Material transfer is the boundary that defines the cleanroom. Air showers, gowning rooms, and pass boxes are the three elements that control what crosses the boundary, and the wrong design is the most common reason a cleanroom fails the in-operation particle count. The design intent is straightforward: every item that enters the cleanroom has to cross a defined boundary, and the boundary has to remove the contamination that the item carries. The implementation is harder than the intent, and the failure modes are well documented in the audit findings of programs that did not get the design right.
This article is a working guide to the design of the pass box, the airlock, and the material transfer sequence. It assumes the cleanroom is already running with a defined contamination control strategy and a working cleaning and disinfection program. The material transfer design is the input to both, and the wrong design is the most common reason the in-operation count is higher than the at-rest count.
The Three Elements: Pass Box, Airlock, and Air Shower
The material transfer sequence has three elements, and the three elements are the input to the in-operation particle count. The first element is the pass box, which transfers small items between zones of the same class. The second element is the airlock, which transfers personnel and large items between zones of different classes. The third element is the air shower, which removes the particles that the personnel and the items carry. The three elements are linked, and the link is the contamination control strategy.
A working material transfer sequence has the following elements:
- Pass box. A pass box is a small chamber with interlocking doors, used to transfer items between two adjacent zones without the operator crossing the boundary. The pass box is purged with filtered air between transfers, and the purge time is the input to the transfer time. A pass box is the default for items that are too small to require an airlock but too large to pass through the wall directly.
- Airlock. An airlock is a larger chamber with interlocking doors, used to transfer personnel and large items between zones of different classes. The airlock is pressurized to a defined value, and the pressure is the input to the cascade. An airlock is the default for personnel and for items that are too large for a pass box.
- Air shower. An air shower is a chamber with high-velocity filtered air jets, used to remove the particles that the personnel carry. The air shower is typically located between the gowning room and the cleanroom, and the air shower cycle is the input to the gowning time. An air shower is the default for personnel entering ISO 7 or cleaner zones.
The three elements are linked, and the link is the cascade. A cascade that has the three elements in the right order is the input to the in-operation count, and the in-operation count is the input to the audit.
The Pass Box: Design, Interlock, and Purge
A pass box is a small chamber with two interlocking doors, used to transfer items between two adjacent zones. The pass box has three design elements: the design, the interlock, and the purge. The wrong design is the most common reason a pass box fails the in-operation count, and the failure is typically a particle spike at the time of the transfer.
A working pass box design has the following elements:
- Design. The design is matched to the items being transferred. A small pass box (typically 600 mm x 600 mm x 600 mm) is the default for small items. A large pass box (typically 1200 mm x 1200 mm x 1200 mm) is the default for large items. The design also includes the surface finish (typically stainless steel 304 or 316L) and the door type (typically hinged or sliding).
- Interlock. The interlock is the mechanism that prevents both doors from being open at the same time. The interlock is typically mechanical (a latch that prevents the second door from opening until the first door is closed) or electronic (a sensor that detects the door state and signals the interlock). The interlock is the input to the cascade, and the cascade is the input to the in-operation count.
- Purge. The purge is the filtered air that flushes the pass box between transfers. The purge time is typically 30 seconds to 2 minutes, depending on the class of the destination zone. The purge is the input to the transfer time, and the transfer time is the input to the production schedule.
- UV-C (optional). The UV-C is the ultraviolet light that disinfects the pass box between transfers. The UV-C is optional, and is the default for pharmaceutical and biotech applications. The UV-C cycle is the input to the transfer time, and the transfer time is the input to the production schedule.
The four elements are linked, and the link is the pass box specification. A specification that has the four elements is the input to the qualification, and the qualification is the input to the in-operation count.
The Airlock: Pressure, Cascade, and Indicator
An airlock is a larger chamber with two interlocking doors, used to transfer personnel and large items between zones of different classes. The airlock has three design elements: the pressure, the cascade, and the indicator. The wrong pressure is the most common reason an airlock fails the cascade, and the failure is typically a particle migration from the lower-pressure zone to the higher-pressure zone.
A working airlock design has the following elements:
- Pressure. The pressure is the differential pressure between the airlock and the two adjacent zones. The differential pressure is typically 10 to 15 Pa between adjacent zones, with the higher-pressure zone on the clean side. The pressure is the input to the cascade, and the cascade is the input to the in-operation count.
- Cascade. The cascade is the relationship between the pressures of the adjacent zones. A typical cascade is ISO 8 (lowest) > ISO 7 > ISO 6 > ISO 5 (highest), with the pressure increasing as the cleanliness increases. The cascade is the input to the airlock design, and the airlock design is the input to the qualification.
- Indicator. The indicator is the device that shows the operator the pressure state of the airlock. The indicator is typically a pressure gauge on the wall, with a green light indicating the pressure is within spec and a red light indicating the pressure is out of spec. The indicator is the input to the operator behavior, and the operator behavior is the input to the in-operation count.
- Door interlock. The door interlock is the mechanism that prevents both doors from being open at the same time. The interlock is the same as the pass box interlock, and is the input to the cascade.
The four elements are linked, and the link is the airlock specification. A specification that has the four elements is the input to the qualification, and the qualification is the input to the in-operation count.
The Air Shower: Velocity, Cycle, and Filter
An air shower is a chamber with high-velocity filtered air jets, used to remove the particles that the personnel carry. The air shower has three design elements: the velocity, the cycle, and the filter. The wrong velocity is the most common reason an air shower fails the particle removal test, and the failure is typically a sustained particle count in the cleanroom after the personnel have entered.
