USE OF PERFLUOROPOLYETHERS IN MECHANICAL PUMPS

Abstract: A process is disclosed for the generation of a vacuum of </= 5.10⁻⁴ Torr by means of mechanical pumps, wherein a per­ fluoropolyether having neutral end groups and containing light products having an average molecular weight of </= 1000 in amounts of </= 50 ppm is used as operative fluid.

Description: The present invention relates to the use of perfluoropolyethers in mechanical pumps for the generation of a vacuum of ≦ 66.7 mPa (5.10⁻⁴ Torr). The use of perfluoropolyethers in vacuum pumps is well-known. Due to the high stability of these compounds the variation of their viscosity is sufficiently low during the operation of the pumps. Perfluoropolyethers are better suited for use in these vacuum apparatuses than conventional fluids such as mineral and silicone oils since the latter tend to decompose, owing both to the heat generated by friction forces and to the chemical action of substances entering the pump during operation. However, even by using the perfluoropolyethers presently employed in the vacuum technique, high vacuums, i.e. ≦ 66.7 mPa (5.10⁻⁴ Torr) cannot be reached. The vacuum usually obtained with said perfluoropolyethers is of the order of 133 mPa (10⁻³ Torr). Mechanical pumps are, generally, used in the field of microelectronics, in particular, in the manufacture of semiconductors. In these fields, the composition of the residual atmosphere in the vacuum chamber, where the processing of semiconductors takes place, is critical. It has been observed that the perfluoropolyethers utilized at present do not permit that residual atmospheres are obtained which exhibit a sufficiently low degree of pollution, as is required for the processing of semiconductors. As a result, the impurities still present cause a considerable reduction in the number of utilizable semiconductor pieces. Consequently, there was a need for an operative fluid for mechanical pumps which permits that a vacuum of ≦ 66.7 mPa (5.10⁻⁴ Torr) is obtained so as to obtain residual atmospheres having a very low content of impurities. It has, surprisingly, been found that the above requirements can be met by using, as operative fluid for mechanical pumps, a perfluoropolyether containing perfluoropolyethers having an average molecular weight of ≦ 1000, in amounts not exceeding 50 ppm, preferably not exceeding 30 ppm. Any perfluoropolyether with neutral end groups having the above characteristics can be used for carrying out the process of the present invention. Examples are Fomblin® (Montedison) and Krytox® (Du Pont). Said perfluoropolyethers are described in GB-A-1 104 482 and US-A-3 665 041, 3 715 378 and 3 242 218. Further perfluoropolyethers are described in EP-A-148 482. One may also use the perfluoropolyethers described in US-A-4 523 039. Also suitable are difunctional perfluoropolyethers produced by Daikin or Krytox® in which the COF end groups have been neutralized by known methods. Examples are the products described in EP-A-148 482 and 151 877. Furthermore, one may also use functional perfluoropolyethers prepared from neutral perfluoropolyethers according to IT-A-22920 A/85. The perfluoropolyethers (PFPE) utilizable in the process of the present invention are prepared from the raw products described above by fractionated distillation to reduce the content of the perfluoropolyethers having an average molecular weight of ≦ 1000 to the values specified above. The viscosity of the PFPE generally ranges from 40 to 350 mm²/s (cSt) as a function of the type of pump intended for a specific application. Generally, perfluoropolyethers having a narrow viscosity range are preferred. If, for example, the oil utilized for a certain application has to have a viscosity of 270 mm²/s (cSt), the viscosity variation is, preferably, not more than ± 20 mm²/s (cSt). The perfluoropolyethers useful for the present process can also be obtained by the cracking, by means of substances such as AlF₃, of high-viscosity perfluoropolyethers and by subsequent distillation in order to obtain the viscosity values mentioned above. This method is, for example, described in IT-A-21052 A/84 of applicant. Other