Topic: Clean Workstations

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Definition

A wet deck is a clean workstation where wet processes such as acid cleaning, acid etching or rinsing are carried out by immersing the workpiece in high purity chemicals or de-ionized water.

Design Considerations

By definition a wet deck presents a major conflict between established ideas in that clean workstations are usually achieved by supplying the work area with filtered clean air, probably in laminar flow streams and slightly pressurizing the work area to prevent ingress of ambient air, while acid processes are usually contained in extracted booths and, for the safety of the operator, are kept at a negative pressure to prevent egress of harmful vapour or fume towards the operator.

At this point it is useful to remember that the principle of "laminar flow" is based on the fact that air travelling in the same direction, in parallel streams at uniformly low velocity and contained within a constant cross-sectional area is less subject to turbulence and cross stream contamination. This feature is used to advantage on processes which involve the generation of particulate contamination at the workpiece because the airflow carries the offending particles away from the work area. Vertical laminar flow streams have the additional advantage of gravity which assists the falling and settlement of particles away from the workpiece.

"Uniformly low velocity" is quantified by Federal Standard 209 as being 90 feet per minute with a tolerance of + or - 20 feet per minute, although this can be established by test for individual requirements. Since the "constant cross-sectional area" is disrupted by the workpiece and necessary equipment, it follows that some turbulence must be generated and therefore optimum conditions will be achieved at the lowest velocity which produces laminar flow at the work area.

By comparison, let us remember that the principle of containment of hazardous and toxic chemical substances as practised in laboratory fume cupboards depends on a uniformly low velocity of air passing through the working aperture at the face of the cupboard to entrain harmful vapour and fume in an evenly distributed flow away from the operator. In this situation, uniformly low velocity is less stringently defined by British Standard "Draft for Development" DD80 as being between 0.3 metre per second and 1 metre per second (60 feet per minute and 200 feet per minute), although it is acknowledged that the higher flow rate round the operator's body and arms can generate turbulence and induce currents back through the working aperture. Not surprisingly, it seems that lower velocities such as the laminar flow velocity of 90 feet per minute will generate the least turbulence round the operator and can be expected to entrain particles shed by the operator into the airstream.

It can therefore be reasonably assumed that the interests of the operator as well as those of the process are both served by laminar flow velocity which leaves the direction of the flow as the principal problem to be solved.

Ideally, to satisfy the requirements of process and operator, it would be necessary to generate two divergent laminar clean air streams at a point between the operator and the process. Since this is impractical, the next best alternative is a vertical downflow laminar flow air curtain introduced between the operator and the process, arranged to provide a split flow and carry process contaminants into the exhaust system and carry operator contamination to the floor away from the process.

General Design Concept

It must be realised that the choice of wet deck design will significantly affect the overall cleanroom design because safe recirculating systems, for the variety of chemical contaminants generated in silicon wafer processing, are not economically viable. Therefore, extraction volumes are a total burden on the air conditioning which not only affects the capital cost of the plant but also the running costs for ambient conditions.

In some circumstances, it may be economically attractive to supply wet deck laminar flow hoods with clean air, non-humidity controlled, with appropriate savings in running costs. The process must be assessed to determine the degree of cleanliness required and whether ingress of ambient air to the work area can be tolerated. For applications where the ingress of ambient air cannot be tolerated under any circumstances, then the wet deck should be treated as a sealed cabinet with airlocks and gloveport operation.

Where an open-fronted wet deck is to be used, the air flows must be arranged to guarantee that air does not escape from the work area.

To achieve maximum cleanliness, an oversize vertical laminar flow hood can be used to overhang the front of the deck to provide an air curtain past the opening which can be drawn into the deck by the extract and preferably through a by-pass at the front of the work surface.

On less critical applications it may be sufficient to position a standard vertical laminar flow hood forward of the front of the deck to provide an overhang and similar curtain effect. Alternatively, the curtain effect can be artificially induced by strategically placed baffles in the laminar flow stream, or visors, to ensure that any air flow which can entrain harmful vapours or fume moves away from the operator.

From Table 1, it can be seen that there are several basic designs which can be tailored to most requirements and their relative merits or demerits easily compared.

Details of Design

Having chosen the class of wet deck to be installed, attention can be given to the detail design and construction of the laminar flow hood, the wet service module and the equipment layout.

The Vertical Laminar Flow Hood

The Vertical Laminar Flow Hood should be a purpose deigned unit constructed in materials capable of withstanding exposure to the range of chemicals to be used in the accompanying wet service module. Particularly, the use of metals, absorbent materials or absorbent surfaces should be avoided whenever possible to reduce the

materials which meet most requirements and give the added bonus of extended life.

Provision for supporting the VLF should be made in the design of the wet service module, although suspension from the ceiling is acceptable (steel C-frames should be avoided).

