1 PURPOSE
This Code of Practice describes the recommended safety features of chemical workstations (also commonly known as wet benches, wet sinks and wet hoods) used in the semiconductor and microelectronics industries. Chemical workstations are also used in research facilities, photo laboratories, specialist electroplating workshops, optical laboratories and, in particular in the pharmaceuticals industry. Although this document deals specifically with chemical workstations as used in semiconductor manufacturing processes in the UK, the safety features may be applicable to other areas of usage. toc
2 EXPLANATION OF SEMICONDUCTOR MANUFACTURING PROCESSES
2.1 Approximately five major processing steps are universal to all silicon and related semiconductor devices: oxidation, photolithography. etching, doping (diffusion and ion implantation), and metallisation. In addition all processing steps may be preceded by chemical cleaning stages.As part of the photolithographic process, unpolymerised areas of photoresist have to be removed from the surface of the silicon wafer.This is done using a water or solvent based developer which is applied by either immersion, or spraying on to the surface of the coated wafer. 2.3 The developed wafer is then selectively etched using chemicals or by other means. 2.4 The remaining photoresist is then stripped from the wafer prior to further processing. Chemical processes are available, often carried out in temperature-controlled baths. 2.5 Chemical processes may be acid or alkali based, or may use solvents. Whatever the medium, the processes are often carried out at chemical workstations, and this Code of Practice addresses the safety features which should be incorporated.
3. DESIGN PARAMETERS
Two criteria must be met during chemical workstation design, namely, the protection of personnel (operators, maintenance technicians, engineers, etc.) from exposure to process chemicals and emissions, and protection of the product from ambient contamination.
3.1 ACID WORKSTATION
This type of workstation shall be constructed of material which is resistant to the acids or other corrosive chemicals likely to be used, preferably with fire retardant properties (e.g. fire retardant polypropylene).
3.2 SOLVENT WORKSTATION
All solvent workstations shall be constructed to minimise the risk of fire. The main carcass material should, where reasonably practicable, be constructed from stainless steel. Should a thermo-plastic material be used for carcass construction, anti-static and fire retardant properties will be essential. In addition, other precautions will also be necessary (e.g. use of stainless steel liners)
3.3 BATHS
Vessels within the workstation, containing chemicals, may be known as baths, tubs, tanks, or 'hot-pots`. For the purpose of this document the term 'bath' will be used throughout. 3.3.1 Baths must be constructed from materials compatible with proposed contents and conditions of use. A process may require the use of aggressive chemicals or may require heating. 3.3.2 Where baths are manufactured from thermoplastic materials construction may be achieved by any suitable moulding process. For baths constructed from sheet material all joints should be welded and shown to be leak-free. 3.3.3 Where baths are manufactured from stainless steel, all joints must be welded and shown to be leak-free. 3.3.4 Baths may be stand-alone units, or may be connected to a recirculation, reclaim or drainage system. Where piping is to be connected directly to the bath, joints must be leak-free. Joints should be made by seal-welding in the case of thermoplastics. Metal to metal welding of a stub section should be used in the case of stainless steel baths. 3.3.5 Any couplings used in pipework systems must be compatible with the bath contents. Particular care should be taken in the choice of gaskets, o-rings or any other coupling components to ensure their suitability for the conditions of use. Baths may be an integral part of the work surface or may be removable units suitably mounted in the work surface. If the bath is supported by means of a flanged lip, then the lip must be adequately sized and suitably joined in order to support both the bath and its contents. For larger baths base mounting may be preferable.
