Design, installation and qualification of minienvironments
Guideline outlines process of designing and manufacturing minienvironments
By Thomas V. Kahlden and Michael Meier
In the semiconductor industry it is currently state-of-the-art to locally enclose manufacturing equipment in so-called minienvironments. This fact has made it necessary to set up a standardized procedure, which includes all phases of the process from planning to installation and final qualification. It should help reduce the planning phase down to a few iteration steps by clearly determining the specifications in advance. Also included should be an unambiguous procedure in terms of final qualification with all necessary measuring methods.
Within the framework of the joint project Smartfab, the companies Domo Logistic GmbH and CCI-von Kahlden GmbH have established a guideline for the design, installation and qualification of minienvironments. This guideline includes items taken from the Semi-Standards 44-961 and the VDI-Recommendations 2083 Sheet 32, and is also based on our own observations, which we have made while planning, installing and qualifying minienvironments.
Minienvironment project procedures
A planner must execute a minienvironment project in accordance with a process schedule (see figure 1). The first step is to enter data into a checklist at the first meeting with the customer. The checklist will help the planner collect relevant customer data and specifications. Of special importance is the required accessibility in case of manual loading and unloading, as well as for equipment maintenance. The ideal situation in this early stage is when the customer supplies the installation schemes, as well as dimensional drawings and photos of the equipment to be enclosed.
At an on-site meeting, a planner takes measurements and discusses with the customer the handling and maintenance procedures as listed on the checklist.
Having done this, a planner makes a first CAD draft of the minienvironment. In an iteration process with as few steps as possible, he adds alterations to the draft until a drawing is approved.
Derived from the approved drawing, a planner uses the parts list of required components as a basis for making a detailed offer to the customer.
Upon receiving the written order, the manufacturer procures the material and fixes an installation date.
After the installation at the customer`s site, a planner executes an acceptance test according to a pre-determined procedure.
Checklist
The checklist, which a planner completes with the customer, includes the following emphases:
Cleanroom. This section includes the general cleanroom specifications in terms of class of cleanliness, space conditions around the machine, and requirements relating to electrostatics. It also includes particulars concerning the sort of ceiling system on which the minienvironment is mounted. A planner also finds remarks on the required air supply engineering of the cleanroom.
Process and equipment. For designing a minienvironment a planner needs particulars relating to type and topography of the equipment, as well as the contained control elements and medium supply connections. He should be able to access them easily, or reserve sufficient space for them at the enclosure. At this phase photos or drawings of the equipment are quite helpful.
Product handling during the loading and unloading process. Here we make a difference between manual and automatic handling. For manual handling, a planner should document the precise handling so that he can consider it in the design. Product handling should remain undisturbed as far as possible; product protection against contamination is a matter of emphasis.
Maintenance and inspection. With the help of drawings, a planner documents the positions of the maintenance doors on the equipment, as well as their access frequency. The maintenance processes have an influence on the design of the minienvironment.
Designing the minienvironment
The design of the minienvironment, as well as the selection of the required materials for frames, fillings (panes), and seals are based on the data of the checklist. The assignor receives documentation, which includes handling and maintenance processes, as well as concept proposals for the realization of the minienvironment. It also features an explanation of the advantages and disadvantages. These points are discussed with the service and operating personnel, then the preferred CAD design is selected. A planner automatically receives from the CAD a parts list of all components for procurement.
Installation of the minienvironment
Cleaning and packing at the manufacturer. Prior to customer delivery all materials are pre-cleaned and packed. A planner should execute cleaning under a laminar-flow tent. To achieve best cleaning results in terms of optical cleanliness, particle freedom and cleaning duration, the components are cleaned according to a fixed, optimized cleaning procedure.
Installation of the minienvironment. Depending on the circumstances in the cleanroom, installing the minienvironment may restrict production or machine performance in the vicinity. To avoid this as far as possible, a planner should plan the installation procedures ahead of time. The following points should be considered:
Time schedule for releasing equipment for installation.
Preparatory measures.
Cleaning and including materials.
Installation.
Final cleaning.
Final work.
System description and documentation. The minienvironment scope of delivery includes the following documents:
Operation and maintenance instructions — description of individual components — instructions for assembling and disassembling the most important components.
Material specifications of the applied components.
Acceptance test at the minienvironment
The assignor may request an acceptance test after the installation of the minienvironment. A planner must check conformity to the pre-determined contractual specifications.
