Friday, 26 June 2015

T 2255/10 - What's the purpose of it all?


This case involves an appeal against the decision of the Examining Division refusing European patent application No. 01 998 993 on the grounds that the claimed subject-matter of the main request and of the first and second auxiliary requests was not clear (Article 84 EPC) and did not involve an inventive step (Articles 52(1) and 56 EPC).

Catchwords

In accordance with the established case law of the Boards of Appeal the closest prior art for assessing inventive step is normally a prior art document disclosing subject-matter conceived for the same purpose as the claimed invention and having the most relevant technical features in common. (Reasons, point 2.2.2, citing T 482/92, Reasons, point 4.1, third paragraph.)
In establishing the closest prior art, the determination of the purpose of the invention is not to be made on the basis of a subjective selection from among statements of purpose which may be set out in the description of the application, without any reference to the invention as defined in the claims. On the contrary, the question to be asked is, what, in the light of the application as a whole, would be achieved by the invention as claimed.
For this reason, statements of purpose must be read in conjunction with the claims. Merely inserting such a statement into the description does not entitle an applicant effectively to "veto" any inventive step objection based on a document which is unrelated to this purpose, if it is not plausible that the invention as claimed would actually achieve the stated purpose. (Reasons, point 2.2.4.)

Summary of Facts and Submissions

I. The appeal is against the decision of the Examining Division refusing European patent application No. 01 998 993 on the grounds that the claimed subject-matter of the main request and of the first and second auxiliary requests all filed with the letter of 24 February 2010 was not clear (Article 84 EPC) and did not involve an inventive step (Articles 52(1) and 56 EPC).

II. At the end of the oral proceedings held before the Board the appellant requested that the decision under appeal be set aside and that a patent be granted on the basis of one of the following requests:

[...]

IV. Claim 1 of the main request reads as follows:

"A production method for silicon wafers, comprising:

a step for slicing a silicon wafer from an ingot composed of perfect area where no agglomerate of interstitial type point defects and no agglomerate of vacancy type point defects exist;

a step for thinning or stripping an oxide film on a surface of the silicon wafer in case where the thickness of the oxide film is 2 nm or more, wherein the oxide film is thinned so as to be less than 2 nm thick; and

a thermal annealing step for thermal-annealing the silicon wafer in an atmosphere to nitride the surfaces of the silicon wafer; thereby, forming vacancies therein,

wherein the step for thinning or stripping an oxide film is performed before the thermal annealing step,

the atmosphere used in the thermal annealing step comprises a nitride gas comprising NH3 having a lower decomposition temperature than a decomposable temperature of N2,

the NH3 in the nitride gas is 0.5% or more in concentration, or 10 sccm or more in flow rate, and

the temperature in the thermal annealing step is 900°C to 1200°C and the time in the thermal annealing step is 60 seconds or less."

[...]

V. In the contested decision, the Examining Division found essentially as follows:

[...]

The subject-matter of claim 1 of the main request also did not involve an inventive step (Article 56 EPC). Document Dl was the closest prior art, and the subject-matter of claim 1 differed from the method disclosed in this document in that:

"i) the wafer is sliced from an ingot composed of perfect area where no agglomerate of interstitial type point defects and no agglomerate of vacancy type point defects exist; and

"ii) the oxide film is thinned so as to be less than 2 nm thick in the oxide film stripping step."

The problem underlying the invention might be regarded as obtaining a high quality wafer.

Concerning point i), since Dl dealt with a gettering process, the use of an ideal precipitation wafer was obvious and belonged to the general knowledge of the skilled person.

Concerning point ii), no technical advantage could be seen in a step of reducing an oxide thickness to below 2 nm compared to already having a native oxide layer with a thickness in that range (1.3 to 1.5 nm) as in D1. Moreover, for an HF stripped wafer (total oxide removal), a very thin native oxide would immediately re-form on contact with the atmosphere. Hence no inventive merit could be recognised in such a process, and the subject-matter of claim 1 was thus not inventive.

The subject-matter of claim 1 of the 1st auxiliary request and of claim 1 of the 2nd auxiliary request was similarly unclear and also did not involve an inventive step.