In addition to housing the necessary HEPA filter, pre-filter, variable speed fan and fluorescent lights, consideration should be given to the possibility of locating sensitive control gear for deck process equipment in the VLF above the work level and clear of spillages or contamination from the process. For convenience, frequent switching operations can be achieved by fitting air bulbs at waist height or air pedals to activate air switches connected to the control units. Pre-filters should be sited to draw air from areas which will have the least effect on the air flow at the face of the wet deck. Where "oversize", "asymmetric" or "offset" VLF's are used, side screens should be fitted to discourage passing traffic close to the work aperture and consequently minimize turbulence.

The Wet Service Module

The Wet Service Module should also be a purpose designed unit, constructed in similar materials to those used on the related VLF, but with special attention given to the process temperatures, live loads and potential high stress points. Again the same materials should be avoided as listed for the VLF, and in this case polypropylene is recommended as the most suitable economic choice, although process conditions may dictate that alternative materials be used in selected areas within the WSM.

The primary function of the WSM is to safely contain the resident individual processes in clean sanitary conditions and careful design is required to ensure that contamination generated by such processes, both airborne and liquorborne, is removed from the working area without risk of accumulation, subsequent cross-contamination or injury to the operator.

Because volumetric supply services to the WSM, excluding ventilation air, are normally piped and can therefore be dispensed at any point within the WSM, the volumetric discharges must be one of the major influences on WSM design. These discharges are usually exhaust condensation, acid waste, alkali waste, waste water dump, de-ionized water recycle, solvent drain and ventilation exhaust. Since exhaust condensation, acid waste, alkali waste, waste water dump can be plumbed together, there remains four separate discharges to be incorporated into the WSM.

For flexibility, the current practice is to design the work surface as interchangeable plates on a fixed pitch which are each tailored to accommodate the particular process units at that location. Process baths, cascade rinse tanks, dump rinsers, hot plates and glovewashes are then installed flush with the work surface for ease of operation.

Containment of liquid wastes is achieved by constructing a sump below the work surface, into which the contents of the process units are allowed to overflow or are dumped and exhaust condensation and spillages can drain.

Segregation of the liquid wastes within the sump is achieved by incorporating baffle divisions in the sump to form separate compartments which can be individually connected to the appropriate drain.

It is important that all service pipes, tubing or electric cables which require to pass through the sump are sealed by welding or glands to prevent leakage to the compartment below the sump.

Note: It is not sufficient to pass services through standpipes because the air in the sump can have a high vapour content and be at a higher temperature than the normal room air which will result in condensation on service pipes and tubing which then drains off in the lower compartment.

Ventilation exhaust is more complicated to design because it must be complementary to the basic wet deck design class chosen. It is not sufficient to assume that, because "Hot-pots" or chemical baths are located at the rear of the work surface, fumes cannot escape from the WSM. Air directed at a flat surface generates turbulence which makes it possible for delinquent streams to come towards the operator which can contain entrained vapour or fume.

From the Exhaust Ventilation Table 2, it can be seen that there are five basic design arrangements and these are identified as compatible with the appropriate wet deck classes. In addition to the tabulated data, there are some special features related to certain design which should be borne in mind when adopting the ideal match for the overall airflow pattern.

For example, the type "a" exhaust which features the front by-pass, requires that the rear extraction port be designed to produce an equivalent pressure drop to that pertaining at the front port in order to establish a balanced airflow within the work space. This is easily appreciated by remembering that negatively pressurized ducts will draw from the source of least resistance.

The type "b" design depends on slots cut in the walls of the process baths to remove waste air from the work space and is therefore heavily dependent on the process engineer choosing an evenly distributed layout of equipment to ensure that the wet deck airflow is not streamed. In this situation, as with type "a", the sump performs the role of extract plenum with the attendant risk of cross-contamination from convection currents escaping from the warm humid sump environment if the extraction fan is switched off or fails. A further characteristic of the type "b" exhaust is that the close proximity of the extract slot to the surface of the bath solution can stimulate evaporation, resulting in excessive chemical loss and the generation of disproportionate levels of fume and duct condensation.

A major pitfall to be avoided with type "b" exhausts is the assumption that drainage perforations can fulfil the role of exhaust slots and the consequent conclusion that the slots in the bath wall are not necessary. There are two main reasons why perforations cannot meet the exhaust requirement of a wet deck and the first is that the basic surface area left, after locating process units and allowances for support rails and services, is rarely sufficient to give 5% free area. The second reason is that, for airflow purposes, a work surface perforation must be treated as a sharp edged orifice with the maximum entry loss coefficient of 1.78 which is further increased by a second entry loss when the sump performs the role of a plenum extracted by a flanged duct to give a cumulative loss of 2.3. Work surface perforations therefore only provide drainage to the sump and are no substitute for a properly designed extract arrangement.

