- Email: [email protected]
Perfluoropolyether vacuum fluids for safety in semiconductor processing
Abstracts
the physical etching component is maintained via the ion-beam energy variation. The resultant improvements in edge profiles leads to the capability, to produce the fine-line (sub-micron) geometries required for current and future generation microelectronic devices.
Perfluoropolyether vacuum fluids for safety in semiconductor processing M Valente, MonteJluos
Spa, Milan
Introduction The use of PFPE fluids for dry plasma technology (plasma- and reactive-ion etching, plasma stripping, plasma deposition), ion implantation, LPCVD, etc. can contribute to an increase in safety, protecting the operators and the environment they work in. Of course it should not be assumed that the use of Fomblin is a universal panacea negating the need for good housekeeping practice. In fact Fomblin oils should be regarded as only one component in a complex engineering system designed to offer safety and cost-effective working. The aim of this naper is to illustrate this idea by some instances. Safety arising from reduced maintenance at the pumping systems Care must be taken when changing the oil in the vacuum pumps, or dismantling pumps for inspection or repair. Some substances remain harmful after being added to the oil during processing. Pumping chlorinated gases coming from a plasma discharge poses not only corrosion but also safety problems. In cases where we use a Ccl,-based discharge, we have to consider the risk coming from products such as Ccl,, C,CI,, &Cl,, C&I, and other high mol wt C,CI, compounds. Ccl, is carcinogenic and its percutaneous absorption is substantial. The toxic and carcinogenic properties of the other halocarbons are well documented. Phosgene (COCl,) is also present, due to traces of water vapour in the reaction chamber and their reaction with CCI, species. Its concentration can be fairly high if 0, is’ intentionally or accidentally mixed with Ccl,. BCl,-based discharges do not generate chlorocarbons but BCl, is irritative and corrosive, and contains COCI, as impurity. All these materials accumulate in the rotary pump, in the oil, and in the exhaust line (near the exhaust valve). Because of the high density of these materials, nitrogen bleeding is only partially effective in removing their vapours*. Thecold trap used to protect the mechanical pump from corrosive species and to reduce water vapour concentration within the reaction chamber concentrates the hazardous reaction products. However safety and cost reasons, as well as difficulty in implementing a reliable automatic regeneration system with a pressure relief valve, have led some equipment manufacturers to eliminate cold traps in Al dry etching systems. The advantage of the use of Fomblin is that a life of 1 yr (typically) between Fomblin oil changes is feasible and advisable.
At the end of that time the pump should be serviced, i.e. seals replaced and the fluid checked, reclaimed if required, and refilled. Since the mineral oil life is short and the pump is subjected to enhanced corrosion due to degradation of mineral oil,.the safety factors of a once-a-year pump service are obvious, not the least being that familiarity of maybe weekly oil change can breed contempt and carelessness leading to unnecessary and unacceptable accident risk. Furthermore, elimination of cold-traps-a potential hazard-is only possible on a fomblinized pump, because no mineral oil can withstand the highly reactive compounds coming from the plasma discharge for a reasonable time. It is also evident that frequent mineral oil changes expose service personnel to risks that are difficult to evaluate. Data about the effect of a single carcinogenic material, massively absorbed, are available; but who knows the effect of the multiple exposure to multiple suspected carcinogenic materials for even low dose, for a prolonged period of time? Similar considerations are valid for ion implantation where poisonous and flammable gases such as ASH,, PH, etc. together with their decomposition products accumulate in the pumping system. When the mechanical pump is stopped for servicing or oil change, contact of PH, with air may occur, and a fire or air explosion can take place. This risk is also present with Fomblin even though reduced servicing helps to minimize the risk. But, again, Fomblin is only one component of the system. To implement safety, nitrogen bleeding and a burn-box at theexhaust are required, or electrical grounding should be considered. Furthermore phosphorus and arsenic non-volatile compounds contained in the oil and within the pump deserve special care. The chemical form of such materials is not known but inhalation and absorption through skin should be avoided.
