KR100236662B1 - Photoetching method of an ingap layer - Google Patents
Photoetching method of an ingap layer Download PDFInfo
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- KR100236662B1 KR100236662B1 KR1019970034251A KR19970034251A KR100236662B1 KR 100236662 B1 KR100236662 B1 KR 100236662B1 KR 1019970034251 A KR1019970034251 A KR 1019970034251A KR 19970034251 A KR19970034251 A KR 19970034251A KR 100236662 B1 KR100236662 B1 KR 100236662B1
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- 238000000034 method Methods 0.000 title claims abstract description 18
- 238000001259 photo etching Methods 0.000 title 1
- 238000005530 etching Methods 0.000 claims abstract description 53
- 229910052751 metal Inorganic materials 0.000 claims abstract description 28
- 239000002184 metal Substances 0.000 claims abstract description 28
- 230000001678 irradiating effect Effects 0.000 claims abstract 2
- VEXZGXHMUGYJMC-UHFFFAOYSA-N Hydrochloric acid Chemical compound Cl VEXZGXHMUGYJMC-UHFFFAOYSA-N 0.000 claims description 22
- 229910001218 Gallium arsenide Inorganic materials 0.000 claims description 13
- 238000004904 shortening Methods 0.000 abstract 1
- 239000000758 substrate Substances 0.000 description 9
- 239000010408 film Substances 0.000 description 8
- 239000000243 solution Substances 0.000 description 8
- NBIIXXVUZAFLBC-UHFFFAOYSA-N Phosphoric acid Chemical compound OP(O)(O)=O NBIIXXVUZAFLBC-UHFFFAOYSA-N 0.000 description 6
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 6
- 239000004065 semiconductor Substances 0.000 description 6
- 238000004519 manufacturing process Methods 0.000 description 5
- 239000007864 aqueous solution Substances 0.000 description 4
- 229910000147 aluminium phosphate Inorganic materials 0.000 description 3
- 239000010931 gold Substances 0.000 description 3
- QSHDDOUJBYECFT-UHFFFAOYSA-N mercury Chemical compound [Hg] QSHDDOUJBYECFT-UHFFFAOYSA-N 0.000 description 3
- 229910052753 mercury Inorganic materials 0.000 description 3
- 239000010409 thin film Substances 0.000 description 3
- 230000000052 comparative effect Effects 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
- 229910052737 gold Inorganic materials 0.000 description 2
- 239000007788 liquid Substances 0.000 description 2
- 229920002120 photoresistant polymer Polymers 0.000 description 2
- 238000001878 scanning electron micrograph Methods 0.000 description 2
- 229910000980 Aluminium gallium arsenide Inorganic materials 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 150000007513 acids Chemical class 0.000 description 1
- 239000000654 additive Substances 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 238000003475 lamination Methods 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 150000002739 metals Chemical class 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 229910052759 nickel Inorganic materials 0.000 description 1
- 238000000059 patterning Methods 0.000 description 1
- 238000000206 photolithography Methods 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 229910052719 titanium Inorganic materials 0.000 description 1
- 229910052721 tungsten Inorganic materials 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
- 229910052724 xenon Inorganic materials 0.000 description 1
- FHNFHKCVQCLJFQ-UHFFFAOYSA-N xenon atom Chemical compound [Xe] FHNFHKCVQCLJFQ-UHFFFAOYSA-N 0.000 description 1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
- H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
- H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
- H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
- H01L21/302—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to change their surface-physical characteristics or shape, e.g. etching, polishing, cutting
- H01L21/306—Chemical or electrical treatment, e.g. electrolytic etching
- H01L21/30604—Chemical etching
- H01L21/30612—Etching of AIIIBV compounds
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
- H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
- H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
- H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
- H01L21/302—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to change their surface-physical characteristics or shape, e.g. etching, polishing, cutting
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- Condensed Matter Physics & Semiconductors (AREA)
- General Physics & Mathematics (AREA)
- Manufacturing & Machinery (AREA)
- Computer Hardware Design (AREA)
- Physics & Mathematics (AREA)
- Power Engineering (AREA)
- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- General Chemical & Material Sciences (AREA)
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- Junction Field-Effect Transistors (AREA)
Abstract
과제 : 본 발명은 InGaP층을 식각에 의하여 가공하여 반도체 장치를 제조하는 때에 금속마스크를 사용하여도 식각 시간의 단축이 가능하고 가공 정도(加功精度)의 우수한 식각 방법을 제공하는 것을 목적으로 한다.DISCLOSURE OF THE INVENTION The present invention aims to provide an etching method capable of shortening the etching time and processing accuracy by using a metal mask when fabricating an InGaP layer by etching. .