VII. The appellant's arguments, insofar as they remain relevant to the present decision and the current requests, may be briefly summarised as follows:

[...]

In relation to inventive step, the Examining Division had been mistaken to regard document D1 as the closest prior art, as the purpose of the method of document D1 (using vacancies for controlling the distribution of dopant elements) was quite different to that of the present invention.

In the first and second embodiments of document D2, a thermal oxidation step S1 was carried out (page 12, lines 15-23) resulting in an oxide layer which could be inferred to have a thickness preferably greater than at least about 2 nm. It would be an unrealistic approach to choose as a starting point for a production method for silicon wafers comprising a step for thinning or stripping an oxide film, an embodiment in which the formation of an additional oxide layer was envisaged (emphasis added by the appellant). Accordingly the Examining Division's argumentation starting from the first embodiment of D2 amounted to an artificial and unrealistic approach.

The third embodiment of document D2 [it was argued in the statement of grounds of appeal] described on page 18, line 16 to page 19, line 16 was therefore to be regarded as the closest prior art.

The distinguishing features of claim 1 over this embodiment of document D2 could be identified as: the thinning or stripping of an oxide layer to less than 2 nm, the use of a nitride gas comprising NH3 (as opposed to nitrogen with a small partial pressure of oxygen), the concentration and flow rate of NH3, and the temperature (900°C to 1200°C) and time (60 seconds or less) of the thermal annealing step.

All of these distinguishing features contributed to the efficient and uniform injection of vacancies into a perfect wafer, thereby increasing BMD density. There was no apparent reason for one of average skill in the art to modify the third embodiment of the method of manufacturing the handle wafer in D2 in the direction of the claimed production method with a reasonable expectation to inject vacancies into the perfect wafer efficiently and uniformly.

[...]

Reasons for the Decision

1. The appeal is admissible.

2. Main request

[...]

2.2 Closest prior art

2.2.1 The Examining Division considered document Dl to be the closest prior art. The appellant regards this as an unsuitable choice for the reason that the purpose of the present invention and that of document D1 are different. The first question to examine, therefore, is whether document D1 represents an appropriate starting point.

2.2.2 The Boards of Appeal have considered the question of the selection of the closest prior art in numerous decisions, the conclusions being summarised in Case Law of the Boards of Appeal, 7th edition 2013, I.D.3.

In T 482/92, for example, the position is stated as follows:

- "in accordance with the established case law of the Boards of Appeal the 'closest prior art' for assessing inventive step is normally a prior art document disclosing subject-matter conceived for the same purpose as the claimed invention and having the most relevant technical features in common." (T 482/92, Reasons, point 4.1, third paragraph.)

2.2.3 The Board therefore agrees with the appellant that a comparison of the purpose of the invention and that of a potential closest prior art document is an important consideration in this regard. However, this leads to the question of how, precisely, the purpose of an invention is to be determined.

Typically, a patent application may comprise numerous statements of aims, ambitions and alleged advantages, the present application being no exception. In the first paragraph on page 1 (where one would normally expect to find the application described in its most general aspect) the invention is said to relate to "forming vacancies in a silicon wafer" and "forming a denuded zone (DZ) just under the surface of the silicon". Later, more specific aims are mentioned, for example providing a layer with a high density of BMDs (which the appellant apparently regards as the purpose of the invention), suppressing the generation of slips, providing satisfactory surface roughness etc.

The question is, therefore, which of these goals is to be considered as the purpose of the invention for the determination of the closest prior art? The answer, in the opinion of the Board, is provided in the passage quoted from T 482/92, in which it is stated that the closest prior art should relate to "the same purpose as the claimed invention" (emphasis added by the Board).

2.2.4 In establishing the closest prior art, the determination of the purpose of the invention is not to be made on the basis of a subjective selection from among statements of purpose which may be set out in the description of the application, without any reference to the invention as defined in the claims. On the contrary, the question to be asked is, what, in the light of the application as a whole, would be achieved by the invention as claimed.

For this reason, statements of purpose must be read in conjunction with the claims. Merely inserting such a statement into the description does not entitle an applicant effectively to "veto" any inventive step objection based on a document which is unrelated to this purpose, if it is not plausible that the invention as claimed would actually achieve the stated purpose.