3.4 THE PLENUM
3.4.1 A 'plenum' (see illustrations, Appendix A) forms part of the chemical workstation construction as it provides working volume for the installation of baths and their ancilliary services, a containment for spillage, overflow or component failure and a means of fume extraction. 3.4.2 It shall be constructed of a suitable material compatible with the chemicals in use, and must be fully leak-tight to prevent any ingress of chemicals into the structure of the chemical workstation. 3.4.3 The plenum must have sufficient strength and rigidity to support the weight of fluids it may be required to contain. If in addition, it is required to bear the weight of baths and other components, adequate strength must be provided in the construction, usually by the addition of a supporting framework. 3.4.4 All joints in the construction of the plenum must be properly welded and shown to be leak-proof. In view of the arduous conditions of load-bearing and flexing the plenum will need to withstand, it is unlikely that a single bead weld will be adequate. Triple bead welding is considered to be more appropriate. 3.4.5 Special attention is required where pipework or other services pass through the base of the plenum. Liquids or other contamininants may be present in the plenum due to leakage or spillage from the work surface These contaminants may in turn leak via the feed-throughs to the area underneath the plenum. Use may be made of upstands (see diagram, Appendix A), where flexible pipework, cabling or other services are to be brought through the base. 3.4.6 Consideration should be given to providing installed wash-down facilities within the plenum for the purpose of washing away any spillage residues. 3.4.7 Where components (such as valves. level sensors, thermocouples, actuators and sensing devices), require wiring to run through the plenum, then adequate sheathing protection is necessary to prevent the ingress of aggressive liquids or fume. Components mounted within the plenum must be similarly proof against such ingress. 3.4.8 As far as possible, mains voltage services within the plenum itself should be avoided. Preference is given to either the use of non-electrical alternatives (e.g. pneumatically actuated valves), or use of ultra-low voltages. Where the use of mains voltage cannot be avoided for heaters, then the electrical installations must be adequately protected. An unsatisfactory installation may be in breach of the Electricity at Work Regulations 1989.
3.5 DRAINAGE 3.5.1
Due consideration should be given to the siting of drainage lines and associated components. They must be accessible for inspection and maintenance purposes. 3.5.2 Acid and solvent drains must be kept separate. Acids should be drained to a suitable neutralisation point, solvents should be drained to dedicated solvent waste tanks. In all cases drain lines must be constructed of a material compatible with the fluid in transport. 3.5.3 Only fully compatible chemicals shall be run into common drainage lines. Whilst this code refers to 'acid' workstations, other non-acid materials such as alkalis, cyanides or strong oxidisers may also be used in chemical workstations. 3.5.4 In certain cases it may be necessary to use incompatible chemicals in different areas of a workstation. In such cases provision must be made within the plenum for total separation of the drainage catchment areas under each position where those chemicals are being used. Where baths are of the free-standing type, (i.e. no external drains or couplings), these may be emptied by aspiration using an installed water jet pump. In such cases, full provision of suitable compatible pipework shall be made to enable the aspiration process to be carried out safely and effectively. The advantage of aspiration techniques is that dilution of the chemical being ejected occurs at the point of aspiration before it reaches the drainage system.Where aspiration is used for the emptying of baths containing chemicals with a high heat of dilution (e.g. concentrated sulphuric acid) provision should be made to prevent syphoning or back-flow. 3.5.6 Ball valves, diaphragm valves or air operated shutter valves in the drain lines may become fouled by debris from the process preventing positive closure. Where this is likely to occur suitable filters must be inserted in the drain line upstream of the valves. These must be accessible for inspection and maintenance purposes.
