JP2576338B2 - Automatic wafer weighing system - Google Patents
Automatic wafer weighing systemInfo
- Publication number
- JP2576338B2 JP2576338B2 JP4205225A JP20522592A JP2576338B2 JP 2576338 B2 JP2576338 B2 JP 2576338B2 JP 4205225 A JP4205225 A JP 4205225A JP 20522592 A JP20522592 A JP 20522592A JP 2576338 B2 JP2576338 B2 JP 2576338B2
- Authority
- JP
- Japan
- Prior art keywords
- wafer
- mass
- weighed
- electronic balance
- reference wafer
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
Landscapes
- Testing Or Measuring Of Semiconductors Or The Like (AREA)
- Length Measuring Devices With Unspecified Measuring Means (AREA)
Description
【発明の詳細な説明】DETAILED DESCRIPTION OF THE INVENTION
【０００１】[0001]
【産業上の利用分野】本発明はウエハの自動秤量システ
ムに関し、特に、生産ラインにおいて成膜前後のウエハ
の秤量結果に基づいてその膜厚を測定するような精密測
定に適した自動秤量システムに関する。BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic weighing system for a wafer, and more particularly, to an automatic weighing system suitable for precision measurement such as measuring the film thickness based on the weighing result of a wafer before and after film formation in a production line. .
【０００２】[0002]
【従来の技術】ウエハ表面に成膜された薄膜の厚さを測
定する方法として、従来、光学的方法によるもの、渦電
流を利用する方法、あるいは精密天びんを用いて成膜前
後にウエハの質量を測定して、その差から膜厚を算出す
る方法が知られている。2. Description of the Related Art Conventional methods for measuring the thickness of a thin film formed on a wafer surface include an optical method, a method using eddy current, and a method using a precision balance to measure the mass of a wafer before and after film formation. Is known, and the film thickness is calculated from the difference.
【０００３】[0003]
【発明が解決しようとする課題】以上の各方法のうち、
光学的方法は、膜の材質が透過光を利用できるものに限
定され、渦電流を用いる方法は高精度の測定ができない
という欠点がある。また、この両者の方法では、いずれ
も、局部的な膜厚を測定することから、凹凸のある膜の
平均膜厚を求める必要のある生産ラインには適していな
い。SUMMARY OF THE INVENTION Among the above methods,
The optical method is limited to a method in which the material of the film can use transmitted light, and the method using the eddy current has a disadvantage that high-precision measurement cannot be performed. Further, both of these methods measure a local film thickness, and thus are not suitable for a production line in which it is necessary to obtain an average film thickness of a film having irregularities.
【０００４】また、精密天びんを用いて成膜前後の質量
を測定する方法は、平均膜厚を求めるには適している
が、生産ライン上で用いる場合には測定の自動化が不可
欠であるとともに、極めて正確にウエハ質量を測定する
必要がある。A method of measuring the mass before and after film formation using a precision balance is suitable for obtaining an average film thickness, but when used on a production line, automation of measurement is indispensable. It is necessary to measure the wafer mass very accurately.
【０００５】ところで、高精度の天びんを用いて正確に
ウエハ質量を測定するためには、ある程度頻繁に感度校
正を行う必要がある。一方、測定の自動化の要求を満た
すためにはウエハの天びんへの載せ降ろし機構を設ける
必要があるばかりでなく、上記の感度校正のためには分
銅の加除機構を設ける必要があって、装置が大がかりな
ものとなるとともに、以下に示すような問題が生じる。Incidentally, in order to accurately measure the mass of a wafer using a high-precision balance, it is necessary to perform sensitivity calibration to some extent. On the other hand, in order to satisfy the demand for automation of measurement, it is necessary not only to provide a mechanism for loading and unloading wafers on the balance, but also to provide a mechanism for adding and removing weights for the above-mentioned sensitivity calibration. In addition to the large scale, the following problems occur.