Fomblin and use of pure silane Another situation of interest is connected to the use of pure silane (SiH,) for semiconductor fabrication. According to the Semiconductor Safety Association, most users think that in some cases the use of 100% SiH, is absolutely necessary. Reports and rumours from USA and Japan indicate that fires in ducts due to ignition by silane of oil silica deposits which have built up for years are likely to occur. Installing oil separators at the outlet of the mechanical pump is advisable. Adopting a regular cleaning procedure for silane exhaust pipes and using stainless steel instead of plastics as a duct material is also recommended. But the only effective remedy seems to be fomblinization of mechanical pumps. This has been the approach of many big semi-conductor companies, as it appears from the relatively huge investments to fomblinize hundreds of pumping groups at one time. In Japan, for instance, concern for safety has led to a change from hydrocarbon to Fomblin fluids for all semiconductor factory vacuum systems even where, technically, their use is not essential. This is in order to avoid any aspect of accidental contamination by hazardous oils in the bulk of the systems where PFPE must be used as an insurance against risk.
* This is especially true for phosgene. According to our experience, COCI, concentrations as high as 10 ppm can be detected near the exhaust valve
Oxygen service
and inside the pump. Monitoring of COCl, by Drager servicing a pump is therefore highly recommended.
As explosions
equipment
before
can
and
do
occur
in vacuum
pumps
using 511
Abstracts
hydrocarbon vacuum fluids* when pumping oxygen, the vacuum pump must be converted to use inert fluid. This can be achieved by thoroughly cleaning the pump and the rest of the pumping system (if the equipment has already been operated with a mineral oil) in order to remove hydrocarbon contamination. Then the pump should be charged with a suitable fluid. Chlorotrifluoroethylene oils are a candidate, as they have a good thermal stability and no fire or flash point. Unfortunately their viscosity index is very poor, and very often cold starting is a severe problem. Many modifications are required in order to adjust a mechanical pump to these fluids (e.g. increasing electric motor size, enlarging clearances, providing a fluid heater, etc.). Also ultimate pressure and backstreaming rate are not very satisfactory. Conversion to Fomblin oil is much easier because Fomblin oil is fabricated with viscosity-vapour pressure characteristics matching those of mineral oil and only minor modifications-if any-are required?. The use of Fomblin in this area has been approved by various national agencies such as BAM (Bundes Anstalt fur Material Prtifung) in West Germany and EMPA in Switzerland, and by private companies such as Air Products and BOC. * The most endangered area is around the exhaust valves where the gases are compressed to atmospheric pressure and can reach higher temperatures. Typical compression ratios for rotary vane-pumps are in the range of 1 x lo5 up to 1 x 106. t Attention must be paid to careful cleaning with Algofrene 113.
particles becomes difficult to identify. Close monitoring of particles is beginning to have a ‘knock-on’ effect, putting greater demands on equipment suppliers to develop machines which minimize particle generation. Careful monitoring of incoming materials is equally important. The semiconductor industry has long recognized that control of dissolved metallic impurities in chemicals is important, but only more recently has the importance of undissolved impurities been highlighted. A case study is presented which was initiated to help to identify the more important sources of particulate contamination during a typical fabrication sequence. Particulate counting was performed using a Tencor Surfscan. This machine scans the wafer surface with a HeNe laser, and records the number of scattering centres greater than agiven diameter (the minimum diameter selectable is 1 pm). The major drawback of this technique is that it cannot distinguish between surface topography, point defects and particulates. However, by measuring a given wafer before and after a particular step (in conjunction with control unprocessed wafers) the cleanliness of the step can be inferred. A variety of techniques such as scanning electron microscopy with elemental analysis facilities and X-ray diffraction were used to identify the composition of the particles and hence the source of the contamination. Results from the study of a typical photoresist track process and wet etching processes will be presented. In the case of wet etching, it was found that undissolved particles in the starting materials could become troublesome even if the supplier used a 0.2 pm filter. The source of such particulate contamination has been traced to the type of packaging material used. It is recommended that glass bottles should be replaced by plastic bottles to store chemicals for VLSI processes.
