T 2255/10 - Closest prior art

T 2255/10

Board 3.4.03 - [C] - For the decision, click here

Key point

• The closest prior should have the same purpose "as the claimed invention" , but how to define the purpose of the invention? The present Board decides to focus on "what would be achieved by the invention as claimed", emphasizing " as claimed". 
• The present case concerns an examination appeal. Nevertheless the headnote seems useful for opposition cases as well. 

EPO Headnote

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.)

Reasons for the Decision

1. The appeal is admissible.

2. Main request

2.1 Amendments (Article 123(2) EPC)

2.1.1 Claim 1 of the main request is based on a combination of claims 1-3 and 5-7 as originally filed. The feature relating to the "perfect area" is based on the passages from page 17, line 8 to page 19, line 17. The feature "to nitride the surfaces of the silicon wafer" may be seen as being based on numerous passages throughout the description, for example page 12, lines 7-11. Hence, the subject-matter of claim 1 of the main request is considered to meet the requirements of Article 123(2) EPC.

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

2.3.1 Document D1 discloses a production method for silicon wafers (see e.g. page 6, line 8), and it is implicit to a skilled person that such wafers would be sliced from an ingot.

Moreover, the use of "in case where" in claim 1 means that two alternatives are defined: when the thickness of the oxide film is 2 nm or more, it is thinned or stripped, and when it is not 2 nm or more, it is not thinned or stripped. The wafers of document D1 have only a natural oxide layer with a thickness of about 13 Angstroms (1.3 nm, see page 6, line 19; page 7, second paragraph), and this is generally not stripped (although as an alternative it could be - see page 7, second paragraph). Hence, document D1 discloses the second claimed alternative in relation to thinning/stripping.

2.3.2 Document D1 therefore discloses the following features of claim 1 of the main request:

A production method for silicon wafers, comprising:

a step for slicing a silicon wafer from an ingot;

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 (see point 2.3.1, above); 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 (see e.g. page 2, line 9 to page 3, line 19),

wherein the step for thinning or stripping an oxide film is performed before the thermal annealing step (see point 2.3.1, above),

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 (page 8, second paragraph),

the NH3 in the nitride gas is 0.5% or more in concentration, or 10 sccm or more in flow rate (page 8, second paragraph, 10000 ppm corresponds to 1%), 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 (page 8, second paragraph).

2.3.3 The method of claim 1 therefore differs from that of document D1 in that the ingot is composed of perfect area where no agglomerate of interstitial type point defects and no agglomerate of vacancy type point defects exist.

According to the application the wafer can be sliced from an ingot grown by a "normal CZ method" (page 17, lines 8-9), or alternatively from an ingot according to the above distinguishing feature (page 17, line 9 et seq.), i.e. corresponding to the region [P] of figure 5.

The appellant essentially argued that if the aim were to produce a wafer with a significant number of vacancies, a skilled person would naturally consider it advantageous to start from a wafer already containing vacancies (region [V]), rather than a wafer that is basically free of any defects (region [P]).

However, if this argument were to be considered valid, it would have to be asked why it would not apply with equal force to the claimed invention. Why, in other words, in the context of an invention which aims to provide a silicon wafer with injected vacancies, is a feature included in the claim which, according to the appellant, is disadvantageous in precisely this respect? Which technical effect (other than the adverse one pointed out by the appellant) would it provide? Which problem would it solve?

During the oral proceedings the appellant conceded that this choice might appear counter-intuitive, especially as the process for forming such ingots involved extra cost and effort. Nevertheless, the appellant asserted that it had been found worthwhile to start from such an ingot. When pressed on this point, however, the appellant did not offer any specific technical effects provided by such a choice.

2.3.4 The Board notes that there is no dispute that the technical content of figure 5 of the present application and of the associated text is well-known, and essentially summarises the relationship (first pointed out by Voronkov in the 1980s) between the levels of vacancy and interstitial type defects and the ratio of the pulling rate to the temperature gradient (V/G). It is also not in dispute that a skilled person would, at the priority date, have been well able to produce ingots corresponding to any of the regions depicted in the figure.

Given the aim of injecting vacancies into the wafer of document D1, the skilled person would clearly not be tempted to select the region [I] depicted in figure 5, in which interstitial type point defects predominate.

Choosing the "perfect area [P]" would result in a low number of vacancy type point defects (admittedly a disadvantage when injecting vacancies is the aim), but also in a low incidence of agglomerates of point defects, i.e. macroscopic defects such as COPs (see page 19, lines 4-8) which are generally harmful. Alternatively, working in the area [V] would result in a greater number of desirable vacancy type point defects, but also in a correspondingly greater number of undesirable agglomerates of point defects.

The skilled person would make a selection from this very limited number of alternatives on the basis of the well-known advantages and disadvantages of each, and without exercising any inventive activity.

The subject-matter of claim 1 of the main request does not, therefore, involve an inventive step within the meaning of Article 56 EPC 1973.