3.6 FUME EXTRACT SYSTEM
3.6.1 The design of the fume extract system will depend on the application involved, but in all cases must be sufficient to prevent hazardous fumes and vapours from reaching the operatorís breathing zone.Efficient fume extraction is also necessary to prevent damage to equipment from corrosive fumes and to avoid build-up of flammable vapours. 3.6.2 In a conventional laboratory fume cupboard, fume containment is achieved by ensuring sufficient extraction airflow across the face of the fume cupboard, usually in the range 0.5 to 0.9m/s (100-175 fpm). Open faced chemical workstations in maintenance and chemical handling areas outside the cleanroom, in which vertical laminar flow (VLF) clean air is not provided, should meet this criterion. Chemical workstations in the cleanroom rely on high capture velocities at the worksurface being achieved through worktop and plenum design, and do not require such high face velocities. On the contrary, high face velocities are avoided, to minimise the amount of particle contaminated air liable to be drawn in from the operator area. Units with integral VLF canopies are set up so that the extract volume flow exceeds the clean air flow through the filter by about 10% This excess flow provides an ingress or air at the front edge of the workstation to avoid the escape of fumes, but limits the amount of dirty airflow to the front of the work surface. The work within the station and the layout of baths can be planned to avoid this front edge. The volume airflow required for efficient fume capture in free-standing cleanroom chemical workstations should be determined by design study and airflow pattern with smoke tests during commissioning, as it will vary with worktop layout and plenum construction. As a guide. [for preliminary estimation only], a volume airflow of up to 80cfm per square foot of worksurface is a typical requirement. 3.6.3 All stations should have physical barriers (sometimes known as eye shields or face shields) to provide splash protection and to enhance fume extraction. These shields should be constructed of at least 6mm thick clear PVC or equivalent and should be easily removable for maintenance access. Eye shields on solvent stations should be constructed of wire re-enforced glass or stainless steel for fire protection. 3.6.4 As well as fitted eye shields, chemical workstations should be equipped with full top and side panels, where the basic module design allows it, to maximise local extraction ventilation performance. The fume extraction should be capable of controlling both emissions from the baths and any emission resulting from process transfer. 3.6.5 Provision should be made in the extract ductwork for drainage of residual liquids. Spray from processes within the workstation, when extracted, tend to accumulate and will flow to the lowest point in the extract system. 3.6.6 Provision must be made in the extract ductwork for airflow measurement and adjustment. The fitting of airflow or pressure sensing devices to detect extract failure, linked to an audible alarm system, is recommended. 3.6.7 Where chemical workstations are fitted with integral VLF canopies, it is recommended that the power supply to the blower fan in the canopy is interlinked with the extract failure alarm to avoid fumes being blown into the cleanroom in the event of extract failure. 3.6.8 Extract and VLF adjustment controls should be designed and mounted to avoid unauthorised adjustment.
3.7 LAYOUT
3.7.1 Where possible, the process baths should be in a single line. It is important to consider the variable length of reach of the workforce; baths in tandem may tempt the operator to duck below the chemical workstation safety shield in order to reach the rear bath. Where it is not possible to rearrange the baths in this way, it is important that the baths containing particularly hazardous chemicals are sited to the rear of the working surface. This will avoid the need for operators to reach over hazardous materials when placing work in the baths at the rear of the chemical workstation.Rinse baths of de-ionised water should therefore be situated at the front of the chemical workstation working surface along with other ancilliary items such as glove wash units, spray guns, vacuum wands, etc. 3.7.2 All baths shall be clearly marked to indicate their contents. This may be achieved by the use of indelible signs located on the front of the module. 3.7.3 Drainage pipework should be designed using the minimum number of joints in order to lessen the risk of leakage. Provision should be made for drainage and thorough flushing out of all chemical pipework prior to maintenance or modification. 3.7.4 Chemical workstations should not be equipped with storage racks on the rear face of the module, as these could constitute a splash hazard should any items fall from the shelving into the chemical baths. 3.7.5 When standing at the workstation an operator needs to be as close as possible to the front edge of the working surface. To facilitate this. provision should be made for a recess along the bottom edge of the module enabling the operator's feet to fit comfortably under the module. 3.7.6 Careful consideration shall be given to the positioning of controls such as process timers, displays, warning butters, temperature controllers, automatic dump/rinse controls. This should take into account accessibility for the operation and maintenance of such controls and the fact that such controls may need to be protected from any aggressive fumes or splashes.Visual displays should be located at eye level above the rear of the relevant bath. Where control devices are necessary, the means of operating these, whether pneumatic or electrical, should be situated on the front panel below the work top level. To allow maintenance to be carried out outside the clean area of the workstation, consideration should be given to siting controls electronics, valves and other ancillary systems so as to be accessible in the adjoining service chase adjacent to the chemical workstation module.