해결수단 : 표면에 마스크로서 금속막을 형성한 InGaP층을 식각액중에 침지하고 동시에 광조사하는 것에 따라 상기 목적이 달성된다.Solution: The above object is achieved by immersing an InGaP layer having a metal film as a mask on its surface in an etchant and simultaneously irradiating with light.
Description
제1도는 본 발명의 식각(食刻)방법을 나타내는 도면이다.1 is a view showing an etching method of the present invention.
제2도는 종래의 식각방법에 의한 식각형상을 나타내는 도면이다.2 is a view showing an etching shape by a conventional etching method.
제3도는 본 발명에 의한 식각형상을 나타내는 도면이다.3 is a view showing an etching shape according to the present invention.
제4도는 본 발명에 의한 식각형의 SEM 사진의 모식도이다.4 is a schematic diagram of an SEM image of an etching according to the present invention.
제5도는 종래의 식각방법에 의한 식각형상의 SEM 사진의 모식도이다.5 is a schematic diagram of an SEM image of an etched shape by a conventional etching method.
* 도면의 주요부분에 대한 부호의 설명* Explanation of symbols for main parts of the drawings
1 : 금속마스크 22 : 하지(下地)의 p-GaAs층Reference Signs List 1: metal mask 22: p-GaAs layer of base
3 : n-InGaP층의 표면이 평탄한 부분 4 : n-InGaP층의 사면부분3: portion where the surface of the n-InGaP layer is flat 4: portion of the slope of the n-InGaP layer
[발명이 속하는 기술분야][TECHNICAL FIELD OF THE INVENTION]
본 발명은 반도체 장치의 제조방법에 관하여 특히 InGaP층의 식각방법에 관한 것이다.The present invention relates to a method of manufacturing a semiconductor device, in particular to an etching method of an InGaP layer.
[종래의 기술][Prior art]
InGaP는, AlGaAs에 대체되는 재료로서 GaAs계의 반도체 장치로의 응용이 넓게 연구되고 있다. GaAs기판 또는 GaAs층상에 형성된 InGaP층을 소정의 형상으로 가공하려면 통상 식각(etching)법에 따라 형성한다. 식각액으로서는 GaAs기판과의 선택성이 좋은 것으로서 특히 염산계의 식각액이 많이 사용되고 있다.InGaP has been widely studied for application to a GaAs-based semiconductor device as a material to replace AlGaAs. InGaP layers formed on GaAs substrates or GaAs layers are usually formed by etching in order to process them into a predetermined shape. As an etchant, a good selectivity with a GaAs substrate is used. Especially, an etchant of hydrochloric acid is used.
여기서 식각의 마스크로서 금속막을 사용하면 이 금속막을 데바이스의 전극으로서 사용할 수 있으므로 포톨리소그래피 공정을 줄일수 있는 이점이 있다. 그러나 마스크로서 금속막을 사용한 경우, 금속막의 주변에서 식각을 저해하는 반응이 일어나 금속막 주변에 에칭잔차(殘差)가 아이랜드상으로 남어서 가공 정도가 충분히 얻을 수 없는 문제가 있었다.In this case, when a metal film is used as the mask for etching, the metal film can be used as an electrode of the device, thereby reducing the photolithography process. However, when a metal film is used as a mask, a reaction that inhibits etching occurs around the metal film, and etching residues remain around the metal film, resulting in a problem that the degree of processing cannot be sufficiently obtained.
[발명이 해결코저 하는 과제][Problem Solving Invention]
본 발명은 InGaP층을 식각에 의하여 가공하여 반도체 장치를 제조하는때 금속 마스크를 사용하여도 식각 시간의 단축이 가능하고, 가공 정도의 우수한 식각 방법을 제공하는 것을 목적으로 하고 있다.An object of the present invention is to provide an etching method that can reduce the etching time even when using a metal mask when manufacturing a semiconductor device by processing an InGaP layer by etching, and having an excellent processing degree.