Particle control in VLSI Acknowledgements J Patel, GEC Research Laboratories The semiconductor industry has recently given significant attention to particulate contamination, motivated by the trend toward smaller feature sizes and higher packing densities in very large scale integration (VLSI). Particles with a diameter of the order of a tenth of the minimum feature size have been reported to affect the performance of an integrated circuit’. It is thus imperative to monitor and control the generation of particles during integrated circuit fabrication processes. The demand for tighter control of contamination is severe: a contamination hierarchy exists within the microelectronic industry whereby certain aspects of contamination are given extensive research and others are given second preferences. The cleanliness of water and air have obviously been given the most attention. Elaborate systems have been built to remove ionic species, organics and particulates from deionized water supplies, and advances have been made in the design of clean rooms with vertical laminar air flow and HEPA (high efficiency particulate air) filter systems. The chemical industry has also met the requirement by supplying higher purity chemicals with stringent specification on the level of dissolved metallic impurities. The tolerance requirements for particle control in VLSI technology are extremely difficult both to visualize and implement effectively since particles may be generated at each step of the process. Every piece of equipment used in the fabrication process can have its own inherent particle generating mechanism, thus monitoring should be carried out on all existing machines and between each transfer process. If no monitoring is performed, particles will not be detected until a later processing stage and the source of 512
The author is grateful to Mr H Pate1 and Miss Y West who helped to monitor the particlecontamination and Mr V Phelan and Mr T Kendrick from the Materials Characterisation Division for chemical analysis. Reference
’ J M Duffalo and J R Monkowski,
Solid Stare Techno/. p 109 (March
1984).
The nondestructive
characterization of silicon-on-sapphire films
M G Pitt, T B Peters and C Dineen, GEC Research Laboratories Silicon-on-sapphire (SOS) is the primary dielectrically isolated substrate technology for CMOS integrated circuit manufacture. However, SOS layers have a high density of defects, principally microtwins, extending from the sapphire interface. These result in considerably reduced transistor mobilities and thus reduced circuit operating speeds. The development of techniques for nondestructively assessing SOS wafer quality has aided the development of improved epitaxial growth conditions as well as providing a basis for processcontrol ofepitaxy. Two techniques have proved to be particularly beneficial in this respect: determination of twin concentration using an X-ray diffraction technique and ultraviolet (uv) reflectometry. The principal crystalline defect present in as-grown silicon-onsapphire films is twins. A technique, based on work reported by
the physical etching component is maintained via the ion-beam energy variation. The resultant improvements in edge profiles leads to the capability, to produce the fine-line (sub-micron) geometries required for current and future generation microelectronic devices.
Perfluoropolyether vacuum fluids for safety in semiconductor processing M Valente, MonteJluos
Spa, Milan
Introduction The use of PFPE fluids for dry plasma technology (plasma- and reactive-ion etching, plasma stripping, plasma deposition), ion implantation, LPCVD, etc. can contribute to an increase in safety, protecting the operators and the environment they work in. Of course it should not be assumed that the use of Fomblin is a universal panacea negating the need for good housekeeping practice. In fact Fomblin oils should be regarded as only one component in a complex engineering system designed to offer safety and cost-effective working. The aim of this naper is to illustrate this idea by some instances. Safety arising from reduced maintenance at the pumping systems Care must be taken when changing the oil in the vacuum pumps, or dismantling pumps for inspection or repair. Some substances remain harmful after being added to the oil during processing. Pumping chlorinated gases coming from a plasma discharge poses not only corrosion but also safety problems. In cases where we use a Ccl,-based discharge, we have to consider the risk coming from products such as Ccl,, C,CI,, &Cl,, C&I, and other high mol wt C,CI, compounds. Ccl, is carcinogenic and its percutaneous absorption is substantial. The toxic and carcinogenic properties of the other halocarbons are well documented. Phosgene (COCl,) is also present, due to traces of water vapour in the reaction chamber and their reaction with CCI, species. Its concentration can be fairly high if 0, is’ intentionally or accidentally mixed with Ccl,. BCl,-based discharges do not generate chlorocarbons but BCl, is irritative and corrosive, and contains COCI, as impurity. All these materials accumulate in the rotary pump, in the oil, and in the exhaust line (near the exhaust valve). Because of the high density of these materials, nitrogen bleeding is only partially effective in removing their vapours*. Thecold trap used to protect the mechanical pump from corrosive species and to reduce water vapour concentration within the reaction chamber concentrates the hazardous reaction products. However safety and cost reasons, as well as difficulty in implementing a reliable automatic regeneration system with a pressure relief valve, have led some equipment manufacturers to eliminate cold traps in Al dry etching systems. The advantage of the use of Fomblin is that a life of 1 yr (typically) between Fomblin oil changes is feasible and advisable.