The acceptance test is the prerequisite and basis for qualification with resulting legal consequences: The warranty time of the assignee starts, and the property goes into the assignor`s possession.
Qualification consists of four sections:
Completeness check.
Functional testing.
Risk analysis.
Functional measurements.
In principle, a planner executes a cleanroom acceptance test in one of these three conditions:
As built: Cleanroom engineering installed and in operation. This condition does not exist with minienvironments because a planner always installs the equipment first, and then encloses it.
At rest: Minienvironment installed and in operation; equipment installed, not in operation, however, no personnel.
In operation: Minienvironment and equipment installed, both in operation, with personnel and/or including SMIF in the cleanroom environment.
In the case of minienvironments, it is common to test in the condition “at rest”. The manufacturer of the minienvironment can only guarantee the cleanliness and proper functioning up to the upper part of the equipment. The minienvironment manufacturer is not liable for interference due to handling and processes. A planner records possible occurring defects in a list, and simultaneously determines the responsible authority and sets a deadline for defect correction.
Completeness check. When checking for completeness a planner should be able to prove that he has completed the delivery within the contractual scope. He should also have integrated the minienvironment considering the technical specifications of the supplier, as well as the aspects of design.
In the presence of the assignor, a planner compares the actual condition of the minienvironment with the approved drawing. He gives special consideration to testing the proper positioning of the openings for the loading and unloading process, and maintenance work.
Functional testing. Together with an operator or service engineer of the assignor, a planner checks if he can carry out the handling process at the enclosed equipment, as pre-defined in the checklist. A planner also simulates maintenance work at the equipment and tries to open the flaps or disassemble components of the enclosure at the minienvironment.
Risk analysis prior to measurements. General. To achieve a best possible acceptance test in terms of time, a planner should perform a risk analysis prior to the functional measurements. The purpose of risk analysis is to detect possible weak points of the minienvironment so that a planner can check them by making measurements. This lets him limit the number of measuring points.
Procedure. Include the following interfaces at risk analysis:
Gaps between the ceiling and the minienvironment.
Gaps between equipment and the minienvironment.
Openings in the minienvironment at loading and unloading areas.
Check these points for interference due to airflow, for example, turbulence or induction of environmental air in the minienvironment. In the case of unstable flow conditions, a planner will have to reckon with particle entry from the outside.
These potential weak points are influenced decisively, depending on the type of operation:
Normal operation with loading and unloading, including the opening of flaps (only in case of manual loading and unloading).
Normal operation, closed mini environment.
Maintenance work, opening of the required flaps.
If possible, a planner should simulate these conditions during risk analysis.
Theoretical discussion. A planner can define possible weak points by using the available CAD drawings or by inspecting the actual on-site installation situation. For preparing the functional measurement he can include these points in the measuring pattern thus avoiding a number of additional measuring points.
Practical execution. Flow visualization helps a planner to confirm or refute the alleged weak points in the interfaces named under the heading “Procedures” above (see photo 1). In case he spots a weak point, a planner should examine it in detail during the subsequent particle measurement. In this way he determines the essential measuring points for the functional measurement.
Functional measurement. Determining the preferred measurements. In agreement with the assignor, a planner determines and establishes by contract the scope of the functional measurement. The following measurements and tests may be included:
1. Flow visualization by way of fog intake. Always required!
2. Measurement of the flow velocity of air, and calculation of the change of air rate.
3. In search of particle entry from outside, scanning the gaps between ceiling and minienvironment, as well as between equipment and minienvironment.
4. Measurement of airborne particles in the minienvironment.
5. Particle measurement on the surfaces of the minienvironment.
6. Measurement of electrostatic charge of the surfaces.
Flow visualization. With this test a planner visualizes accurately by way of a fog generator the flow conditions, which have already been examined at random during risk analysis, and document everything on video or photos. Since he normally finds bright walls and ceiling systems installed in cleanrooms, and the generated fog is white, a planner might have to work with a black background (see photo 2).
Outflow into the environment. A planner should check the flow conditions at the gaps between minienvironment and equipment, as well as ceiling. Air of low class of cleanliness should not flow from the surrounding cleanroom into the minienvironment at any point. This test is especially important at the following points:
Junction point minienvironment-ceiling.
Junction point minienvironment-equipment.
Maintenance and handling openings.
A planner should check the junction points along the whole length (see photo 3).
Air flow in the minienvironment. Here a planner detects the required airflow in the inside of the minienvironment. The flow is visualized at several points depending on size and geometry of the enclosure (see photo 4).