[과제를 해결하기 위한 수단][Means for solving the problem]
본 발명은 표면에 마스크로서 금속막을 형성한 InGaP층을 식각액중에 침지하고 동시에 광조사하는 것을 특징으로 하는 InGaP층의 식각 방법에 관한 것이다.The present invention relates to an etching method of an InGaP layer, wherein the InGaP layer having a metal film formed thereon as a mask is immersed in an etching solution and simultaneously irradiated with light.
그 식각액으로서 염산계 식각액을 사용하면 제어성이 좋고 InGaP를 식각될수 있으므로 특히 좋다.The use of a hydrochloric acid-based etching solution as the etching solution is particularly advantageous because it provides good controllability and allows InGaP to be etched.
전기 InGaP층이란 InGaP층만이 아니고 InGaP의 기판도 포함하는 것이다. InGaP의 층을 GaAs기판 또는 층상에 형성하고, InGaP층을 소정의 패턴으로 가공하는 경우에 있어서도 상기 염산계 식각액을 사용하는 것에 따라 InGaP층을 선택적으로 식각할 수가 있다.The electric InGaP layer includes not only an InGaP layer but also an InGaP substrate. Even when the InGaP layer is formed on a GaAs substrate or layer, and the InGaP layer is processed into a predetermined pattern, the InGaP layer can be selectively etched by using the hydrochloric acid-based etching solution.
[발명의 실시형태]Embodiment of the Invention
본 발명의 식각방법으로서는 예를들면 제1도와 같이 InGaP층과 이 위에 형성한 금속마스크를 갖는 시료를 식각액의 속에 적시면서 수은등으로 빛을 조사한다.As an etching method of the present invention, for example, as shown in FIG. 1, a sample having an InGaP layer and a metal mask formed thereon is irradiated with mercury lamp while wetted in an etching solution.
제2도는 종래의 방법에 의해 GaAs층상의 InGaP층을 금속마스크를 사용하여 식각하였을 때의 형상을 모식적으로 나타낸 것이다. 즉 금속마스크 근방의 InGaP층의 식각이 저해되고 금속마스크에서 떨어진 부분과 비교하여 식각속도가 극히 늦어지는 결과, 패턴 형상이 마스크 패턴보다 넓혀지는 형상을 보인다.FIG. 2 schematically shows the shape when the InGaP layer on the GaAs layer is etched using a metal mask by a conventional method. That is, the etching of the InGaP layer near the metal mask is inhibited and the etching speed is extremely slow compared to the part away from the metal mask. As a result, the pattern shape is wider than the mask pattern.
본 발명자는 종래의 식각방법에서는 금속막 및 InGaP와 식각액이 접촉하였을 때 발생하는 정(正)의 전하가 반도체층의 표면에 축적되고 이것이 식각을 방해하는 것으로 추정하고 이 정전하를 중화하는 방법을 검토한 결과 본 발명에 이른 것이다. 즉 본 발명에서 식각할 때에 광조사하면 InGaP의 표면에 부(負)의 전하가 예기(
본 발명에 사용되는 염산계 식각액이란 InGaP의 식각에 사용되는 염산을 포함하는 수용액으로서 염산수용액 이외에, 예를들면 염산과 인산의 수용액, 염산과 황산의 수용액, 염산과 인산과 황산의 수용액등과 같이 식각에 악영향을 주지 않는 기타의 산을 포함하여도 좋고, 또한 식각을 개선하기 위한 기타의 첨가제를 포함하여도 좋다. 이들 식각액은 예를들면 InGaP층의 하지로 되는 기판 또는 층, 사용하는 금속마스크, 제조공정 등을 고려하여 적의 선택된다.The hydrochloric acid-based etching solution used in the present invention is an aqueous solution containing hydrochloric acid used for etching InGaP. In addition to an aqueous hydrochloric acid solution, for example, an aqueous solution of hydrochloric acid and phosphoric acid, an aqueous solution of hydrochloric acid and sulfuric acid, an aqueous solution of hydrochloric acid, phosphoric acid and sulfuric acid, etc. Other acids which do not adversely affect the etching may be included, and other additives for improving the etching may be included. These etchant is appropriately selected in consideration of, for example, a substrate or layer serving as the base of the InGaP layer, a metal mask to be used, a manufacturing process, and the like.
