T 0424/90 11-12-1991

Reasons for the Decision

1. Inventive Step - Claim 1 - Main Request

1.1. It was not contested by the Respondent that document D1 is accurate evidence of the technical contents of the corresponding paper which was orally presented in January 1981. From the oral disclosure evidenced by document D1 there is known, in accordance with the wording of Claim 1:

"A plasma processing apparatus for the selective etching of a workpiece (D1, Figure 5), the apparatus including an evacuable chamber (D1, Figure 1), means for evacuating the chamber ("Roots Blower" and "Diffusion Pump" in Figure 1), at least one source of reactant gas ("Gas inlet"), means for introducing said at least one radiant gas at low pressure into the chamber (D1, page 166, left column, last line, in combination with the indication CHF3 Pressure: 0.05 Torr in Figure 3), an electrode ("Cathode" in Figure 1) in the chamber having at least one workpiece support surface (the "wafer" in Figure 1) electrically insulated from the chamber (see "Teflon" in Figure 1), means for applying a voltage to the workpiece support surface (1.3 KV in Figure 1), and means for generating an electron-trapping magnetic field ("Sm-Co Permanent Magnet Assembly" in Figure 1), whose lines of magnetic force co- operate with the electrode surfaces to form a completely enclosed region (Figure 2)."

1.2.1. Starting from document D1 as the closest prior art, the objective problem underlying the subject-matter as claimed in Claim 1 is, in the Board's view, restricted to avoiding the disadvantages of this known apparatus, such as relative movement between the plasma loop and the wafer in order to cover the complete wafer surface, and a sloped etch profile in regions along the side edges of the known plasma loop, and to improve thereby the efficiency of the reactive species generation.

1.2.2. The reducing of the kinetic energy of the reactive ions impinging on the substrate surface, as a result of increasing the magnetic field for trapping plasma generating electrons is known from curve "VDC" in Figure 3 and the corresponding description of document D1.

1.2.3. As is admitted in the introductory part of the patent in suit, when evaluating the relevant prior art -in particular in column 3, lines 15, 16, 27, 28 and 38 to 40 - it is known in the art to reduce the loading effect and resulting mask undercutting by increasing the reactive species generation rate, for instance by increasing the plasma density. Document D1, page 168, lines 6 and 7, indicates explicitly that the electron trapping magnetic field increases the ionisation efficiency - i.e. the reactive species generation rate by a factor of ten. Therefore, due to the known critical importance of the quality of the edge profile in VLSI-circuits, a skilled person working according to the teaching of document D1, in the Board's view, would both expect and be able to observe, concomittantly with the increase of the reactive species generation rate, a reduction of the mask undercutting caused by the loading effect.

1.2.4. Hence, the reduction of the kinetic energy of the reactive ions and that of the loading-effect-induced mask under- cutting are qualitatively known and foreseeable expectations which are inherent in the closest prior art. Moreover, the Respondent has submitted no evidence of a quantitative improvement in these two effects when replacing the plasma loop of document D1 by the plasma belt of document D2. For these reasons -contrary to the Respondent's view in paragraph VIII (c) and (e) - and following the established jurisprudence of the Boards of Appeal, these two effects cannot be included into the formulation of the objective problem when assessing the question of inventive step.

1.3. Though the claimed apparatus is no doubt intended to be used in the reactive ion etching of VLSI circuits, the Board does not accept the Respondent's submission set out in paragraph VIII (a) above that, for the evaluation of inventive step, the competent skilled person is merely a semiconductor specialist. The Board considers that in real life, a semiconductor specialist would consult a plasma specialist, if his problem concerns providing a technical improvement of an ion generating plasma apparatus as in the present case. The semiconductor specialist would be expected to form a team with the plasma specialist and to entrust him with the task of increasing the efficiency of the reactive species generation. Thus, in the Board's view, it is appropriate in the present case to follow Decision T 32/81, OJ EPO 1982, 225, and to base the assessment of whether the claimed solution involves an inventive step on the knowledge and abilities of the specialist in that technical field in which the objective problem prompts the person skilled in the art to seek its solution. Hence, for the above reasons in the present case the competent skilled person is held to be a plasma specialist.

1.4. The Board considers that a plasma specialist would have been aware that the objective problem of increasing the efficiency of the reactive species generation meant the task of enlarging the plasma source area which emits ions for an interaction with a target and of homogenising the density of the emitted ions. It was clearly part of the general knowledge of the plasma specialist that any later use of the emitted ions for a chemical etch process or for a physical sputter process would not technically influence the generation process of the ions. Thus, the plasma specialist would look for advice in each special technical field, where he would expect to find or where he knows of the existence of his own problem, i.e. to produce ions for a homogenous interaction with a large target surface. According to the established jurisprudence of the Boards of Appeal such technical fields are considered as neighbouring fields and are considered as part of the relevant state of the art when assessing inventive step; see in particular Decision T 176/84, OJ EPO 1986, 50, paragraph 5.3.1. In the Board's view, the Appellant is right in his submission set out in paragraph IV (a) and (c) above, that sputtering processes and reactive ion etching processes are often carried out in the same apparatus. Hence, a plasma specialist has a realistic practical motivation to be aware of technical developments in the field of sputtering devices and thus to know of the existence in this field of problems which are similar to his own, as well as how such problems have been solved. For the above reasons, the Board does not accept the Respondent's submission as set out in paragraph VIII (b) above, and holds that the competent skilled person would indeed take into account the teaching of document D2 when seeking to solve problems arising in the apparatus disclosed in document D1.