Flow velocity measurement. With this measurement a planner determines the velocity distribution across the surface of the minienvironment, and/or detects the required or adjusted flow velocity. Keep in mind that 80 percent of the measured value should be within a tolerance of ± 20 percent, and 20 percent within a tolerance of ± 30 percent of the target value2. A planner should execute the measurement in an area with calm flow, meaning that he keep an appropriate distance to the immediate outflow area, and avoids positions directly below ceiling profiles.
A planner uses either a hot-wire probe (independent of direction) or a hydrometric vane as a measuring device. He calculates the air change rate based on the mean value of the flow velocity, and the area and/or volume of the minienvironment.
Particle measurement. Scanning the junction points at the minienvironment. In addition to flow visualization, the particle measurement at the gaps of the minienvironment helps to prove its operability. To do the measurement, a planner fastens the air particle probe at a telescopic rod and moves it along the gap at the inner side of the pane. A planner should keep the following points in mind:
The operator should not be able to disturb the measurement (proper distance).
Avoid bumping the probe against the enclosure.
To avoid air intake from outside, a planner should keep a level distance of about 100 mm to the gap.
A laser particle counter with a lower detection limit of 0.1 micron is used as a measuring device.
Airborne particles. A planner executes particle measurements in the minienvironment in order to prove the required class of cleanliness.
The minienvironment manufacturer can only guarantee the class of cleanliness when a planner performs the measurements with the equipment at rest, and/or until right above the equipment, since the process might cause various disturbances (operator access, etc.).
A planner performs targeted particle measurements at the following points of the minienvironment:
beneath the air outlet area.
above the process level.
at the outside of the minienvironment to detect the environmental con centration.
In addition, a planner executes measurements at the points specified during risk analysis.
If a planner detects particle concentrations in the minienvironment that are higher than the specification, he will be able to define the source based on the measured environmental concentration. For this purpose a planner also uses a laser particle counter with a lower detection limit of 0.1 micron.
Particles on surfaces. A planner measures the particle concentration on the glass panes of a minienvironment by using a special sampling probe. He performs several measurements per pane. Since a planner dedusts the pane while sampling, he must measure at various points of the pane.
The particle counter applied for this purpose should have a lower detection limit of 0.3 micron.
Measurement of electrostatic charges and leak resistors. There are requirements relating to ESD protection for certain processes in semiconductor production, for example in the case of wet processes. In this case, a planner must prove conformance with the specifications within the scope of the functional measurement by applying respective measuring methods.
The methods listed below are standard measuring methods, which must be modified and tested for minienvironment applications. The measurable variables gained this way, however, are the most accurate for the qualification of minienvironments.
Surface charge measurement. A planner can make non-contact measurements of the electrical charge on the surface of materials by using a field-induced measuring device. The charge is indicated in V/m.
Discharging behavior measurement of material. There are materials, which are rated as static dissipative but cannot completely reduce their electrical charge. These charges may cause quality reductions in semiconductor production.
With this method a planner does targeted charging of the surfaces by way of point ionization. After charging a planner does non-contact measuring of the potential by using a field-induced model, and records the discharging behavior over the decay time. He rates materials as being dissipative when the targeted charge is reduced to zero within one second.
Leak resistance of surface measuring points. This method3 is meant for horizontal surface applications. A planner measures the electrical resistance between the surface measuring point and the earth potential with a probe featuring a total mass of 2.5 ± 0.5 kg made of brass gilt or stainless steel, which leans on the surface, and a high-value resistance measuring device. He reads the lowest value within 15 seconds.
Thomas von Kahlden and Michael Meier work for CCI-von Kahlden GmbH in Stuttgart, Germany. They can be reached at ++49/711/78 19 43-0
References
1. Semi E44-96 “Guide for procurement and acceptance of minienvironments.”
2. VDI Guideline 2083 Part 3; Measurements in the cleanroom air.
3. ” Leitfaden für ESD-Schutz”, Fachausschub Elektrostatik VDI/VDE-Gesellschaft für Mikroelektronik.
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1) Detection of weak points by way of flow visualization. Example: From under the microscope clean air should flow out of the minienvironment to protect the product.
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Figure 1.
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2) Using a black background.
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3) Above left, flow in the minienvironment.
4) Above, outflow of air into the environment.
This paper was presented at CleanRooms Europe `98. For a copy of the CleanRooms Europe conference proceedings (available for $95 ground shipping), please contact Libbey Duggar at (603) 891-9462.