Trimethylsilanol (TMSOH) can cause damage to surfaces of scanner lenses in the semiconductor industry, and there is a critical need to measure and control airborne TMSOH concentrations. This study develops a thermal desorption (TD)-gas chromatography (GC)-mass spectrometry (MS) method for measuring trace-level TMSOH in occupational indoor air. Laboratory method optimization obtained best performance when using dual-bed tube configuration (100 mg of Tenax TA followed by 100 mg of Carboxen 569), n-decane as a solvent, and a TD temperature of 300°C. The optimized method demonstrated high recovery (87%), satisfactory precision (<15% for spiked amounts exceeding 1 ng), good linearity (
Trimethylsilanol (TMSOH, CAS No. 1066-40-6) in industrial sectors has gained wide attention due to the widespread use of silicon materials and their detrimental effects on equipments and products [
The nonhealth hazards from siloxanes use are of more concern in industrial processes. Most widely, the concern arises from siloxanes in biogas emitting from landfills and wastewater treatment [
Even trace levels of TMSOH can accumulate and form salts on surfaces of scanner lenses over time. The scanner lens is an expensive key device in the semiconductor wafer production line, and TMSOH salts can cause severe and sometimes irreversible damage [
Analytical methods for siloxanes often focus on D4–D6 cyclical siloxanes and linear siloxanes, including hexamethyldisiloxane (L2), octamethyltrisiloxane (L3), and decamethyltetrasiloxane (L4), without considering TMSOH [
High-sensitivity methods have not been designed specifically for airborne TMSOH in industrial environments that have special needs to control TMSOH, for example, the semiconductor manufacturing workshops. This study develops a thermal desorption (TD)-gas chromatography (GC)-spectrometry (MS) method for trace-level TMSOH analysis in occupational indoor air. The method is optimized by comparing adsorbent configurations and analysis solvents, choosing an appropriate desorption temperature and applying a more sensitive selected ion monitoring (SIM) mode in MS. The method performance is evaluated in laboratory and in a semiconductor fabrication factory.
Trimethylsilanol (98.5%) was obtained from Apollo Scientific Ltd., Cheshire, UK; methanol (99.9%) from Duksan, Ansan, Republic of Korea; n-pentane (99%) from Junsei, Tokyo, Japan; n-hexane (96%) from Kanto Chemical, Tokyo, Japan; n-decane (99.5%) from Dae Jung, Siheung, Republic of Korea. Tenax TA (poly(2,6-diphenyl-p-phenylene) oxide, 60–80 mesh, 35 m2/g, for C5–C26 VOCs), Carbopack B (graphitized carbon black, 60–80 mesh, 100 m2/g, for C5–C12 VOCs), Carbopack C (graphitized carbon black, 60–80 mesh, 10 m2/g, for C12–C20 VOCs), Carboxen 569 (carbon molecular sieve, 20–45 mesh, 485 m2/g, for C2–C5 VOCs), and Carbosieve-SIII (carbon molecular sieve, 60–80 mesh, 820 m2/g, for C2–C5 VOCs) were obtained from Supelco, Bellefonte, PA, USA. Empty glass thermal desorption tubes (18 cm long, 4 mm I.D.) were obtained from Gerstel, Mülheim, Germany.
To determine the adsorbent or adsorbent combination that has the highest recovery, we prepared three sets of adsorbent tubes based on adsorbents’ affinity abilities:
Clean tubes were removed from the refrigerated storage container and equilibrated at room temperature before use. An adsorbent tube was then spiked with 1
An appropriate solvent is critical to GC-MS analysis as the solvent is the carrier of analytes. Methanol is a widely used solvent for VOC analysis because of its fast elution, significant low background, non-aggressive behavior into GC column, and acceptable level of compound solvatation, as well as easy separation with other chemicals. However, preliminary tests showed that methanol had detrimental effects on TMSOH separation, including formation of artifacts from chemical reactions. Thus, four candidate solvents, methanol, n-pentane, n-hexane, and n-decane, were tested to obtain the best separation. Standard TMSOH solution was diluted in these solvents to form 200
Dual-bed tubes were spiked with 100 or 200 ng of TMSOH in n-decane, and then thermally desorbed at 150, 200, 250, and 300°C, respectively. Duplicate samples were used for each test.