At the end of that time the pump should be serviced, i.e. seals replaced and the fluid checked, reclaimed if required, and refilled. Since the mineral oil life is short and the pump is subjected to enhanced corrosion due to degradation of mineral oil,.the safety factors of a once-a-year pump service are obvious, not the least being that familiarity of maybe weekly oil change can breed contempt and carelessness leading to unnecessary and unacceptable accident risk. Furthermore, elimination of cold-traps-a potential hazard-is only possible on a fomblinized pump, because no mineral oil can withstand the highly reactive compounds coming from the plasma discharge for a reasonable time. It is also evident that frequent mineral oil changes expose service personnel to risks that are difficult to evaluate. Data about the effect of a single carcinogenic material, massively absorbed, are available; but who knows the effect of the multiple exposure to multiple suspected carcinogenic materials for even low dose, for a prolonged period of time? Similar considerations are valid for ion implantation where poisonous and flammable gases such as ASH,, PH, etc. together with their decomposition products accumulate in the pumping system. When the mechanical pump is stopped for servicing or oil change, contact of PH, with air may occur, and a fire or air explosion can take place. This risk is also present with Fomblin even though reduced servicing helps to minimize the risk. But, again, Fomblin is only one component of the system. To implement safety, nitrogen bleeding and a burn-box at theexhaust are required, or electrical grounding should be considered. Furthermore phosphorus and arsenic non-volatile compounds contained in the oil and within the pump deserve special care. The chemical form of such materials is not known but inhalation and absorption through skin should be avoided.
Fomblin and use of pure silane Another situation of interest is connected to the use of pure silane (SiH,) for semiconductor fabrication. According to the Semiconductor Safety Association, most users think that in some cases the use of 100% SiH, is absolutely necessary. Reports and rumours from USA and Japan indicate that fires in ducts due to ignition by silane of oil silica deposits which have built up for years are likely to occur. Installing oil separators at the outlet of the mechanical pump is advisable. Adopting a regular cleaning procedure for silane exhaust pipes and using stainless steel instead of plastics as a duct material is also recommended. But the only effective remedy seems to be fomblinization of mechanical pumps. This has been the approach of many big semi-conductor companies, as it appears from the relatively huge investments to fomblinize hundreds of pumping groups at one time. In Japan, for instance, concern for safety has led to a change from hydrocarbon to Fomblin fluids for all semiconductor factory vacuum systems even where, technically, their use is not essential. This is in order to avoid any aspect of accidental contamination by hazardous oils in the bulk of the systems where PFPE must be used as an insurance against risk.
* This is especially true for phosgene. According to our experience, COCI, concentrations as high as 10 ppm can be detected near the exhaust valve
Oxygen service
and inside the pump. Monitoring of COCl, by Drager servicing a pump is therefore highly recommended.
As explosions
equipment
before
can
and
do
occur
in vacuum
pumps
using 511
Abstracts
hydrocarbon vacuum fluids* when pumping oxygen, the vacuum pump must be converted to use inert fluid. This can be achieved by thoroughly cleaning the pump and the rest of the pumping system (if the equipment has already been operated with a mineral oil) in order to remove hydrocarbon contamination. Then the pump should be charged with a suitable fluid. Chlorotrifluoroethylene oils are a candidate, as they have a good thermal stability and no fire or flash point. Unfortunately their viscosity index is very poor, and very often cold starting is a severe problem. Many modifications are required in order to adjust a mechanical pump to these fluids (e.g. increasing electric motor size, enlarging clearances, providing a fluid heater, etc.). Also ultimate pressure and backstreaming rate are not very satisfactory. Conversion to Fomblin oil is much easier because Fomblin oil is fabricated with viscosity-vapour pressure characteristics matching those of mineral oil and only minor modifications-if any-are required?. The use of Fomblin in this area has been approved by various national agencies such as BAM (Bundes Anstalt fur Material Prtifung) in West Germany and EMPA in Switzerland, and by private companies such as Air Products and BOC. * The most endangered area is around the exhaust valves where the gases are compressed to atmospheric pressure and can reach higher temperatures. Typical compression ratios for rotary vane-pumps are in the range of 1 x lo5 up to 1 x 106. t Attention must be paid to careful cleaning with Algofrene 113.
