Particle Counter Sampling Locations: A Practical Guide to Probes, Ports, and Grid Design

Particle counter sampling locations are the most under-specified element of an environmental monitoring program, and the wrong location is the most common reason an audit finds a cleanroom that meets the in-operation class but fails the audit. The in-operation class is a number, the audit finding is a record, and the record is built from the sampling locations that the program documents. This article is a working guide to the design of the sampling location plan for a particle counter program in a semiconductor or pharmaceutical cleanroom.

The default mistake is to put the sample probe in the most convenient location, not the most representative location. The convenient location is the return air grille, the supply air diffuser, or the corner of the room. The representative location is the work area, the transfer point, or the personnel path. The program that documents the convenient location is a program that does not survive the audit.

The Three Classes: Probe, Port, and Grid

A particle counter sampling program has three classes of location: the probe, the port, and the grid. The three classes are the input to the in-operation count, and the in-operation count is the input to the audit. A program that does not have all three classes is the most common audit finding in environmental monitoring.

A working sampling program has the following elements:

1. Probe. A probe is a sample point at the work area, where the work is performed. The probe is the location that represents the contamination that the product sees, and the probe is the input to the in-operation count. A typical cleanroom has 1 to 3 probes per work area, depending on the size of the area and the class of the area.
2. Port. A port is a sample point at the transfer point, where the items or personnel cross the boundary. The port is the location that represents the contamination that the transfer introduces, and the port is the input to the cascade. A typical cleanroom has 1 port per transfer point, and the transfer point is the input to the cascade.
3. Grid. A grid is a sample point at a regular interval across the cleanroom, used to verify the cleanliness of the area. The grid is the location that represents the contamination that the cleanroom as a whole carries, and the grid is the input to the qualification. A typical cleanroom has a grid of 6 to 12 points, depending on the size of the area and the class of the area.

The three classes are linked, and the link is the sampling plan. A plan that has the three classes is the input to the qualification, and the qualification is the input to the audit.

The Probe: Work Area, Breathing Zone, and Critical Zone

The probe is the most important class of sampling location, and the probe is the input to the in-operation count. A working probe design has three elements: the work area, the breathing zone, and the critical zone. The wrong design is the most common reason a probe fails the in-operation count, and the failure is typically a count that is higher than expected at the work area.

A working probe design has the following elements:

1. Work area. The work area is the location where the work is performed, typically 1 to 2 meters above the floor and within 1 meter of the work surface. The work area is the input to the in-operation count, and the count is the input to the audit.
2. Breathing zone. The breathing zone is the location where the personnel breathe, typically 1.5 meters above the floor and within 1 meter of the worker’s face. The breathing zone is the input to the personnel contribution, and the contribution is the input to the in-operation count.
3. Critical zone. The critical zone is the location where the product is exposed, typically at the work surface or in the open container. The critical zone is the input to the product risk, and the risk is the input to the in-operation count.
4. Probe orientation. The probe is oriented vertically, with the inlet pointing up. The vertical orientation is the input to the ISO 14644-1 sampling procedure, and the procedure is the input to the in-operation count.

The four elements are linked, and the link is the probe specification. A specification that has the four elements is the input to the qualification, and the qualification is the input to the audit.

The Port: Transfer Point, Air Lock, and Pass Box

The port is the second class of sampling location, and the port is the input to the cascade. A working port design has three elements: the transfer point, the air lock, and the pass box. The wrong design is the most common reason a port fails the cascade, and the failure is typically a count that is higher than expected at the transfer point.

A working port design has the following elements:

1. Transfer point. The transfer point is the location where the items or personnel cross the boundary, typically at the door of the airlock or the pass box. The transfer point is the input to the cascade, and the cascade is the input to the in-operation count.
2. Air lock. The air lock is the location where the personnel cross between zones, typically inside the airlock chamber. The air lock is the input to the personnel contribution, and the contribution is the input to the cascade.
3. Pass box. The pass box is the location where the items cross between zones, typically inside the pass box chamber. The pass box is the input to the material contribution, and the contribution is the input to the cascade.
4. Port orientation. The port is oriented horizontally, with the inlet pointing into the chamber. The horizontal orientation is the input to the transfer procedure, and the procedure is the input to the cascade.

The four elements are linked, and the link is the port specification. A specification that has the four elements is the input to the qualification, and the qualification is the input to the audit.

The Grid: Room Volume, Recovery Time, and Uniformity

The grid is the third class of sampling location, and the grid is the input to the qualification. A working grid design has three elements: the room volume, the recovery time, and the uniformity. The wrong design is the most common reason a grid fails the qualification, and the failure is typically a recovery time that is longer than expected at one or more points in the room.

A working grid design has the following elements:

1. Room volume. The room volume is the input to the number of grid points. A typical cleanroom has 1 grid point per 10 to 20 square meters of floor area, with a minimum of 4 points and a maximum of 12 points. The room volume is the input to the qualification, and the qualification is the input to the audit.
2. Recovery time. The recovery time is the time it takes for the cleanroom to return to the at-rest class after a release of particles. The recovery time is the input to the grid, and the grid is the input to the qualification. A typical recovery time is 15 to 30 minutes for ISO 7, and 5 to 15 minutes for ISO 6.
3. Uniformity. The uniformity is the consistency of the count across the grid. A uniformity that is within 20% of the average is the typical spec, and the spec is the input to the qualification.
4. Grid layout. The grid layout is the arrangement of the grid points across the room. A typical layout is a regular grid, with the points at the corners and the center of the room. The grid layout is the input to the qualification, and the qualification is the input to the audit.