【０００６】すなわち、通常の基準分銅の材料としてに
用いられるステンレスないしは真鍮の比重約８．０とウ
エハの比重約２．７との差が大きく、気圧や気温の変化
によって両者に浮力差が生じることになる。例えば質量
約２５ｇの６インチのウエハと、比重８．０の同質量の
分銅とは、空気中における浮力差は約８ｍｇあり、気温
が１０°Ｃ変化すると、この浮力差は約０．３ｍｇ変化
する。このため、分銅による校正では、成膜前後の質量
差を求めるような正確な測定ができない、という問題が
ある。That is, the difference between the specific gravity of stainless steel or brass, which is used as a material for a normal reference weight, is approximately 8.0 and the specific gravity of a wafer is approximately 2.7, and a change in air pressure or temperature causes a difference in buoyancy between the two. Will be. For example, a 6-inch wafer having a mass of about 25 g and a weight having the same mass having a specific gravity of 8.0 have a buoyancy difference in the air of about 8 mg. When the temperature changes by 10 ° C., the buoyancy difference changes by about 0.3 mg. I do. For this reason, there is a problem in that calibration using a weight cannot perform accurate measurement such as determining the difference in mass before and after film formation.
【０００７】本発明はこのような点に鑑みてなされたも
ので、生産ライン上において、ウエハの質量を正確に、
しかも自動的に測定することができ、成膜前後の質量差
からその膜厚を高精度に測定することのできるウエハ自
動秤量システムの提供を目的としている。[0007] The present invention has been made in view of such a point, and accurately measures the mass of a wafer on a production line.
In addition, it is an object of the present invention to provide an automatic wafer weighing system that can automatically measure the thickness and accurately measure the film thickness from the difference in mass before and after film formation.
【０００８】[0008]
【課題を解決するための手段】上記の目的を達成するた
め、本発明のウエハ自動秤量システムは、電子天びん機
構と、この電子天びん機構の秤量皿上にウエハを１枚ず
つ載せ降ろしするハンドリング機構と、質量既知の基準
ウエハを載置するための基準ウエハ載置部と、その基準
ウエハの質量値を記憶し、かつ、電子天びん機構および
ハンドリング機構を制御する制御部を有し、制御部から
の指令に基づくハンドリング機構の動作により、電子天
びん機構であらかじめ設定された枚数の被測定ウエハを
秤量するごとに、基準ウエハを秤量するよう構成されて
いるとともに、基準ウエハを秤量後、次回に基準ウエハ
を秤量するまでの間、その秤量結果とその質量値を用い
て被測定ウエハの秤量結果を補正するよう構成されてい
ることによって特徴づけられる。In order to achieve the above object, an automatic wafer weighing system according to the present invention comprises an electronic balance mechanism and a handling mechanism for loading and unloading wafers one by one on a weighing dish of the electronic balance mechanism. And a reference wafer mounting unit for mounting a reference wafer having a known mass, and storing a mass value of the reference wafer, and having a control unit for controlling an electronic balance mechanism and a handling mechanism, from the control unit The operation of the handling mechanism based on the command of the above is configured to weigh the reference wafer every time a predetermined number of wafers to be measured are weighed by the electronic balance mechanism, and the reference wafer is weighed next time after the reference wafer is weighed. Until the wafer is weighed, the weighing result and the mass value are used to correct the weighing result of the measured wafer. It is marked.
【０００９】[0009]
【作用】電子天びん機構の校正を、分銅を用いずに、被
測定ウエハと同等の質量既知のウエハを用いることによ
り、電子天びん機構に対してウエハを載せ降ろしするハ
ンドリング機構を、校正質量の加除機構に兼用させるこ
とができる。The electronic balance mechanism is calibrated by using a wafer having a known mass equivalent to the wafer to be measured without using a weight, and a handling mechanism for loading and unloading the wafer from the electronic balance mechanism is added to and subtracted from the calibration mass. It can be shared with the mechanism.
【００１０】また、基準ウエハの秤量結果およびその既
知質量から被測定ウエハの秤量結果を補正することで、
浮力の差を考慮する必要がなくなり、所期の目的を達成
できる。Further, by correcting the weighing result of the measured wafer from the weighing result of the reference wafer and its known mass,
There is no need to consider the difference in buoyancy, and the intended purpose can be achieved.