For the three-tube configurations, each was spiked with 200 ng of TMSOH in n-decane and then analyzed following the same TD-GC-MS procedure. The triple-bed tubes contained high surface area (820 m2/g) Carbosieve-SIII, which may retain VOCs after the thermal desorption, a phenomenon called the memory effect. To examine this effect, triple-bed tubes were analyzed again after the first thermal desorption. Tests were repeated three times for each tube configuration.
Adsorbent tube samples were analyzed on a thermal desorption system (TDS, Gerstel, Mülheim an der Ruhr, Germany) followed by GC-MS (Agilent 7890A/5975C, Agilent Technologies, Santa Clara, CA, U.S.A.). The TDS was mounted on top of a cooled injection system that was used as a cryotrap. TMSOH and other VOCs were cryofocused and concentrated at −30°C using liquid nitrogen, after which they were transferred to the capillary column without discrimination or loss of analytes. The TD-GC-MS conditions (Table
Thermal desorption (TD)-GC-MS conditions.
TD parameters | |
TD model | Gerstel thermal desorption system (TDS)/cooled injection system (CIS) |
1st desorption temperature (°C) | 300 |
1st desorption holding time (min) | 5 |
1st desorption flow rate (mL/min) | 80 |
Transfer line temperature (°C) | 280 |
CIS cryofocusing temperature (°C) | −30 |
CIS 2nd desorption temperature (°C) | 300 |
2nd desorption holding time (min) | 5 |
Cryofocusing liquid | Liquid nitrogen (N2) |
| |
GC parameters | |
GC model | Agilent 7890A GC |
Split ratio | 20 : 1 |
Column | HP-5MS (60 m Length, 0.25 mm I.D., 0.250 |
Oven temperature program | 40°C, hold for 2 min |
8°C/min to 180°C, hold for 2 min | |
10°C/min to 250°C, hold for 1 min | |
15°C/min to 300°C, hold for 5 min | |
Run time (min) | 37.83 |
| |
MS parameters | |
MS model | Agilent 5975C inert XL MSD with triple-axis detector |
Electron ionization voltage (eV) | 70 |
Quadrupole temperature (°C) | 150 |
Source temperature (°C) | 230 |
Mass mode | Full scan and SIM (selected-ion monitoring), |
Mass range (m/z) | 35~550 in full scan |
45, 47, and 75 in SIM (for TMSOH) |
The laboratory performance experiments were aimed to determine recovery, establish calibrations, check instrument linearity, and determine analysis precision, method detection limits (MDLs), adsorbent retainability, and storage stability. The performance evaluation procedure generally followed the U.S. EPA’s guidelines for analyzing VOCs, for example, TO-15 [
The recovery was calculated as the ratio of abundance for a given amount of TMSOH from TD-GC-MS to that generated from direct injection of the same amount of TMSOH followed by the same GC-MS analysis. The fraction reflects the combined efficiency of adsorption of gaseous compounds and thermal desorption. Recovery experiments were conducted for a wide range of spiked amounts, including 5, 20, 100, and 200 ng. Recovery tests also included redesorption of the tube to check the memory effect. All tests used duplicates. As an indicator of accuracy, recovery is expected to be within ±30% of the true amount [
The initial 7-point calibration was established using loadings of 0.1, 0.5, 1.0, 5.0, 20, 100, and 500 ng, respectively. Calibration solutions were prepared by diluting pure TMSOH to 100 mL of high-purity n-decane. This resulted in series solutions of 0.1, 0.5, 1, 5, 20, 100, and 500
Precision is commonly expressed as relative standard deviation (RSD) for multiple replicates or percent difference (%
The MDL was determined by analyzing 7 replicate tubes spiked with a low concentration of TMSOH that was expected to be near the MDL to avoid an artificially high MDL [
These experiments were aimed to test how well the single- and dual-bed tubes could retain TMSOH. For each tube configuration, three tubes were connected in series, and the first tube was spiked with 200 ng of TMSOH. Then a 100 mL/min flow of N2 gas or air was pulled through the tube series for 10, 50, 100, and 200 min, respectively, corresponding to total volumes of 1, 5, 10 and 20 L, respectively. The same procedures were repeated for 5 ng loadings. In each test, amounts of TMSOH obtained from analysis of the front, 1st backup, and 2nd backup tubes were calculated as the percentages of the initial amount spiked to the front tube.