본 발명의 금속막으로서는 식각액에 대하여 전혀 침해되지 않든지, InGaP에 비하여 식각속도가 충분히 늦은 것으로서, 용이하게 가공될 수 있는 것이 사용된다. 이와같은 것으로는 예를들면 Au, Ti, W, Ni 및 AuGe 등의 금속의 박막(薄膜)및 이들 금속의 박막을 적층(積層)한 것을 둘 수가 있다.As the metal film of the present invention, one which does not intrude into the etching solution at all, or which has a sufficiently slow etching speed as compared with InGaP, is used. As such a thing, the thin film of metals, such as Au, Ti, W, Ni, and AuGe, and the lamination | stacking of these metal thin films can be used, for example.
이들 금속마스크 패턴의 형성방법은 특별한 제한은 없고, 예를들면 포토레지스트등을 사용하는 리프트오프(lift-off)에 따라 형성할 수가 있다.The formation method of these metal mask patterns does not have a restriction | limiting in particular, For example, it can form by the lift-off using a photoresist etc., for example.
본 발명에서 조사하는 빛은 InGaP의 밴드 캡이상의 에너지(650㎚이하의 파장)을 갖는 빛이라면 사용할 수가 있으나, 특히 자외선을 사용하는 것이 좋다. 광원으로서는 특히 제한은 없고, 수은 등의 이외에 크세논램프, 레저 등을 사용할 수도 있다.The light irradiated in the present invention can be used as long as the light has an energy of more than the band cap of InGaP (wavelength of 650 nm or less). There is no restriction | limiting in particular as a light source, Xenon lamp, leisure, etc. can also be used other than mercury.
또한 빛의 조사 강도는 광조사에 의하여 예기되는 전자수가 식각에 의하여 축적되는 정부하를 지워 없애는데 필요한 양 이상이 되게끔 설정한다. 상기의 광원의 일반적인 것은 통상은 충분한 광강도를 갖고 있으나, 필요에 따라서 다시 강력한 광원을 사용하여도 좋다.In addition, the irradiation intensity of the light is set so that the number of electrons expected by the irradiation of light is more than the amount necessary to erase the stationary load accumulated by etching. In general, the light source generally has sufficient light intensity, but a strong light source may be used again as necessary.
또한 식각할 때의 온도, 시간등은 식각액의 조성등을 고려하여, 식각 형상이 최적하게 되게끔 설정할 수가 있다.In addition, the temperature, time, etc. at the time of etching can be set so that an etching shape may be optimized considering the composition of an etching liquid.
또한 본 발명은 InGaP의기판 또는 InGaP층의 어디에도 적용이 가능하다. 특히 GaAs의 기판 또는 층상에 형성된 InGaP층의 패터닝에 호적하게 사용된다. 즉 본 발명은 예를들면 HBT(헤테로접합바이폴라 트란지스터), HEMT(High Electron Mobility Transistor)등의 반도체장치의 제조에 사용하면, 프로세스 시간과 원료에 대한 제품의 비율의 개선을 도모할 수가 있다.In addition, the present invention can be applied to either InGaP substrate or InGaP layer. In particular, it is used suitably for the patterning of the InGaP layer formed on the board | substrate or layer of GaAs. That is, the present invention can improve the process time and the ratio of the product to the raw material when used in the manufacture of semiconductor devices such as HBT (heterojunction bipolar transistor) and HEMT (High Electron Mobility Transistor).
이하에 실시예를 나타내고 본 발명은 다시 상세히 설명한다.An Example is shown to the following and this invention is demonstrated to it in detail again.
[실시예 1]Example 1
우선 기판 GaAs의 표면에 100㎚ p-GaAs층과 250㎚의 n-InGaP층을 적층하였다.First, a 100 nm p-GaAs layer and a 250 nm n-InGaP layer were laminated on the surface of the substrate GaAs.
이 n-InGaP층의 표면에 포토레지스트를 도포하고, 로광(露光), 현상하여 금속마스크를 형성하는 위치에 레지스트의 개구를 설정하였다. 이어서 50㎚의 Au(금)와 20㎚의 Ti의 박막을 적층한 후 리프트오프에 의하여 소정의 패턴상으로 금속마스크를 가공하였다.The photoresist was apply | coated to the surface of this n-InGaP layer, and the opening of a resist was set in the position which furnace light develops and forms a metal mask. Subsequently, 50 nm of Au (gold) and 20 nm of Ti thin films were laminated, and a metal mask was processed into a predetermined pattern by lift-off.