A working air shower design has the following elements:
- Velocity. The velocity is the speed of the air jets, typically 20 to 25 m/s. The velocity is the input to the particle removal, and the particle removal is the input to the in-operation count.
- Cycle. The cycle is the time the personnel are exposed to the air jets, typically 15 to 30 seconds. The cycle is the input to the gowning time, and the gowning time is the input to the production schedule.
- Filter. The filter is the HEPA or ULPA filter that removes the particles from the air jets. The filter is typically H13 (99.95% at 0.3 µm) for ISO 7, and H14 (99.995% at 0.3 µm) for ISO 6. The filter is the input to the air shower performance, and the performance is the input to the qualification.
- Nozzle layout. The nozzle layout is the arrangement of the air jets around the personnel. A typical layout has nozzles on three sides (two side walls and the ceiling), with the nozzles angled at 30 to 45 degrees to the personnel. The nozzle layout is the input to the particle removal, and the removal is the input to the qualification.
The four elements are linked, and the link is the air shower specification. A specification that has the four elements is the input to the qualification, and the qualification is the input to the in-operation count.
The Cascade: From ISO 8 to ISO 5 in Four Steps
The cascade is the relationship between the pressures of the adjacent zones, and the cascade is the input to the in-operation count. A typical cascade is four steps: ISO 8 to ISO 7 to ISO 6 to ISO 5, with the pressure increasing as the cleanliness increases. The cascade is documented in the contamination control strategy, and the strategy is the input to the qualification.
A working cascade has the following elements:
- ISO 8 (corridor). The ISO 8 zone is the corridor outside the cleanroom, with a pressure of 0 Pa relative to the building. The ISO 8 zone is the source of the contamination, and the cascade is designed to prevent the contamination from migrating to the cleaner zones.
- ISO 7 (gowning room). The ISO 7 zone is the gowning room, with a pressure of +10 to +15 Pa relative to the ISO 8 corridor. The gowning room is the first boundary, and the air shower is the second boundary.
- ISO 6 (cleanroom support). The ISO 6 zone is the cleanroom support area, with a pressure of +10 to +15 Pa relative to the ISO 7 gowning room. The support area is the second boundary, and the pass box is the third boundary.
- ISO 5 (cleanroom critical). The ISO 5 zone is the cleanroom critical area, with a pressure of +10 to +15 Pa relative to the ISO 6 support area. The critical area is the destination, and the cascade is the input to the in-operation count.
The four elements are linked, and the link is the cascade specification. A specification that has the four elements is the input to the qualification, and the qualification is the input to the in-operation count.
The Monitoring: Pressure, Particle, and Interlock
The monitoring program has three elements: the pressure, the particle, and the interlock. The three elements are the input to the in-operation count, and the in-operation count is the input to the audit. A monitoring program that does not have all three elements is the most common audit finding in material transfer design.
A working monitoring program has the following elements:
- Pressure monitoring. The pressure is monitored continuously at each airlock and each pass box. The pressure is recorded in the building management system (BMS), and the record is the input to the trend plot. A pressure that is out of spec is the signal for an investigation.
- Particle monitoring. The particle is monitored continuously at the cleanroom critical area. The particle is recorded in the environmental monitoring system, and the record is the input to the trend plot. A particle that is out of spec is the signal for an investigation.
- Interlock monitoring. The interlock is monitored continuously at each airlock and each pass box. The interlock state is recorded in the BMS, and the record is the input to the audit trail. An interlock that is bypassed is the signal for an investigation.
- Alarm. The alarm is the audible and visual signal that alerts the operator to an out-of-spec condition. The alarm is the input to the response procedure, and the response is the input to the corrective action.
The four elements are linked, and the link is the monitoring specification. A specification that has the four elements is the input to the audit, and the audit is the input to the certification.
The Semiconductor Equivalent: Wafer and Tool Transfer
Semiconductor R&D and pilot lines do not have a regulatory mandate to follow a material transfer design program, but the principle is the same. The wafer and the tool are the items being transferred, and the contamination is the particle that the wafer and the tool carry. The wrong design is the most common reason a fab fails the wafer defect correlation, and the failure is typically a particle adders that tracks to the transfer event.
A working semiconductor material transfer program has the same three elements (pass box, airlock, air shower) as a pharmaceutical program, with the design adjusted to the wafer and tool handling. The pass box is replaced by a wafer transfer box, the airlock is replaced by a tool enclosure, and the air shower is replaced by a particle counter on the tool. The monitoring is the input to the wafer defect correlation, and the correlation is the input to the engineering decision.
Closing: The Boundary Is the Program
The mental shift that makes the material transfer design work is to stop treating it as a sequence of chambers and start treating it as the boundary that defines the cleanroom. The pass box, the airlock, and the air shower are the three elements that control what crosses the boundary, and the wrong design is the most common reason the in-operation count is higher than the at-rest count. A working pass box, a working airlock, a working air shower, and a working cascade are the four elements that turn a sequence of chambers into a boundary, and the boundary is the input to the next quarter’s yield.
If you are building a material transfer design from scratch, or reviewing an existing one, we can share a draft pass box specification, an airlock qualification worksheet, an air shower velocity calculation, and a cascade design template, typically within two business days. Reach out with your current zone layout, your current class limits, and the date of your most recent material transfer qualification.