particles becomes difficult to identify. Close monitoring of particles is beginning to have a ‘knock-on’ effect, putting greater demands on equipment suppliers to develop machines which minimize particle generation. Careful monitoring of incoming materials is equally important. The semiconductor industry has long recognized that control of dissolved metallic impurities in chemicals is important, but only more recently has the importance of undissolved impurities been highlighted. A case study is presented which was initiated to help to identify the more important sources of particulate contamination during a typical fabrication sequence. Particulate counting was performed using a Tencor Surfscan. This machine scans the wafer surface with a HeNe laser, and records the number of scattering centres greater than agiven diameter (the minimum diameter selectable is 1 pm). The major drawback of this technique is that it cannot distinguish between surface topography, point defects and particulates. However, by measuring a given wafer before and after a particular step (in conjunction with control unprocessed wafers) the cleanliness of the step can be inferred. A variety of techniques such as scanning electron microscopy with elemental analysis facilities and X-ray diffraction were used to identify the composition of the particles and hence the source of the contamination. Results from the study of a typical photoresist track process and wet etching processes will be presented. In the case of wet etching, it was found that undissolved particles in the starting materials could become troublesome even if the supplier used a 0.2 pm filter. The source of such particulate contamination has been traced to the type of packaging material used. It is recommended that glass bottles should be replaced by plastic bottles to store chemicals for VLSI processes.
Particle control in VLSI Acknowledgements J Patel, GEC Research Laboratories The semiconductor industry has recently given significant attention to particulate contamination, motivated by the trend toward smaller feature sizes and higher packing densities in very large scale integration (VLSI). Particles with a diameter of the order of a tenth of the minimum feature size have been reported to affect the performance of an integrated circuit’. It is thus imperative to monitor and control the generation of particles during integrated circuit fabrication processes. The demand for tighter control of contamination is severe: a contamination hierarchy exists within the microelectronic industry whereby certain aspects of contamination are given extensive research and others are given second preferences. The cleanliness of water and air have obviously been given the most attention. Elaborate systems have been built to remove ionic species, organics and particulates from deionized water supplies, and advances have been made in the design of clean rooms with vertical laminar air flow and HEPA (high efficiency particulate air) filter systems. The chemical industry has also met the requirement by supplying higher purity chemicals with stringent specification on the level of dissolved metallic impurities. The tolerance requirements for particle control in VLSI technology are extremely difficult both to visualize and implement effectively since particles may be generated at each step of the process. Every piece of equipment used in the fabrication process can have its own inherent particle generating mechanism, thus monitoring should be carried out on all existing machines and between each transfer process. If no monitoring is performed, particles will not be detected until a later processing stage and the source of 512
The author is grateful to Mr H Pate1 and Miss Y West who helped to monitor the particlecontamination and Mr V Phelan and Mr T Kendrick from the Materials Characterisation Division for chemical analysis. Reference
’ J M Duffalo and J R Monkowski,
Solid Stare Techno/. p 109 (March
1984).
The nondestructive
characterization of silicon-on-sapphire films
M G Pitt, T B Peters and C Dineen, GEC Research Laboratories Silicon-on-sapphire (SOS) is the primary dielectrically isolated substrate technology for CMOS integrated circuit manufacture. However, SOS layers have a high density of defects, principally microtwins, extending from the sapphire interface. These result in considerably reduced transistor mobilities and thus reduced circuit operating speeds. The development of techniques for nondestructively assessing SOS wafer quality has aided the development of improved epitaxial growth conditions as well as providing a basis for processcontrol ofepitaxy. Two techniques have proved to be particularly beneficial in this respect: determination of twin concentration using an X-ray diffraction technique and ultraviolet (uv) reflectometry. The principal crystalline defect present in as-grown silicon-onsapphire films is twins. A technique, based on work reported by