when the diffracted X-ray is detected changes.

  Furthermore, Formula 1 is transformed into Formula 2.

If ω _B is measured when the irradiation region 17 is the region C and the region D, the angle between the surfaces of the region C and the region D, that is, the angle change of the surface due to warpage is obtained from the difference. It shows that you can.

  Although the case where the single crystal substrate 16 is warped upward has been described above, the change of the angle formed by the warp can be measured by the same principle even when the single crystal substrate 16 is warped downward. (Description is omitted). When the single crystal substrate 16 is warped downward, the above formulas 1 and 2 are changed to the following formulas 3 and 4.

That is, it is possible to determine the position of the codes of the change in the warp direction of the omega _B (convex downward convex or above).

  The above is the description of the principle of the method of measuring the warpage of the single crystal substrate using X-ray diffraction.

In the above description, the sample is a single crystal substrate, but the present invention intends to measure the warpage of a packaged semiconductor chip (a single crystal substrate used therein). In order to measure the warpage of a packaged semiconductor chip, X-ray diffraction must be measured through the packaging mounting material. In general, when X-rays having an intensity of ₁₀ enter the material and travel through the material, the X-rays are absorbed by the material, and the intensity is expressed by the depth (the thickness through which the X-rays are transmitted) as shown in Equation 5. Decay exponentially with respect to t.

Density here ρ is material, the mu _m is the mass absorption coefficient of X-ray. mu _m is dependent on the energy of the elements and the incident X-rays that make up the substance. That is in the mu _m is a greater tendency as elemental atomic number Z is high, the higher the energy E of the incident X-ray mu _m is small. These dependencies are approximately expressed as in the following Equation 6 except for the vicinity of the absorption edge.

  Here, k is a proportionality coefficient. Therefore, by using higher energy X-rays, the absorption of X-rays by the package material can be reduced, and as a result, X-ray diffraction measurement through a mounting material such as a package becomes possible.

  FIG. 6 is a cross-sectional view of an example of a sample 60 to be specified by the warpage measurement apparatus according to the embodiment of the present invention.

  In the illustrated semiconductor package, four Si chips 62 to 65 manufactured using a single crystal substrate are stacked on a resin substrate 61. These stacked Si chips 62 to 65 are wired and sealed with a molding material 15.

  Next, a warpage measurement method using the measurement apparatus according to the present embodiment will be described. The configuration of the apparatus is the same as in FIG. For example, an electron synchrotron can be used as the X-ray source 11. That is, high-energy X-rays (energy: 18 keV) obtained from electron synchrotron radiation can be used as the incident X-rays 18. Moreover, it is preferable to use X-rays having a wavelength shorter than 0.16 nm.

  The X-ray source 11 and the entrance slit 12 function as an X-ray irradiation apparatus that irradiates the sample 60 with X-rays. The sample stage 13 functions as a movement / adjustment unit that moves the sample along the X axis, the Y axis, and the Z axis, and as a rotation unit that rotates these axes about the center of rotation. The sample stage 13, together with the light receiving slit 14, causes the detector 15 to detect only a desired diffracted X-ray (from the single crystal substrate) among the plurality of diffracted X-rays from the sample 60.

  FIG. 7 is a plan view showing the positional relationship between the incident X-ray 18, the diffracted X-ray 20, and the sample 60 (X-ray irradiation region 17).

  The opening size of the incident slit 12 that defines the size of the incident X-ray 18 is set to an appropriate size for detecting the diffracted X-ray 20, for example, 0.2 mm × 0.05 mm.

  When the opening size of the entrance slit 12 is 0.2 mm × 0.05 mm, the irradiation region 18 of incident X-rays on the Si chip is, for example, Si (004) diffraction obtained from a Si (001) substrate as diffracted X-rays. When X-rays are used, the area is approximately W1 × W2 = 0.2 mm × 0.27 mm (area shown in white in FIG. 7).

  Hereinafter, the procedure of warpage measurement will be described with reference to FIGS.

  First, as shown in FIG. 8A, the sample 60 is set on the sample stage 13 so that the incident X-ray 18 is irradiated to the end portion of the sample 60. Here, it is assumed that the X-ray source 11 and the detector 15 are adjusted so that the incident X-ray 18 and the diffracted X-ray 20 travel on the XZ plane.

Next, the sample stage 13 is rotated by a predetermined angle (changing the rotation angle ω) with the Y axis (front and back in the figure) as the center of rotation, and the intensity of the X-ray detected by the detector 15 is measured (rocking). Measure the curve). When the measurement result is graphed as a function of the rotation angle ω, a rocking curve as shown in FIG. 8B is obtained. The rotation angle ω at which the peak of the rocking curve is obtained is ω _{B at the} measurement point (irradiation region 17).