3.8 REDUCING RISK OF PERSONAL EXPOSURE TO HAZARDOUS CHEMICALS
Chemical workstations are often used as a means of avoiding personal exposure to hazardous materials, in compliance with the Control of Substances Hazardous to Health Regulations 1988 (COSHH). This section of this Code of Practice addresses the design parameters for physical controls; the use of Personal Protective Equipment (PPE) will be pursued further in section 8. 3.8.1 Immersion and agitation of jigs or boats carrying process product in baths should be automated whenever possible. Automation minimises the risk of hazardous chemical exposure to personnel. 3.8.2 Pouring of chemicals can be avoided by the use of closed chemical delivery systems (see 3.13). When pouring and mixing are unavoidable, the chemical workstation should have sufficient free space available for that operation to be carried out with minimum risk to the operator. Allowance must be made for any fume generation, which may occur during open pouring/mixing processes. 3.8.5 Where chemical delivery systems, and waste collection systems are located separately from the chemical workstation, fume extraction may be necessary to minimise exposure during filling and venting. 3.8.6 Air flow patterns at the workstation should be monitored at frequent intervals. One possible way of doing this would be by using proprietary ësmokeí tubes to show the effectiveness of fume extraction.
3.9 STORAGE
It may be practical and convenient to design chemical storage in to the chemical workstation module, below the work surface. If so, suitable provision must be made for spillage containment and fume extraction.
3.10 FIRE PRECAUTIONS
3.10.1 All chemical workstations should have the means to prevent the spread of fire through the exhaust ductwork. This may be through the use of sprinkler systems, cut-off dampers, etc.Where sprinkler systems are to be used, it is important that these are incorporated into the chemical workstations at the design stage, as retrofitting is difficult and costly. 3.10.2 Where fire suppression systems are fitted, they should be connected to an independent electrical supply, separate from the chemical workstation and should be labelled accordingly. Consideration should be given to the provision of an un-interruptible power supply. 3.10.3 The Montreal Protocol is phasing out the production of halons. Where new systems are to be installed the use of halons as an extinguishing medium is not recommended. 3.10.4 As an effective risk management measure, consideration should be given to fire resistant trunking in extract systems. A chemically resistant ductwork, which is also fire resistant, is commercially available. 3.10.5 Where fire detection systems are in operation, periodic testing is essential.
3.11 LABELS
3.11.1 It is essential to set up a clear system of labelling for chemical workstations and baths to show the chemicals in use and associated hazards.Labels must be easily read at normal working distances, and resistant to chemical damage. They should be positioned carefully to avoid any ambiguity or uncertainty. Suitable warning signs should be posted to indicate all the chemical hazards present in the workstation. Emergency and First Aid measures (such as showers, eyewash and emergency shut-off) should all be clearly marked.All safety signs must comply with the Safety Signs Regulations 1980 (SI 1980/1471), and must conform to British Standard 5378 in colour and design. Note that new Safety Sign Regulations are expected in 1994, implementing the European Parliament Safety Sign Directive 92/58/EEC.