【００１１】[0011]
【実施例】図１は本発明実施例の平面図で、図２はその
Ａ−Ａ矢視図である。架台１上に電子天びん２と、この
電子天びん２の全体を収容し、かつ、扉３ａを備えた秤
量室３、ウエハハンドリング装置４、カセット載置台
５、およびパーソナルコンピュータ６が載せられてい
る。そして、秤量室３の天井面に基準ウエハ載置部７が
設けられている。1 is a plan view of an embodiment of the present invention, and FIG. 2 is a view taken along the line AA of FIG. An electronic balance 2, a weighing room 3 having a door 3 a, a weighing chamber 3, a wafer handling device 4, a cassette mounting table 5, and a personal computer 6 are mounted on a gantry 1. A reference wafer mounting unit 7 is provided on the ceiling surface of the weighing chamber 3.
【００１２】ウエハハンドリング装置４は水平方向およ
び鉛直方向に変位することができる公知のハドリングロ
ボットで、カセット載置台５上に置かれたカセット内の
被測定ウエハと、基準ウエハ載置部７上の基準ウエハＲ
のいずれかを、１枚ずつ把持して電子天びん２の秤量皿
２ａに対して載せ降ろしすることができる。The wafer handling device 4 is a known hadling robot that can be displaced in the horizontal direction and the vertical direction. The wafer handling device 4 has a wafer to be measured in a cassette placed on a cassette mounting table 5 and a reference wafer mounting portion 7. Reference wafer R
Can be gripped one by one and placed on the weighing dish 2a of the electronic balance 2 and dropped.
【００１３】秤量室３の扉３ａはシリンダ３ｂの駆動に
よって開閉され、このシリンダ３ｂとウエハハンドリン
グ装置４、および電子天びん２は、パーソナルコンピュ
ータ６からの指令によってシーケンシャルに制御される
ように構成されている。The door 3a of the weighing chamber 3 is opened and closed by driving a cylinder 3b. The cylinder 3b, the wafer handling device 4, and the electronic balance 2 are configured to be sequentially controlled by a command from a personal computer 6. I have.
【００１４】パーソナルコンピュータ６は、あらかじめ
基準ウエハＲの質量値ＷR を記憶しているとともに、電
子天びん２による被測定ウエハの秤量結果は順次このパ
ーソナルコンピュータ６内に記憶されていくが、この秤
量結果は以下に示すように基準ウエハＲの質量値とその
秤量結果によって補正される。[0014] personal computer 6, with stores mass value W R in advance reference wafer R, but weighing result of measurement wafer by electronic balance 2 is gradually stored sequentially in the personal computer 6, the weighing The result is corrected based on the mass value of the reference wafer R and the weighing result as described below.
【００１５】図３はパーソナルコンピュータ６のプログ
ラムに基づく各部の動作手順を示すフローチャートで、
以下、この図を参照しつつ本発明実施例の作用を述べ
る。スタート指令を与えると、秤量室３の扉３ａが開か
れた後、ハンドリング機構４によって基準ウエハＲが取
り出されて電子天びん２の秤量皿２ａ上に載せられる。
その状態で扉３ａが閉じられて基準ウエハＲの質量が測
定され、その測定結果Ｗ1 がパーソナルコンピュータ６
に記憶される。その後、扉３ａが開かれ、ハンドリング
装置４によって基準ウエハＲが基準ウエハ載置部７上に
戻される。FIG. 3 is a flowchart showing the operation procedure of each unit based on the program of the personal computer 6.
The operation of the embodiment of the present invention will be described below with reference to FIG. When a start command is given, the reference wafer R is taken out by the handling mechanism 4 and placed on the weighing dish 2a of the electronic balance 2 after the door 3a of the weighing chamber 3 is opened.
Its mass of reference wafer R door 3a is closed in the state is measured, the measurement result W 1 is a personal computer 6
Is stored. Thereafter, the door 3a is opened, and the reference wafer R is returned onto the reference wafer mounting unit 7 by the handling device 4.