The type "c" exhaust is a multiple slotted rear plenum which operates on a low slot velocity and is only suitable for use in wet decks with baffled laminar flow streams which will prevent short circuiting of the air supply into the extract. For maximum effectiveness, the slot sizes should be graded to equalize the resistance to flow and maintain a balanced extraction within the work space. Ideally, an adjustment facility could be provided to allow for varying process conditions.

In general, this is a flexible arrangement which can service a wide variety of equipment ranging from flush fitted baths to hot plates with beakers generating fume well above the work surface.

The type "d" exhaust is readily identified as a high velocity slot extract which, as the description suggests, operates on the principle that a high velocity stream of air passing over a surface will entrain fume generated at or below that surface. Although the effective operating velocity for this type of exhaust is usually 2000 feet per minute, the operational characteristics are such that the velocity rapidly decays and it has been found to be an acceptable system in non-baffled airflow wet decks.

It is worth noting that a properly designed high velocity slot extract can influence the air flow characteristics of the deck because of the volume of air required for containment. The size of the slot is usually calculated on the basis of 1 inch height per foot of work surface to be drawn across, with a minimum of 2 inches.

Applying these figures to a 2'6" deep work surface results in an extract volume requirement of 416 cubic feet per minute per foot length of wet deck which, when compared with the extract volumes tabulated against the wet deck classes, is seen to exceed the volumes necessary to control the air flow. Special care must therefore be taken to provide the appropriate volume of conditioned air, otherwise the operator contaminated air may be drawn into the work area.

The type "e" exhaust is a variation on the slotted rear plenum exhaust which offers the same features, but with the added advantage of full access to the rear plenum for cleaning and also the facility to independently adjust the individual slot sizes to suit the fume generation pattern of any particular process.

A further advantage of this design is that a chemical vapour coalescer and mist eliminator can be located in the rear extract plenum to prevent accumulation of chemical cocktails in the extract ducting with the attendant hazards and risk to maintenance personnel. This coalescer may be installed for dry operation, or irrigated for maximum effect with the irrigation liquor draining into the sump prior to discharge.

The Equipment Layout

The process engineer will obviously dictate the ergonomic layout of the equipment in the WSM, but the final decision should take cognisance of any possible detrimental effects on the air movement within the work area. As a general rule it is wise to site potentially hazardous processes at the rear of the work surface with rinsing tanks and less hazardous equipment at the front of the work surface. This serves the dual role of protecting the operator while keeping processes which generate the most contamination, downstream of the cleaner procedures.

The orientation of the equipment which protrudes above the work surface, or has lift-up access covers, should be such that the minimum obstruction is presented to the flow to the extract port, especially in the circumstances of covers where the main risk to the operator occurs when opening the cover.

Service equipment such as glovewash units, nitrogen dusters, de-ionized water spray pistols and control valves should be positioned as conveniently and unobtrusively as possible towards the front of the work surface. Aspirators and chemical dispense points should be sited at the rear of the work surface with their control valves placed within easy reach from the front of the work surface for safe operation.

Multiple Wet Deck Installations - Integration

Since very few installations involve only one wet deck it is usual to manifold the individual ducts from each wet deck into one system with one extract fan, to avoid the cost and inconvenience of separate duct systems. This may be the simplest method for installation, but operationally it gives rise to problems at individual wet decks, because it is difficult to ensure that equal volumes of exhaust are removed from each wet deck.

To over come this problem , dampers are fitted in the duct from each wet deck which can be adjusted to increase the resistance to flow in that duct and so reduce the volume being extracted at that wet deck. Obviously the wet deck closest to the extract fan will be subject to the highest pressure and therefore the greatest suction, which means that, if the operator on that deck opens the damper fully, then the other decks will suffer a loss of volume.

The problem can be further aggravated by the fact that the fans in the VLF's can have variable speed motors to allow the speed to be increased to compensate for particulate build-up in the HEPA filter and this facility is open to abuse by unwitting or obstreperous operators. It is therefore preferable that these controls should be sited out of easy reach to avoid interference by enthusiastic meddlers who may believe that the velometer readings are inaccurate and decide to have a go independently.

Another possible solution is to design the extraction ductwork as a through system with a balance dampered inlet of fresh air at one end and the conventional extract fan at the other. The wet decks are then fitted with their own separate variable speed fan which discharges into the main manifold dust and can be adjusted as required, independent of the other wet decks on the same system.

Conclusions

From this brief guide it will be appreciated that the subject of wet deck design is complex and requires an expert team of engineers to conceive and specify a design which resolves apparent conflicts of interest, and produces the optimum design for the particular application. Close co-operation is necessary between all interested parties, and to avoid oversights, it is useful to prepare a data sheet for all wet decks as per the typical layout shown.

The choice of manufacturer for the wet decks should be given careful consideration and, apart from normal financial constraints, should be selected on technical expertise in wet deck design and not just on ability to manipulate the specified materials.

New developments will continue to take place, and these will be most beneficial if the basic design principles are not ignored.

 

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