3.12 ELECTRICAL SAFETY
3.12.1 Protection from hazardous electrical potentials in a chemical workstation must be achieved, wherever necessary, by measures such as earth bonding of exposed conducting materials. The fitting of circuit breakers for circuit protection and the use of residual current devices are recommended. 3.12.2 Consideration may be given to the provision of low voltage supplies (e.g. supplies from a 11OV centre-tapped transformer or isolation transformer). 3.12.3 All electrical components and wiring should be clearly identified by labelling, colour-coding and numbering, with reference to "as installed" drawings and wiring diagrams 3.12.4 All electrical components should accessible for repair and maintenance connections should be readily accessible for repair and maintenance. Electrical connections should be shrouded. 3.12.5 All electrical equipment must be suitably protected against ingress from moisture, flammable liquids and vapours and from corrosive fumes. 3.12.6 Lighting over chemical workstations must be adequately housed to prevent corrosion of electrical fittings. Trim and other components that are likely to corrode should be avoided to prevent contamination of the chemical workstation by falling debris. 3.12.7 All wiring should preferably be multi-stranded. 3.12.8 When mounting Emergency stop buttons, reference should be made to BS 5304. Safety of Machinery, Section 5, for details of correct positioning of such devices. 3.12.9 Consideration should be given to inert gas purging of electrical/ electronic control equipment, where necessary, to prevent ingress of fumes, vapours, etc
3.13 CHEMICAL SUPPLY UNITS (CSU's)
3.13.1 Many of the more modern chemical workstations have chemical supply systems installed. These have the advantage of avoiding the hazards associated with carrying and pouring chemicals by hand, and can be designed to minimise product contamination. CSUís may be located adjacent to the chemical workstation, or may be installed in service chases or chemical handling rooms at a distance from the cleanroom (even on different floor levels). 3.13.2 Chemical supply lines should be installed within containment pipework or ducting, which should be connected to suitable drainage. A leak detection system should be provided. Additional leakage containment should be installed where pipework runs over occupied areas and walk-ways. 3.13.3 Pumped and pressurised systems should be designed to include means of over-pressure protection. Where corrosive and otherwise hazardous materials are involved, all components of the system should be suitably housed to protect personnel in the event of component failure. 3.13.4 Valving should be provided for depressurisation and isolation of pipework in the event of component failure. Removable enclosures (doors, access panels, etc.) should be interlocked to depressurise/isolate the system on removal. 3.13.5 Where highly flammable liquids are to be dispensed, suitable containment (housing, co-axial pipework) must be provided to control spillage. Ventilation must be installed as necessary to prevent the accumulation of flammable vapours. Precautions (such as electrical bonding and choice of suitable materials) should be taken to avoid the accumulation of static charge.
4 HEATERS
When electrical heating elements are used to heat liquids in baths, special precautions must be taken to prevent overheating. Failure of an automatic temperature controller to shut oft the power to a heating element can cause increased vaporisation of the liquid in the bath. This can cause a drop in the liquid level and subsequent exposure of the heating element. If the liquid level drops below the heating element, the liquid will begin to cool causing a continuous "on" signal to the heating element control. This will further accelerate the overheating which can result in:- 1. Ignition of:(a) the liquid in the bath(b) the bath or bath pipework(c) residues in the bottom of the bath. 2. Chemical decomposition of:(a) the liquid in the bath the bath or bath pipework(c) residues in the bottom of the bath. Generation of noxious fume or vapour.4. Ignition of exhaust ducts. 4.2 To avoid the dangers of overheating, the following control measures should be incorporated.Operating controls should include:A liquid temperature sensor and controller;A liquid level sensor with automatic heater cut-off;An independent excess temperature sensor linked to automatic heater cut-off. Additional safety features may include a heater sheath over-temperature control, and the use of programmable timer switches to cut off power supply outside normal working hours.
5. EYE-WASH STATIONS AND SAFETY SHOWERS
5.1 Whilst eye-wash stations and safety showers may not be considered to be a part of chemical workstation construction, the provision of such items shall be allowed for in the layout of a total chemical workstation facility. An eyewash station should, ideally, be situated as close as practicable to all chemical workstations. If it is possible to design an eyewash station into a chemical workstation module then every precaution must be taken to ensure - (a) that the position is adequately protected from the presence of any nearby hazard, and (b) that the position is clearly designated as an eyewash station not to be used for any other purpose. 5.2 Safety showers shall also be situated as close as possible to the chemical workstation area where work with hazardous chemicals is being carried out. An ideal position for a safety shower would be between two chemical workstations as a overall part of the construction with an exit from the shower unit passing into a service or less critical area. This way the clean room environment would not be compromised during any emergency involving the use of the shower.