이 시료를 식각액[염산 : 인산 : 황산 : 물=1 : 1 : 1 : 1(R.T.)]에 담그고, 동시에 100W의 수은등을 사용하여 약 15㎝의 거리에서 자외선을 조사하였다. 그리고 식각을 2분동안 계속하였다.The sample was immersed in an etchant [hydrochloric acid: phosphoric acid: sulfuric acid: water = 1: 1: 1: 1: 1 (R.T.)] And irradiated with ultraviolet rays at a distance of about 15 cm using a 100 W mercury lamp. The etching was continued for 2 minutes.
식각후 이 기판표면을 SEM(주사형전자현미경)으로 식각현상을 관찰하였더니 제4도에 모식적으로 나타낸 것과 같은 현상으로 되어 있었다. 즉, 금속마스크(I)이외의 표면은 p-GaAs층(2)이 노출되어 있고, 마스크형상을 반영하여 n-InGaP층이 식각되고 있는 것을 알았다.After etching, the surface of the substrate was etched by SEM (Scanning Electron Microscope), and it was as shown in FIG. That is, it was found that the p-
[비교예 1]Comparative Example 1
실시예 1에 대하여 자외선 조사를 하지 않은 이외는 실시예 1를 반복하였다.Example 1 was repeated except that the ultraviolet irradiation was not performed with respect to Example 1.
식각 현상은 제5도에 모식적으로 나타낸 것처럼 금속마스크에 인접하여 n-InGaP층의 표면이 평탄한 부분(3)이 존재하고 약간 떨어져서 n-InGaP층의 사면부분(4)이 존재하고 금속마스크로 부터 1~2㎛ 떨어진 곳으로부터 p-GaAs층(2)이 노출되어 있다. 이 비교예에서 자외선 조사가 없는 경우에는 금속마스크의 주위는 에칭이 저해되고 식각 상태가 나빠지는 것을 알았다.The etching phenomenon is as shown in FIG. 5, where the surface of the n-InGaP layer has a
[발명의 효과][Effects of the Invention]
본 발명에 의한 InGaP를 식각에 의하여 가공하여 반도체 장치를 제조하는 때에 금속마스크를 사용하여도 식각 시각이 짧고, 또한 가공 정도의 우수한 식각 방법을 제공할 수가 있다.When manufacturing a semiconductor device by processing the InGaP according to the present invention by etching, even if a metal mask is used, the etching time is short and the etching method excellent in the degree of processing can be provided.
Claims (3)
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP8193338A JPH1041285A (en) | 1996-07-23 | 1996-07-23 | Etching method of indium-gallium-phosphorus layer |
| JP1996-193338 | 1996-07-23 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| KR980011975A KR980011975A (en) | 1998-04-30 |
| KR100236662B1 true KR100236662B1 (en) | 2000-01-15 |
Family
ID=16306240
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| KR1019970034251A KR100236662B1 (en) | 1996-07-23 | 1997-07-22 | Photoetching method of an ingap layer |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPH1041285A (en) |
| KR (1) | KR100236662B1 (en) |
| FR (1) | FR2751788B1 (en) |
| GB (1) | GB2315597B (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP5228298B2 (en) * | 2006-08-04 | 2013-07-03 | カシオ計算機株式会社 | Semiconductor thin film processing method and semiconductor device manufacturing method |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4414066A (en) * | 1982-09-10 | 1983-11-08 | Bell Telephone Laboratories, Incorporated | Electrochemical photoetching of compound semiconductors |
| US5270245A (en) * | 1992-11-27 | 1993-12-14 | Motorola, Inc. | Method of forming a light emitting diode |
-
1996
- 1996-07-23 JP JP8193338A patent/JPH1041285A/en not_active Withdrawn
-
1997
- 1997-07-21 GB GB9715325A patent/GB2315597B/en not_active Expired - Fee Related
- 1997-07-21 FR FR9709226A patent/FR2751788B1/en not_active Expired - Fee Related
- 1997-07-22 KR KR1019970034251A patent/KR100236662B1/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| GB2315597A (en) | 1998-02-04 |
| FR2751788A1 (en) | 1998-01-30 |
| JPH1041285A (en) | 1998-02-13 |
| GB2315597B (en) | 1999-02-24 |
| KR980011975A (en) | 1998-04-30 |
| GB9715325D0 (en) | 1997-09-24 |
| FR2751788B1 (en) | 2000-07-07 |
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