【００１６】次に、カセット載置台５上に置かれたカセ
ット内のウエハがハンドリング装置４によって１枚ずつ
取り出され、電子天びん２の秤量皿２ａ上に載せられ
る。その状態で扉３ａが閉じられてそのウエハの質量が
測定され、その測定結果Ｗ2 がパーソナルコンピュータ
６に転送される。パーソナルコンピュータ６では、その
測定結果Ｗ2 を、基準ウエハＲの測定結果Ｗ1 およびそ
の質量値ＷR を用いて、 Ｗ＝Ｗ2 ×ＷR ／Ｗ1 によって補正し、その補正後の値Ｗをそのウエハの質量
として記憶する。Next, the wafers in the cassette placed on the cassette mounting table 5 are taken out one by one by the handling device 4 and placed on the weighing dish 2a of the electronic balance 2. And the door 3a is closed in this state is measured mass of the wafer, the measurement result W 2 is transferred to the personal computer 6. The personal computer 6 corrects the measurement result W 2 by using the measurement result W 1 of the reference wafer R and the mass value W R by W = W 2 × W R / W 1 , and the corrected value W Is stored as the mass of the wafer.
【００１７】このようにしてカセット内のウエハをあら
かじめ設定された枚数だけ秤量すると、前記と同様にし
て基準ウエハＲの質量が測定され、基準ウエハＲの測定
結果Ｗ1 が更新され、以下、同様にして被測定ウエハの
測定と、その測定結果の補正演算およびその補正後の値
の記憶が実行されていく。[0017] In this way weighed number of sheets set in advance the wafers in the cassette, the mass of the reference wafer R in the same manner as the are measured, the measurement result W 1 of the reference wafer R is updated, and the same Then, the measurement of the wafer to be measured, the correction calculation of the measurement result, and the storage of the corrected value are executed.
【００１８】以上の動作により、電子天びん２の感度が
多少ずれても、基準ウエハＲの質量測定結果およびその
真の質量に基づく補正演算により、各ウエハの質量測定
結果には電子天びん２の感度変化の影響が及ばない。ま
た、基準ウエハＲの質量測定による補正により、分銅を
用いた感度構成や補正を行う場合に比して浮力差の影響
が及ばず、従ってパーソナルコンピュータ６には常に正
確なウエハ質量が格納されていくことになる。With the above operation, even if the sensitivity of the electronic balance 2 is slightly deviated, the sensitivity measurement of the electronic balance 2 is performed on the mass measurement result of each wafer by the correction operation based on the mass measurement result of the reference wafer R and its true mass. The effects of change are not significant. Further, the correction by the mass measurement of the reference wafer R does not affect the buoyancy difference as compared with the case where the sensitivity configuration and the correction using the weight are performed, so that the personal computer 6 always stores the accurate wafer mass. Will go.
【００１９】以上のような測定動作を、ウエハに対する
成膜の前後において実行し、両者の差を算出することに
より、成膜による質量増加量、ひいてはウエハ上の平均
膜厚が高精度に求められる。The above-described measuring operation is performed before and after the film formation on the wafer, and the difference between the two is calculated, whereby the amount of increase in mass due to the film formation and, consequently, the average film thickness on the wafer can be obtained with high accuracy. .
【００２０】なお、以上の実施例では、何枚かの被測定
ウエハの秤量ごとに基準ウエハＲを秤量するようにして
いるが、一枚ごとに基準ウエハＲの秤量を行うようにし
てもよい。In the above embodiment, the reference wafer R is weighed for every weighed number of wafers to be measured. However, the reference wafer R may be weighed for each wafer. .
【００２１】また、以上の実施例では、基準ウエハＲの
既知質量値と各回の質量測定結果を用いてパーソナルコ
ンピュータ６において被測定ウエハの質量測定結果を補
正したが、基準ウエハＲの既知質量と質量測定結果を用
いて、電子天びん２の感度校正を行うようにしてもよ
い。この場合、上記した補正演算は実質的に電子天びん
２内で行われることになり、パーソナルコンピュータ６
における補正演算は不要となる。In the above embodiment, the mass measurement result of the wafer to be measured is corrected in the personal computer 6 by using the known mass value of the reference wafer R and the result of each mass measurement. The sensitivity calibration of the electronic balance 2 may be performed using the mass measurement result. In this case, the above-described correction operation is substantially performed in the electronic balance 2, and the personal computer 6
Does not require the correction calculation.