catalysts utilizable for said cracking are the oxyfluorides and the fluorides of the transition metals, in particular, those of Ti, Co and Ni or the oxides, preferably, those of Ti and Al. Said catalysts are described in the cited patent application. A further advantage deriving from the use of the PFPE according to the present invention is the fact that the oil consumption during the operation of the pump is minimal. In fact, even after long running periods of the pump, almost no oil consumption can be observed (see example 1). This is a remarkable advantage since these types of oil are very expensive. Mechanical pumps of any type can be utilized to generate the vacuum to be reached by the process of the present invention. Mechanical blade pumps are particularly suitable. The following examples illustrate the invention but do not limit it in any way. EXAMPLE 1 Of two identical pump systems, each consisting of a mechanical pump (type 2063 CP, manufactured by Cit Alcatel) equipped with a D filter and a 500 m³/h Roots Blowers pump the first system was charged with 40 kg of Fomblin® 27 HP and the second system was charged with an equal amount of Fomblin® Y L-VAC 25/6. After a run of three months, 24 hours a day, under identical conditions, the amount of oil necessary to make up for the losses experienced during the operation of the pump was 1 kg/week in the system charged with L-VAC 25/6. By contrast, there was no need to add oil to the system charged with Fomblin® 27 HP, which had been run under the same conditions. A 25% saving in terms of oil consumption per week was, thus, attained. EXAMPLE 2 Oils of the Fomblin® type are predominantly utilized as fluids for vacuum pumps in the fieid of microelectronics, where the composition of the residual atmosphere in the reaction chamber is of great importance. The vacuum pump described in example 1 was used. By means of the neutron activation technique it is possible to determine directly the amounts of polluting substances present on the surface of the substrates placed in the vacuum chambers. Table 1 compares the results of the analysis carried out by the above technique on substrates placed in two vacuum chambers. In the first, the vacuum was generated by a pump system charged with Fomblin® 27 HP while, in the second chamber, the vacuum was generated by a system charged with Fomblin® Y L-VAC 25/6. 27 HP 25/6 Na 20 ppb 160 ppb K 50 ppb 1000 ppb Cl 1500 ppb 3700 ppb Li 0 very high It is evident that the amounts of the elements considered to be the most dangerous due to their remarkably lowering the yields of utilizable products (semiconductors), namely Na, K and Li, are far lower in the 27 HP oil than in the 25/6 oil. The vacuum generated by the pump which utilized Fomblin® 27 HP was 26.7 mPa (2.10⁻⁴ Torr) while the vacuum obtained with Fomblin® L-VAC 25/6 was 800 mPa (6.10⁻³ Torr). The amount of perfluoropolyethers having an average molecular weight of ≦ 1000 was 15 to 20 ppm in Fomblin® 27 HP and 90 ppm in Fomblin® L-VAC. The viscosities of both Fomblin® 27 HP and Fomblin® L-VAC were 270 ± 20 mm²/s (cSt). The above example was repeated, using Fomblin® 15 HP (viscosity is 150 ± 20 mm²/s (cSt) and the vacuum obtained was 40 mPa (3.10⁻⁴ Torr), the fraction of light perfluoropolyethers (MW = ≦ 1000) being 20 ppm. The amount of impurities (see table 1) was of the same order of magnitude as in the case of Fomblin® 27 HP.

Claims: A process for generating a vacuum of ≦ 66.7 mPa (5.10⁻⁴ Torr) by means of mechanical pumps, wherein a perfluoropolyether having neutral end groups and containing light perfluoropolyethers having an average molecular weight of ≦ 1000 in amounts of ≦ 50 ppm is used as operative fluid. The process according to claim 1, wherein the content of perfluoropolyethers having an average molecular weight of ≦ 1000 is ≦ 30 ppm. The process according to claim 1 or 2, wherein the viscosity of the perfluoropolyether ranges from 40 to 350 mm²/s (cSt), depending on the pump employed.

Inventors: CALLONI, ENZO, STOPPA, LUIGI, TASCA, ADRIANA
Applicants: AUSIMONT S.P.A.
Application year: 1986
Main classification: C10M107
IPC codes: C10M107, F04C29, F04B37, F04C25