1.5. Moreover, the explicit teaching in document D2, column 2, line 67, and column 3, lines 39,40, that the plasma belt produced by the magnetic pole means disclosed in this document, extends over broad target areas and produces a relatively even sputtering - i.e. ion impingement - across the target, in the Board's view makes it obvious that these means solve the objective problem as defined in paragraph 1.2.1 above. This teaching of document D2 would therefore actively suggest to the skilled person the replacement of the magnet assembly and its scanning mechanism in the reactive ion etching apparatus according to Figure 1 of document D1 by the magnetic pole means disclosed in document D2. Thus the teaching of document D2 would suggest a solution to the objective problem, which solution is characterised according to the wording of Claim 1 in that:

"... the means for providing an electron trapping magnetic field (see D2, column 3, lines 30 to 37) comprise first magnetic pole means (D2, Figure 3, 104, upper parts 106, 108) at one end of the workpiece support surface (between 62 and 72 in Figure 3) and second magnetic pole means (102, lower parts 106, 108 in Fig. 3) at the other end of the workpiece support surface, with the workpiece support surface being located between the first and second magnetic pole means (see Figures 3 and 4), which project outwardly therefrom (108 versus 72 in Figure 3), the first and second magnetic pole means being of opposite polarity (column 7, lines 59 to 62) thereby to provide lines of force which leave said electrode at the first pole means, extend across the workpiece support surface in the gap between the first and second pole means and re-enter said electrode at the second pole means (see dashed curves in Figures 3 and 4, and column 7, lines 37 to 40) to enclose the workpiece support surface in an electron-trapping magnetic field (column 7, lines 10 to 15)."

1.6. The technical facts submitted by the Respondent according to paragraph VIII (d) represent - as shown below - no technical hindrance which would prevent a skilled person from making use of the above-described known advantages of the magnetic pole means of document D2 in the closely analogous situation of the apparatus of document D1. Document D1 teaches particularly on page 167, lines 15-18, to keep the kinetic energy of the ions as small as possible in order to preserve the mask pattern. Furthermore, from the direction of the field lines in Figure 3 of document D2 a skilled person would easily derive the parallel relationship between the equipotential lines of the border area of the enclosed plasma and the neighbouring target surface, and would thus also derive the movement of the impinging ions which is throughout perpendicular to the target surface.

1.7. The Respondent's submission in paragraph VIII (e) that a skilled person would not be able to adapt the process parameters when transferring magnetic pole means from a sputtering to a reactive ion etching apparatus was not supported by any technical fact. The Board does not accept this submission for the following reasons: in the reactive ion etching apparatus of document D1, the electron trapping magnetic field of the pole means disclosed in document D2 would be confronted mainly with two different experimental conditions: a different working gas to be ionised (see also paragraph IV (a)) and a lower acceleration (diode) voltage in order to produce ions with a lower kinetic energy (see D1, page 167, lines 15 to 18). It is regarded as generally known expert knowledge that a possible lower average velocity of ionising electrons - such as in a reactive ion etching apparatus as compared to a sputtering apparatus as a consequence of the lower ion acceleration voltage to be supplied - can easily be compensated by an increase of the magnetic field strength in order to achieve a comparable degree of electron deflection, hence of electron confinement. Moreover, in the Board's view, Figure 3 of document D1 gives a skilled person sufficient advice in particular with regard to what strength of magnetic field and what radio frequency power is needed in order to achieve an appropriate kinetic ion energy for reactive etching, and to compensate for resulting differences in the ionisation energy between inert atoms for sputtering and molecules for reactive etching. This view is also supported by the fact that the range of magnetic field strength in Figure 3 of document D1 is identical with the one indicated in the patent in suit, column 7, lines 21-24.

1.8. For the reasons stated above, in the Board's judgment Claim 1 of the Respondent's main request therefore lacks an inventive step within the meaning of Article 56 EPC.

2. Inventive Step - Claim 1 - Auxiliary Request

2.1. The wording of Claim 1 according to the Respondent's auxiliary request adds to the subject-matter of Claim 1 of the main request only features which are known either from document D1 or from document D2.

2.2. From document D1 the following features are known: "in a reactive ion etching process for the selective chemical etching of a masked surface (see D1, Figure 6) of a workpiece ... mounting the workpiece to be etched on the workpiece support surface with the masked surface exposed (see "wafer" in Figure 1); introducing at least one reactant gas at low pressure into the chamber (Figure 1 and Figure 3); and applying a voltage to the workpiece support surface (Figure 1) to produce a uniform plasma of reactive species and ions in the electron trapping magnetic field closely adjacent the workpiece support surface (Figure 4 with the corresponding description)."

2.3. It is also known from document D2 that "the magnetic field lines (of the electron trapping magnetic field) extend substantially parallel to and closely adjacent the workpiece support surface .... and spaced therefrom by a plasma sheath electric field having lines of force perpendicular to the masked surface of the workpiece (see Figures 3 and 4 in D2 and note that the plasma is equipotential)."

2.4. Hence, a skilled person arrives at the subject-matter of Claim 1 of the auxiliary request by making use of the magnetic pole means disclosed in document D2 in the closest prior art according to document D1. Such a use has to be regarded as obvious for the reasons set out above. Therefore, in the Board's judgment the subject-matter of Claim 1 according to the Respondent's auxiliary request also does not involve an inventive step within the meaning of Article 56 EPC.

3. Claims 2 to 9 of both requests fall because of their dependency on the respective Claims 1.

4. Method Claim 10 exclusively comprises measures which are completely self evident in view of the prior art according to document D1. In particular, the claimed step of "adjusting the voltage applied to the workpiece support to the minimum value necessary to produce a dense glow discharge" represents the adjustment of an optimum between two conflicting parameters, reactive ion generation rate and target temperature, which is regarded to fall within a skilled person's normal abilities. For these reasons, Claim 10 is not allowable under Article 56 EPC.