6. MAINTENANCE PROCEDURES
6.1 As previously stated in the introduction to section 3.8, chemical workstations are often used to ensure compliance with the COSHH Regulations. If so, a systematic programme of examination and testing of the workstation is essential. The recommendations in the Approved Code of Practise relating to Regulation 9 of the COSHH Regulations should be followed.Fume exhaust systems and plant should generally be thoroughly examined and tested at least every 14 months, and key performance characteristics, such as volume flow and static pressure, compared with design parameters. Detailed records of tests should be kept, and should be retained for at least five years.During maintenance and inspection the risk of exposure to chemical and other hazards may tend to be greater than in normal use. Precautions should be taken to minimise this risk.For example, formal written procedures should be established which will alert personnel to potential hazards, and which set out appropriate measures such as the wearing of personal protective equipment, decontamination, electrical isolation, shut-down of dispensing systems, and posting of warning notices.Consideration should be given to instituting a formal "Permit-to-Work" scheme. 6.3 Functional tests on any installed safety or risk management systems should be carried out periodically. These systems may include:-Fire Protection System, including fire sensors, alarm sounders. and damper operation [Note that extinguishant discharge must be suppressed during test.]Emergency Off ButtonExtract Failure AlarmLeakage Detection System. 6.4 Electrical testing of the workstation should include earth loop impedance tests, and trip current and trip time measurements on RCDs. 6.5 A visual inspection of the workstation should also be carried out periodically, and should include:-- Removal of worktop panels for examination of the plenum- Removal of any debris in plenum or elsewhere, liable to cause obstruction to extract or drain apertures- Inspection of carcasses and ductwork for physical damage and deterioration- Inspection of baths, quartzware and accessories for damage and deterioration- Inspection of plenum base, storage base, ducting and other spillage containment for signs of leakage, which may indicate weld failure or other damage.
7. PERSONAL PROTECTION
In addition to the comments in Section 3.8 of this Code of Practice, it is important that the use of proper and appropriate personal protection, when working with hazardous chemicals in chemical workstations, is thoroughly understood. Effective protection is only achieved by use of suitable PPE, correctly fitted, maintained and properly used. 7.2 Care must be taken to ensure that whilst wearing PPE the operator is not put at risk through limited mobility, or limited visibility. The operator must be free to perform tasks within the confines of the chemical workstation, which may require the lifting of heavy components. 7.3 Consideration should be given to the design and layout of the workstation with the possibility that respiratory protection equipment (RPE) may be required to be worn for specific operations. 7.4 Both the PPE at Work and COSHH regulations require employers to provide appropriate PPE and training in its usage to their employees whenever there is a risk to health and safety that cannot be controlled by other means.
8. BIBLIOGRAPHY
The following publications may provide useful information and guidance relating to the safe use of chemical workstations. Approved Codes of Practice (available from HMSO and HSE Books)L5 - Control of Substances Hazardous to Health (General ACOP) and Control of Carcinogenic Substances (Fourth Edition) 1993 HSE Guidance NotesHS(G)37 Introduction to local exhaust ventilationHS(G)51 Storage of flammable liquids in containers.HS(G)54 Maintenance, examination and testing of local exhaust ventilation. British StandardsBS7258: 1990 Laboratory Fume Cupboards Other PublicationsControlling airborne contaminants in the workplace; British Occupational Hygiene Society, Technical Guide No. 7; Science Reviews Ltd. 1980.Control of Chemical in the Workplace ñ a Guide for Managers; Hayes and Iungus, Tolley Publications Co. 1993 AddressesHSE Books, PO Box 1999, Sudbury, Suffolk CO10 6FS.Tel: 0787 881165; Fax: 0787 313995.
APPENDIX A: 1 TYPICAL CHEMICAL WORKSTATION ILLUSTRATION
Code of Practice - SAFETY FEATURES OF CHEMICAL WORKSTATIONS