Next, the irradiation region 17 is moved along the X axis, and as shown in FIG. 8C, the incident X-ray 18 is irradiated to a region different from the case of FIG. Then, again, the X-ray intensity detected by the detector 15 is measured while changing the rotation angle ω of the sample stage 13. As a result, a rocking curve as shown in FIG. 8D is obtained, and ω _{B at the} measurement point is obtained.

  As shown in FIG. 8D, the deviation (difference in rotation angle ω) between the rocking curve obtained this time (shown by a solid line) and the previously obtained rocking curve (shown by a broken line) and measurement From the distance between points (between irradiation areas), the direction and size of warpage can be obtained.

  Now, the rocking curves shown in FIGS. 8B and 8D each have only one peak. However, since the sample 60 is formed by stacking four Si chips 62 to 65, actually, a rocking curve having a plurality of peaks as shown in FIG. 9 is usually obtained. Each peak corresponds to one of the stacked S chips 62 to 65. In such a case, it is necessary to associate the peak with the Si chip.

  The correlation between the peak of the rocking curve and the stacked Si chip can be performed based on the strength. That is, it can be determined that the diffraction X-rays 20 are from the first Si chip 65, the second Si chip 64, and the third Si chip 64 from the top in descending order of intensity. This is because the diffracted X-ray from the lower Si chip has a greater intensity attenuation due to absorption.

  In FIG. 9, the interval between the peaks is 0.1 ° to 0.15 °. In order to easily identify analytical X-rays of a plurality of Si chip shells, the divergence angle of incident X-rays 17 should be small. Specifically, when measuring the warpage of the Si chip mounted in the MCP, the divergence angle of incident X-rays is desirably 0.1 milliradians or less.

  In a situation where a plurality of peaks are observed, it is not impossible to obtain warpage for each of a plurality of Si chips by the method described above with reference to FIG. However, if the irradiation region 17 of the incident X-ray 18 is changed by moving the sample 60, the peaks of the diffracted X-rays that have been observed separately until then overlap, resulting in difficulty in determining the peak position. The problem can occur.

  Therefore, in this embodiment, after obtaining a rocking curve having a plurality of peaks as shown in FIG. 9, the height position of the sample and the opening width of the light receiving slit 14 are adjusted, and the peaks as shown in FIG. To obtain a single rocking curve. That is, the diffracted X-ray 20 from a desired single Si chip is incident on the detector 15. Hereinafter, a specific adjustment method will be described with reference to FIGS. 11 and 12. Here, in order to simplify the explanation, it is assumed that the sample includes two Si chips 62 and 63, but even if the sample includes three or more Si chips, Diffracted X-rays 20 from a desired single Si chip can be detected.

  First, as shown in FIG. 11, it is assumed that the diffracted X-rays 71 and 72 from the two Si chips 62 and 63 are detected by the detector 15.

  Next, as shown in FIG. 12, a light receiving slit 14 is inserted between the detector 15 and the sample stage 13 (sample 60). The opening width of the light receiving slit 14 is a width through which the diffracted X-rays 71 or 72 from one of the Si chips 61 or 62 can pass. Or you may insert the light-receiving slit 14 which can adjust the opening width of a slit previously. In this case, the opening width is initially widened so that both diffracted X-rays 71 and 72 from the two Si chips 61 and 62 can be detected, and then one diffracted X-ray 71 or 72 is selectively detected. It can be so.

  Next, as shown in FIG. 13, the position of the sample 60 is adjusted by moving the sample stage 13 in the Z-axis direction, and diffracted X-rays (here, Si chip 62) from a desired single Si chip (here, Si chip 62). Then, only the diffracted X-rays 72) are allowed to reach the detector 15.

After adjusting to detect diffracted X-rays from one of the plurality of Si chips included in the sample 60 as described above, the irradiation region 17 is set as described with reference to FIG. Move to obtain ω _{B of} each measurement point.

  The measurement is performed at regular intervals from one edge to the other edge, for example, so as to cross the surface of the sample. In the middle of the measurement, the interval between the measurement points is made small so as not to be mistaken for diffracted X-rays from other Si chips, that is, to continue to detect diffracted X-rays from the same Si chip. For example, a single movement amount of the sample stage is set to 0.1 mm.

For the actual sample, repeat the steps of “moving the sample stage by 0.1 mm along the X axis” and “obtaining a rocking curve and obtaining ω _B ”, and from one edge of the sample to the other edge The results of obtaining ω _{B up} to are shown in FIGS. 14 (a), (b) and (c). The sample included four Si chips, and ω _B could be obtained for the upper three of them.

FIGS. 14A, 14B, and 14C show the dependence of ω _B on the distance X from the center of the chip. FIG. 15 is a graph showing a difference Δω _B (X) = ω _B (X) −ω _B (X = 0) when ω _B at the center of the chip (X = 0) is used as a reference (= 0).