2.2.5 It must be borne in mind that Article 113(2) EPC guarantees that the manner in which the invention is defined in the claims of a European patent application is entirely in the hands of the applicant.

Where a specific purpose of the invention is stated in the description, the applicant is free to include in an independent claim those features by means of which this purpose is achieved. In this case a document unrelated to this purpose (or at least a similar purpose) would generally not be a suitable choice of closest prior art, for the reasons indicated above.

Alternatively, the applicant may decide to seek protection for the invention according to a broader definition by omitting from the claim one or more of the features mentioned above so that the claimed invention would no longer achieve the specific purpose referred to. In this case a different purpose must be sought by asking what would in fact be achieved by the invention as claimed.

It will often be the case that a more general purpose will have to be ascribed to the broader claim, and that the number of prior art documents relating to this more general purpose is correspondingly greater. By defining the invention in broader terms the applicant aims for a larger scope of protection, but at the same time exposes the claimed invention to possible inventive step attacks based on a larger number of potential candidates for closest prior art. As noted above, the choice is entirely in the hands of the applicant.

2.2.6 Turning to the present main request, the appellant stresses the aspect of providing a high density of oxygen precipitates (BMDs). The Board accepts that this aim is mentioned in the application, the invention being described, for example, as providing a "high quality silicon wafer having a DZ layer, which is suitable for forming a device, as the surface layer, and also having a high BMD density area having a proximity gettering effect" (see page 6, lines 9-12, and in general, page 6, line 2 to page 7, line 11; page 19, lines 4-8 etc.).

The appellant concludes that document D1 is not a suitable choice of closest prior art, as the purpose of the method disclosed in this document is different, namely to control the distribution of dopant elements in the silicon wafer, thereby controlling resistivity.

2.2.7 The Board notes that the method disclosed in the present application comprises at least two steps, as follows:

- a first step involving subjecting a silicon wafer to rapid thermal annealing in an atmosphere comprising NH3 to inject vacancies into the wafer (page 3, line 5 - page 4, line 20; page 10, line 21 - page 12, line 11), and

- a second step involving a second thermal anneal at a lower temperature in an atmosphere comprising O2 resulting in a denuded zone at the wafer surface and a layer having a high density of oxygen precipitates (BMD layer) suitable for proximity gettering (page 6, lines 2-12; page 12, lines 12-21).

The purpose highlighted by the appellant (a high density BMD layer) is therefore achieved by carrying out both steps of this procedure.

In claim 1 of the main request, however, the appellant has chosen to define the invention in terms of the first step only. The result achieved by carrying out this claimed method would be a silicon wafer with injected vacancies. There would be no denuded zone and no BMD layer.

In the light of the analysis above, it cannot be legitimately asserted that the purpose of the invention as claimed is to provide a silicon wafer having BMD layer with high density, or for that matter having a denuded zone. The purpose of the claimed invention can only be defined in terms of what would be objectively achieved by carrying out the claimed method, namely providing a silicon wafer with injected vacancies. It follows, therefore, that any document which describes a process having the purpose of injecting vacancies into a silicon wafer, for whatever reason, is a potentially suitable choice as closest prior art, or at least cannot be excluded on the grounds that it relates to a different purpose.

There has never been any dispute that document D1 discloses a method by which vacancies are injected into a silicon wafer. Moreover, the method of document D1 has many technical features in common with the claimed invention (see point 2.3.2, below). The Board therefore considers that document D1 is a perfectly suitable choice as closest prior art for the main request.

2.3 Main request: inventive step starting from document D1

[Remainder of decision omitted, according to which the main request lacked inventive step, but wherein claims 1 and 2 according to a 2nd auxiliary request were considered to be allowable]

Order

For these reasons it is decided that:

1. The decision under appeal is set aside.

2. The case is remitted to the department of first instance with the order to grant a patent in the following version:

- Claims 1 and 2 according to the 2nd auxiliary request, filed during oral proceedings before the Board;

[...]


This decision T 2255/10 (pdf) has European Case Law Identifier ECLI:EP:BA:2015:T225510.20150423. The file wrapper can be found here. Photo "Purpose" by Seth Sawyers obtained via Flickr under CC BY 2.0 license (no changes made).

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