The four elements are linked, and the link is the grid specification. A specification that has the four elements is the input to the qualification, and the qualification is the input to the audit.

The Sampling Procedure: Volume, Flow, and Interval

The sampling procedure has three elements: the volume, the flow, and the interval. The three elements are the input to the in-operation count, and the in-operation count is the input to the audit. A procedure that does not have all three elements is the most common audit finding in environmental monitoring.

A working sampling procedure has the following elements:

1. Volume. The volume is the amount of air sampled at each location, typically 1 to 100 liters depending on the class of the cleanroom. The volume is the input to the detection limit, and the detection limit is the input to the in-operation count. A typical volume is 28.3 liters (1 cubic foot) for ISO 5, and 100 liters for ISO 7.
2. Flow. The flow is the rate at which the air is sampled, typically 1 to 100 liters per minute. The flow is the input to the sampling time, and the sampling time is the input to the procedure. A typical flow is 28.3 liters per minute (1 cubic foot per minute) for ISO 5, and 50 liters per minute for ISO 7.
3. Interval. The interval is the frequency at which the sampling is performed, typically continuous for ISO 5, hourly for ISO 6, and daily for ISO 7. The interval is the input to the in-operation count, and the count is the input to the audit.
4. Alarm. The alarm is the audible and visual signal that alerts the operator to an out-of-spec count. 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 procedure specification. A specification that has the four elements is the input to the audit, and the audit is the input to the certification.

The Instrument: Counter, Probe, and Tubing

The instrument has three elements: the counter, the probe, and the tubing. The three elements are the input to the in-operation count, and the in-operation count is the input to the audit. An instrument that does not have all three elements is the most common audit finding in environmental monitoring.

A working instrument setup has the following elements:

1. Counter. The counter is the device that measures the particle count, typically a laser-based optical particle counter. The counter is the input to the in-operation count, and the count is the input to the audit. A typical counter is a GCC-MST-5100XPro or equivalent, with a detection limit of 0.3 µm and a flow rate of 28.3 liters per minute.
2. Probe. The probe is the inlet that draws the air into the counter. The probe is typically stainless steel, with a 1/4 inch or 1/2 inch barb fitting. The probe is the input to the sampling location, and the location is the input to the in-operation count.
3. Tubing. The tubing is the connection between the probe and the counter. The tubing is typically conductive silicone or stainless steel, with an inner diameter matched to the probe. The tubing length is the input to the transport time, and the transport time is the input to the in-operation count. A typical tubing length is 1 to 3 meters, with a transport time of 1 to 3 seconds.

The three elements are linked, and the link is the instrument specification. A specification that has the three elements is the input to the audit, and the audit is the input to the certification.

The Qualification: Installation, Operation, and Performance

The qualification has three phases: the installation qualification (IQ), the operational qualification (OQ), and the performance qualification (PQ). The three phases are the input to the audit, and the audit is the input to the certification. A qualification that does not have all three phases is the most common audit finding in environmental monitoring.

A working qualification has the following elements:

1. IQ. The IQ verifies that the instrument is installed correctly, with the right probe, the right tubing, and the right location. The IQ is the input to the OQ, and the OQ is the input to the audit.
2. OQ. The OQ verifies that the instrument operates correctly, with the right flow, the right volume, and the right alarm. The OQ is the input to the PQ, and the PQ is the input to the audit.
3. PQ. The PQ verifies that the instrument performs correctly in the cleanroom, with the right count at the right location. The PQ is the input to the certification, and the certification is the input to the audit.

The three phases are linked, and the link is the qualification specification. A specification that has the three phases is the input to the audit, and the audit is the input to the certification.

The Semiconductor Equivalent: Wafer Scan and Tool Correlation

Semiconductor fabs do not have a regulatory mandate to follow a sampling location program, but the principle is the same. The wafer is the product, the tool is the work area, and the defect is the contamination. The wrong sampling location is the most common reason a fab fails the wafer defect correlation, and the failure is typically a defect adders that tracks to the wrong tool.

A working semiconductor sampling program has the same three classes (probe, port, grid) as a pharmaceutical program, with the design adjusted to the wafer and tool handling. The probe is replaced by a wafer scan, the port is replaced by a tool transfer, and the grid is replaced by a tool monitor. The monitoring is the input to the wafer defect correlation, and the correlation is the input to the engineering decision.

The Cross-Link: Particles, Air Shower, and Consumables

The sampling location program is linked to the air shower design and the cleaning and disinfection program by the same boundary principle. The sampling location at the air lock port is the verification of the air shower performance, and the sampling location at the work area probe is the verification of the cleaning and disinfection program. A program that does not link the three is a program that does not survive the audit.

Closing: The Location Is the Program

The mental shift that makes a sampling location program work is to stop treating the probe as a sensor and start treating it as the input to the audit. The probe, the port, and the grid are the three classes of location, and the wrong class is the most common reason the in-operation count is higher than the at-rest count. A working probe, a working port, a working grid, and a working procedure are the four elements that turn a counter into a program, and the program is the input to the next quarter’s certification.

If you are building a sampling location program from scratch, or reviewing an existing one, we can share a draft sampling plan template, a probe specification, a port specification, and a grid layout calculation, typically within two business days. Reach out with your current zone layout, your current class limits, and the date of your most recent environmental monitoring qualification.