In this experiment, 10 dual-bed tubes were initially spiked with 10 ng of TMSOH each. Then duplicate tubes were analyzed immediately, and 1, 3, 7, and 14 days after the initial loading, respectively. Tubes were sealed and stored at 4°C in a VOC-free refrigerator, and an internal standard solution was loaded to each tube right before GC-MS analysis. Using the mean of duplicates, storage stability was expressed as the percentage of the initial measurement.
The field monitoring was conducted in a wafer manufacturing workshop of a semiconductor fabrication factory in Cheong-Ju City, Republic of Korea, every two weeks from June to October, 2010. Samples were collected at a flow rate of 100 mL/min for 60 or 200 min using a microprocessor-controlled air sampling pump (SIBATA Mini-pump, Σ30, Japan). The initial intention was to measure TMSOH concentrations as well as to compare two tube configurations. Thus, each sampling event used a dual-bed tube and a single-bed tube, and samples were collected side-by-side. Single-bed Tenax tubes showed poor performance as observed in laboratory, so only results from dual-bed tubes were reported. A follow-up field sampling was conducted in the same workshop in August 2011. This sampling collected duplicate 6 L samples and distributed volume (6 L and 20 L) samples. Distributed volume samples are two samples with different volumes in parallel at the same monitoring location. The U.S. EPA recommends this strategy for adsorbent sampling to increase method sensitivity as well as to check reproducibility [
Contamination is almost unavoidable in siloxane analysis given many silicon-containing materials used in GC parts; however, the artifacts of concern are cyclic siloxanes [
Column separation of TMSOH using different solvents is displayed in Figure
Solvent effects on TMSOH separation. (a) methanol, (b) n-pentane, (c) n-hexane, and (d) n-decane. MTS: methoxytrimethylsilane. Samples were analyzed in scan mode.
The recoveries of TMSOH were similar at different thermal desorption temperatures, ranging from 92 to 125% (Figure
TMSOH abundances at different thermal desorption temperatures. Error bars show minimum and maximum recoveries.
The recoveries were
In summary, the sampling and analytical method was optimized if using dual-bed tubes as sampling device, n-decane as the analysis solvent, and a desorption temperature of 300°C. The performance of the method was then evaluated using these parameters.
The retention time of TMSOH under the optimized GC condition was 5.143 min with a narrow range from 5.116 to 5.183 min.
The average recoveries were 87% (range 78–96%) and 87% (range 76–99%) at 100 and 200 ng loadings, respectively. The recovery increased to 126% (range 110–142%) when tubes were spiked with 5 ng of TMSOH. The higher recovery might be due to the omission of the IS in recovery tests.
The replicate precision, expressed as relative standard deviation or percent difference, averaged 17.7% over a wide amount range from 0.1 to 500 ng. The precision deteriorated at lower spiked amounts, as reported for other VOCs [
The negative coefficient clearly indicated the discordant relationship, although the association was medium (
The 7-point calibration curves were determined as the following.
The analyses of seven 0.1 ng of TMSOH replicates yielded an MDL of 0.057 ng in SIM mode. This corresponded to MDLs of 2.8 and 9.5 ng/m3 for sample volumes of 20 L and 6 L, respectively. These MDLs were at least 100 times lower than the minimum concentration (1.0
Total ion chromatograms showing signal-to-noise (S/N) ratios obtained from thermal desorption followed by GC-MS analysis of 5 ng of TMSOH in (a) MS scan mode and (b) MS SIM mode.
Results of retainability tests were summarized in Table
Results of retainability tests. Front, Back1, and Back2 were the front, 1st backup and 2nd backup tubes in series. Amounts of TMSOH in three tubes were expressed as the percentages of the initial amount spiked to the front tube.
