Contamination monitoring

Contamination Control Monitoring

This document describes the methods/procedures used to monitor contamination in the M.S.S.L. cleanroom.

Contents

Molecular Contamination

Non Volatile Residue monitoring (NVR)

Particulate Contamination.

Particle Fallout (tapelift)

1. Molecular Contamination Monitoring

Introduction

At M.S.S.L. there has been a requirement to monitor molecular contamination levels of optical instruments, which operate over the wavelength range of 160nm to 550nm. To this end we have introduced a molecular contamination monitoring program utilising an PMS AiM100 Surface Acoustic Wave (S.A.W) instrument. This instrument is used to monitor, in realtime, airborne molecular contamination (AMC), its deposition rate and total mass deposition on the surface of interest in either ng/cm² or Angstrom units.

The SAW instrument allows us to detect the deposition of AMC on a silicon dioxide sensor surface. Due to realtime data collection we can pinpoint exactly at what time the contamination event occured, the contamination layer thickness and correlate optics surface damage with either internal cleanroom activities, external events or process activities. Being able to do this we can then eliminate or control the sources of the AMC. In addition to this we can also investigate futher as to what chemical species make up the contamination layer by subjecting the SAW sensor chip to Time Of Flight/Secondary Ion Mass Spectroscopy (TOF/SIMS) analysis

Method of Measurement

The SAW instrument is positioned near a critical item or process surface where the effects of molecular contamination can result in the degradation of the instrument or item performance.

The method of measurement is based on the fact that a quartz crystal has piezoelectric characteristics. The quartz crystal will oscillate when electricity is passed through it. Both sensors (one sensor is sealed and the other is exposed) in the PMS AiM100 are made of quartz cryastal. The crystals top surfaces are coated with silicon dioxide and facilitates the adsorbtion of AMC. Once AMC is adsorbed onto the Silicon dioxide surface the crystals frequency of oscillation will be less than that of the uncontaminated/sealed crystal. The oscillation frequency will gradually decrease as the mass of the deposited contamination increases. The AiM100 will then calculate the difference in frequency between the exposed sensor chip and the reference (sealed) sensor chip. As mass accumulates on the exposed sensor the difference in frequency between the two crystals will increase proportionately. A software algorithm calculates the mass of the contamination based on this frequency shift and then output to a computer monitoring the data within the cleanroom.

Data Archiving

Both data and data analysis are archived on the dedicated cleanroom computer and is made available on the cleanroom website. Click here (MSSL only) to view the latest SAW data

2. Non Volatile Residue monitoring (NVR)

The MSSL cleanroom is monitored for background NVR using ASTM E 1235-95 and ASTM E 1234 - 94 methods. Results of background NVR for the MSSL cleanroom can be found here (MSSL only)

3. Particulate Contamination Monitoring

This document describes the procedure used at MSSL for the monitoring of Airborne particulate contamination in the Cleanroom

Procedures used in particulate monitoring

Particulate contamination monitoring is carried out using an independently calibrated, cleanroom dedicated, particle detector with the following specification:

Instrument: Climet CI-7300 microprocessor based six channel airborne particle counter

Particle sensitivity: 0.3 micron to 10 microns

Particle discrimination: 0.3, 0.5, 0.7, 1, 5 and 10 microns

Standard sample flow rate: 1.0 cubic ft per minute (1 CFM)

Calibration: Every six months by independent contractors

The cleanroom is continuously monitored for particulate contamination and is logged to a cleanroom dedicated computer via RS 232 communications. This continuous monitoring involves taking a sample of 1 cubic ft of cleanroom air every 3 minutes, analysing it for particles in the above mentioned ranges and sending the results to a dedicated cleanroom computer for monitoring and data archiving. The monitoring of the the results is done via remote link up to the dedicated cleanroom computer using Norton PCanywhere. This negates the need for monitoring personnel to enter the cleanroom for monitoring purposes.

The continuous monitoring takes place at a static reference point located near the entrance to the cleanroom.

If levels of contamination are seen to rise, then the particulate contamination will be monitored at numerous locations within the cleanroom to isolate the source.Corrective action will then be taken. The procedure for this is detailed below.

Isolating particulate contamination sources

Starting in area A of the white cleanroom (closest to the HEPA filters) the particle detector is setup and readings are taken from four different locations in area A at a height of approximately one meter.
At each location four counts are taken from the four different locations, giving sixteen counts for area A.
At each location, after four counts are taken, the particle detector will automatically generate an average reading of the four counts. Whilst this is happening, the detector inlet funnel is moved to the next location ready for the next four counts.
This procedure is then repeated for areas B, C, the anteroom and the dark Cleanroom.
Overall, 100 counts are taken for both the white and dark Cleanroom and the anteroom. The results are then examined and appropriate actions taken if necessary.
The results are filed for record purposes. (see file records below)
The following particulate sizes are recorded together with date and time:
0.3 um, 0.5 um, 0.7 um, 1.0 um, 5.0 um and 10 um.

Particulate records

Particlate counts are archived on the cleanroom computer and are accessed for analysis of particulate levels.The records are saved as ASCII files and can be imported into a word processor or spreadsheet for trend analysis etc.

A cleanroom particulate count archive can be found here.

Note: some of these files are 2Mb + so will take some time to show on a slow connection

4. Particulate fallout monitoring

The MSSL cleanroom is frequently monitored for particulate fallout to ASTM E 1216 - 99 methods. Particulate fallout data for the MSSL cleanroom can be found here.