As described above, since the change in ω _B (X) corresponds to the change in the angle of the Si chip surface due to warping, Δω _B (X) is the angle of the chip surface at the position X measured with respect to the chip surface where X = 0. Represents.

Therefore, when a schematic diagram of the cross section of the Si chip is written as shown in FIG. 16 and the curve representing the warpage of the substrate is expressed as z = f (X), the inclination of f (X) is just tan (−Δω _B (X)) = It is equal to tan (Δω _B (X)). The reason why the minus sign is attached is that Δω _B (X) is not the angle change itself of the substrate surface due to the warp but the amount obtained by adjusting the rotation angle ω of the sample stage so as to cancel it. From the above, a differential equation for obtaining f (X) is obtained as Equation 7 below using Δω _B (X) which is a measured value.

ｚ＝ｆ（Ｘ）はこれを積分することによって以下の数式８が得られる。

z = f (X) is integrated to obtain the following Expression 8.

  Therefore, if integration is performed using the result of FIG. 15 and Equation 8, f (X), that is, the warp of the Si chip can be obtained. The results obtained in this way are shown in FIG. The first to third Si chips from the surface side are warped as a whole, and the vicinity of the center of the chip is warped upward. This is considered to reflect the influence of stress due to the material around the chip, such as a mold material. The information about the chip warpage in the package that has been clarified in this way is useful knowledge for mounting process development, failure analysis, and reliability technology.

  Note that the sample contains four Si chips, and diffraction X-rays from the lowermost (fourth from the top) Si chip could not be detected, but high-energy X-rays were used. Thus, it becomes possible to detect diffracted X-rays from the fourth Si chip from the top.

  As mentioned above, although this invention was demonstrated according to one embodiment, this invention is not limited to the said embodiment, A various deformation | transformation and change are possible without deviating from the main point of this invention. .

11 X-ray source 12 Incident slit 13 Sample stage 14 Receiving slit 15 Detector 16 Single crystal substrate 17 Irradiation area 18 Incident X-ray 19 Wave vector 20 Diffraction X-ray 60 Sample 61 Resin substrate 62, 63, 64, 65 Si chip 66 Mold Material 71, 72 Diffracted X-ray

Claims (8)

A method for measuring warpage of a single crystal substrate, wherein warpage of each of a plurality of single crystal substrates stacked on each other and accommodated in a package is measured without destroying the package,
Irradiating the package with X-rays;
By adjusting the light receiving slit and adjusting the position of the package in the height direction, the diffracted X-rays from a desired single crystal substrate can be obtained from the plurality of diffracted X-rays diffracted by the plurality of single crystal substrates. Selectively detecting only, and
Measuring a rocking curve of diffracted X-rays from the desired single crystal substrate;
Moving the package a predetermined distance in a predetermined direction;
A method for measuring warpage of a single crystal substrate, comprising repeatedly performing the step of measuring the rocking curve and the step of moving the package in a predetermined direction by a predetermined distance.   2. The warpage measurement of a single crystal substrate according to claim 1, wherein the step of irradiating the package with X-rays is a step of irradiating the package with X-rays having a beam divergence angle of 0.1 milliradians or less. Method.   The method for measuring a warpage of a single crystal substrate according to claim 1, wherein the step of irradiating the package with X-rays is a step of irradiating the package with X-rays generated by synchrotron radiation.   4. The single irradiation according to claim 1, wherein the step of irradiating the package with X-rays is a step of irradiating the package with X-rays having a wavelength shorter than 0.16 nm. Method for measuring warpage of crystal substrate. A warp measuring device for a single crystal substrate that measures each warp of a plurality of single crystal substrates stacked on each other and accommodated in a package without destroying the package,
Means for irradiating the package with X-rays;
Detecting means for detecting diffracted X-rays from the package;
A light receiving slit for selectively allowing only the diffracted X-rays from the desired single crystal substrate out of the diffracted X-rays from the package;
Means for adjusting the height position of the package so that diffracted X-rays from the desired single crystal substrate pass through the light receiving slit;
Means for rotating the package about a predetermined axis of rotation to measure a rocking curve of diffracted x-rays from the desired single crystal substrate;
Means for moving the package a predetermined distance in a predetermined direction;
A warp measuring device for a single crystal substrate, comprising:   6. The warpage measurement of a single crystal substrate according to claim 5, wherein the means for irradiating the package with X-rays is means for irradiating the package with X-rays having a beam divergence angle of 0.1 milliradians or less. apparatus.   7. The single crystal substrate warpage measuring apparatus according to claim 5, wherein the means for irradiating the package with X-rays is means for irradiating the package with X-rays generated by synchrotron radiation.   The means for irradiating the package with X-rays is means for irradiating the package with X-rays having a wavelength shorter than 0.16 nm. Crystal substrate warpage measurement device.

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