【００２２】更に、基準ウエハＲの質量測定結果が、そ
の既知質量値に対してある限度を越えて相違している場
合には、システムの異常として警報等を発するように構
成してもよい。Further, when the mass measurement result of the reference wafer R is different from its known mass value by a certain limit, an alarm or the like may be issued as a system abnormality.
【００２３】すなわち、この場合、基準ウエハＲを秤量
するごとに、その秤量結果と既知質量との相違が、あら
かじめ設定された許容範囲内にあるか否かを判別し、許
容範囲内である場合には、前述した実施例と同様に、被
測定ウエハの秤量を行ってその秤量結果を同様にして補
正するとともに、許容範囲を越えている場合には、以下
の被測定ウエハの秤量並びにその補正を行わずに、警報
出力を発するように構成する。これにより、基準ウエハ
Ｒの秤量時における動作の異常や、電子天びん自体の異
常等があっても、誤った測定結果が蓄積されることがな
くなり、システムの信頼性が向上する。That is, in this case, every time the reference wafer R is weighed, it is determined whether or not the difference between the weighed result and the known mass is within a preset allowable range. In the same manner as in the above-described embodiment, the measurement target wafer is weighed, and the weighing result is corrected in the same manner. When the measurement result exceeds the allowable range, the following measurement target wafer weighing and its correction are performed. , And an alarm output is issued. Accordingly, even if there is an abnormality in the operation of weighing the reference wafer R or an abnormality in the electronic balance itself, an erroneous measurement result is not accumulated, and the reliability of the system is improved.
【００２４】[0024]
【発明の効果】以上説明したように、本発明によれば、
測定すべきウエハと同等の基準ウエハを基準質量として
用い、この基準ウエハを被測定ウエハ群の測定の間に逐
次秤量して、各被測定ウエハの秤量結果を補正するの
で、電子天びんに対して被測定ウエハを載せ降ろしする
ハンドリング機構をそのまま用いて基準質量の載せ降ろ
しを行うことができ、装置を大型化することなく自動的
にウエハの質量測定を行うことができ、特に生産ライン
で用いるのに適したウエハ秤量システムとなる。As described above, according to the present invention,
A reference wafer equivalent to the wafer to be measured is used as a reference mass, and the reference wafer is sequentially weighed during measurement of the group of wafers to be measured, and the weighing result of each wafer to be measured is corrected. The handling mechanism for loading and unloading the wafer to be measured can be used as it is to perform loading and unloading of the reference mass, and the mass of the wafer can be automatically measured without increasing the size of the apparatus. It becomes a wafer weighing system suitable for.
【００２５】また、基準分銅に代えて基準ウエハを用い
た補正を行うことから、基準質量体と被測定物との浮力
差が実質的に０となり、温度変化に応じて変化する空気
に対する浮力差による誤差の極めて少ない高精度の質量
測定を行うことができ、成膜前後のウエハの平均膜厚を
正確に求めることが可能となった。Further, since the correction using the reference wafer in place of the reference weight is performed, the buoyancy difference between the reference mass body and the object to be measured becomes substantially zero, and the buoyancy difference with respect to air which changes according to the temperature change. This makes it possible to perform high-precision mass measurement with extremely few errors due to the above, and to accurately obtain the average film thickness of the wafer before and after film formation.
【図１】本発明実施例の平面図FIG. 1 is a plan view of an embodiment of the present invention.
【図２】そのＡ−Ａ矢視図FIG. 2 is a view taken in the direction of arrows AA.
【図３】本発明実施例のパーソナルコンピュータ６のプ
ログラムに基づく各部の動作手順を示すフローチャートFIG. 3 is a flowchart showing an operation procedure of each unit based on a program of the personal computer 6 according to the embodiment of the present invention.