Amount (ng) | Vol (L) | Flow |
Dual-bed tubes | Single-bed tubes | ||||
---|---|---|---|---|---|---|---|---|
Front (%) | Back1 (%) | Back2 (%) | Front (%) | Back1 (%) | Back2 (%) | |||
10 | 1 | N2 | 98.2 | 1.1 | 0.7 | 91.6 | 4.4 | 3.9 |
10 | 5 | N2 | 96.0 | 0.7 | 3.3 | 66.6 | 28.7 | 4.8 |
10 | 10 | N2 | 99.4 | 0.4 | 0.2 | 78.7 | 20.8 | 0.5 |
10 | 20 | N2 | 97.8 | 1.0 | 1.2 | 68.5 | 27.1 | 4.5 |
200 | 1 | N2 | 99.5 | 0.5 | 0.1 | 94.3 | 5.7 | 0.0 |
200 | 5 | N2 | 98.9 | 0.3 | 0.8 | 24.6 | 34.0 | 41.4 |
200 | 10 | N2 | 97.8 | 1.1 | 1.2 | 18.4 | 35.0 | 46.7 |
200 | 20 | N2 | 99.5 | 0.4 | 0.1 | 23.0 | 38.5 | 38.5 |
200 | 20 | Air | 99.7 | 0.3 | n.a. | 35.8 | 64.2 | n.a. |
200 | 20 | Air | 98.4 | 1.6 | n.a. | 40.9 | 59.1 | n.a. |
Notes: aFlow matrix: the gas blown through tubes in retainability tests. n.a.: not available.
The loss of TMSOH in spiked sorbent tubes was 13% of the initial loading within 3 days. However, storage caused a larger loss of 23% for one week, and no further loss was observed afterwards. A previous study showed that the loss averaged 14% after 1-week storage for 51 compounds, and it was negligible from 1 week to upto 6 weeks [
Results of TMSOH samples collected in a semiconductor factory were summarized in Table
TMSOH concentrations measured in a semiconductor fabrication workshop.
Sampling date |
Sample volume (L) |
TMSOH concentration ( |
|
---|---|---|---|
Rep 1 | Rep 2 | ||
06/23/2010 | 6 | 1.58 | n.a. |
07/08/2010 | 6 | 1.32 | n.a. |
07/21/2010 |
|
1.21 | n.a. |
|
2.82 | n.a. | |
08/02/2010 | 6 | 5.98 | n.a. |
08/25/2010 |
|
1.02 | n.a. |
|
2.61 | n.a. | |
09/30/2010 | 6 | 3.91 | n.a. |
10/14/2010 | 20 | 2.74 | n.a. |
08/25/2011 | 6/ |
22.51 | 20.19 |
08/26/2011 |
|
23.80 | 27.30 |
Notes: aSamples at two locations within the same workshop. bCo-located distributed volume replicate samples, cCo-located same volume replicate samples. n.a.: not available.
Total ion chromatograms of a typical field sample collected in a semiconductor fabrication workshop. Notes: IPA: Isopropyl alcohol; PGME: Propylene glycol methyl ether; PGMEA: Propylene glycol monomethyl ether acetate; L2: Hexamethyldisiloxane; D3: Hexamethylcyclotrisiloxane; D4: Octamethylcyclotetrasiloxane; D5: Decamethylcyclopentasiloxane.
Total ion chromatogram in scan mode
Total ion chromatogram in SIM mode
While both the laboratory and field tests showed satisfactory performance for measuring TMSOH, this study had several limitations. The quantification was limited to only TMSOH, which was the interest and/or concern from the manufacturing perspective. The chromatograms of field samples revealed several other siloxane compounds, including L2 and D3–D5 siloxanes (Figure
In this study, we developed a sensitive sampling and analytical method for measuring trace levels of TMSOH in indoor air of semiconductor fabrication environments. Method optimization suggested that best performance could be obtained if using dual-bed (Tenax TA followed by Carboxen 569) adsorbent configuration, n-decane as analysis solvent, and a thermal desorption temperature of 300°C. Laboratory and field evaluation revealed satisfactory performance of the methods: a reasonable recovery of 87%, typical replicate precision of within 15%, high linearity (
This work was supported by the Department of Management of GemVax & KAEL. The data analyses were partially supported by the Collaborative Health Disparity Research Incubator Grant from the Center for Health Equity Research and Promotion (CHERP) at the University of Memphis. The authors thank Eui-Jin Do, Ho-Kyoung Ki, Jae-Hyun Ban, and Sung-Hyun Heo for their technical assistance and significant help. They are grateful to Dr. D. Tsikas and two anonymous referees for constructive comments.