１ 架台 ２ 電子天びん ２ａ 秤量皿 ３ 秤量室 ３ａ 扉 ４ ウエハハンドリング装置 ５ カセット載置台 ６ パーソナルコンピュータ ７ 基準ウエハ載置部 REFERENCE SIGNS LIST 1 base 2 electronic balance 2a weighing dish 3 weighing chamber 3a door 4 wafer handling device 5 cassette mounting table 6 personal computer 7 reference wafer mounting section
───────────────────────────────────────────────────── フロントページの続き (72)発明者 河本 晟 京都府京都市中京区西ノ京桑原町１番地 株式会社島津製作所三条工場内 (56)参考文献 特開 平６−50802（ＪＰ，Ａ) 特開 平５−149741（ＪＰ，Ａ) 特開 平４−2925（ＪＰ，Ａ) 特開 昭49−18346（ＪＰ，Ａ) 実開 昭62−115613（ＪＰ，Ｕ) 実開 昭60−53028（ＪＰ，Ｕ) ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Kawamoto Akira 1 Nishinokyo Kuwabara-cho, Nakagyo-ku, Kyoto City, Kyoto Prefecture Inside the Sanjo Plant of Shimadzu Corporation (56) References JP-A-6-50802 (JP, A) JP-A Heihei 5-149741 (JP, A) JP-A-4-2925 (JP, A) JP-A-49-18346 (JP, A) JP-A 62-115613 (JP, U) JP-A 60-53028 (JP, A) U)
Claims (1)
の秤量皿上にウエハを１枚ずつ載せ降ろしするハンドリ
ング機構と、質量既知の基準ウエハを載置するための基
準ウエハ載置部と、その基準ウエハの質量値を記憶し、
かつ、上記電子天びん機構およびハンドリング機構を制
御する制御部を有し、上記制御部からの指令に基づく上
記ハンドリング機構の動作により、上記電子天びん機構
であらかじめ設定された枚数の被測定ウエハを秤量する
ごとに、上記基準ウエハを秤量するよう構成されている
とともに、上記基準ウエハを秤量後、次回に基準ウエハ
を秤量するまでの間、その秤量結果とその質量値を用い
て被測定ウエハの秤量結果を補正するよう構成されてい
ることを特徴とするウエハ自動秤量システム。An electronic balance mechanism, a handling mechanism for loading and unloading wafers one by one on a weighing dish of the electronic balance mechanism, a reference wafer mounting portion for mounting a reference wafer having a known mass, and Store the mass value of the reference wafer,
And a controller for controlling the electronic balance mechanism and the handling mechanism. The operation of the handling mechanism based on a command from the control section weighs a predetermined number of wafers to be measured by the electronic balance mechanism. Each time, the reference wafer is configured to be weighed, and after the reference wafer is weighed, until the next reference wafer is weighed, the weighed result of the measured wafer is measured using the weighed result and its mass value. An automatic wafer weighing system, wherein the automatic weighing system is configured to correct the following.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP4205225A JP2576338B2 (en) | 1992-07-31 | 1992-07-31 | Automatic wafer weighing system |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP4205225A JP2576338B2 (en) | 1992-07-31 | 1992-07-31 | Automatic wafer weighing system |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH0650803A JPH0650803A (en) | 1994-02-25 |
| JP2576338B2 true JP2576338B2 (en) | 1997-01-29 |
Family
ID=16503486
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP4205225A Expired - Fee Related JP2576338B2 (en) | 1992-07-31 | 1992-07-31 | Automatic wafer weighing system |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JP2576338B2 (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5924058A (en) * | 1997-02-14 | 1999-07-13 | Applied Materials, Inc. | Permanently mounted reference sample for a substrate measurement tool |
| GB0719460D0 (en) | 2007-10-04 | 2007-11-14 | Metryx Ltd | Measurement apparatus and method |
| GB0804499D0 (en) * | 2008-03-11 | 2008-04-16 | Metryx Ltd | Measurement apparatus and method |
| GB201315715D0 (en) * | 2013-09-04 | 2013-10-16 | Metryx Ltd | Method and device for determining information relating to the mass of a semiconductor wafer |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2647585B2 (en) * | 1991-11-28 | 1997-08-27 | 三菱電機株式会社 | Automatic thin film measuring device |
-
1992
- 1992-07-31 JP JP4205225A patent/JP2576338B2/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| JPH0650803A (en) | 1994-02-25 |
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