- Email: [email protected]
Fabrication of a novel micro time-of-flight mass spectrometer
Sensors and Actuators A 97±98 (2002) 441±447
Fabrication of a novel micro time-of-¯ight mass spectrometer Hyeun Joong Yoona,*, Jung Hoon Kimb, Eun Soo Choia, Sang Sik Yanga, Kwang Woo Jungb a
School of Electronics Engineering, Ajou University, San 5 Wonchun-Dong, Paldal-Gu, Suwon 442-749, South Korea b Department of Chemistry, Wonkwang University, 344-2 Shinyong-Dong, Iksan 570-749, South Korea Received 11 June 2001; received in revised form 4 December 2001; accepted 5 December 2001
Abstract This paper presents the fabrication and the tests of a novel micro-mass spectrometer fabricated by micro-machining and emission test of hot electron for ionization. A micro-mass spectrometer consists of a micro-ion source and a micro-ion separator. The micro-ion source consists of a hot ®lament and grid electrodes. Electrons emitted from a hot ®lament are to ionize some sample molecules. The ions are accelerated to an ion detector by an electric ®eld of the ion separator. Mass can be analyzed by using the time-of-®ght depending on the mass-to-charge ratio. The current of hot electron emission from the hot ®lament is measured for various input voltages and the ion separation is tested for acetone. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Micro time-of-¯ight mass spectrometer; Micro-ion source; Hot ®lament
1. Introduction The mass spectrometer is a powerful instrument that serves for establishment of the molecular weight and structure of organic compounds, and the identi®cation and determination of the components of inorganic substances. It is used as a de®nitive method to monitor environmental pollutants, to analyze biological samples and drugs, to detect residual gas, and to determine chemical abuse in athletic competitions. The mass spectrometer uses the difference in mass-to-charge ratio (m/e) of ionized atoms or molecules to separate them from each other. It consists of an ion source that generates ions from the sample, an analyzer that separates the ions according to their m/e values, and a detector that gives the intensity of the ion current for each species. The major types of mass spectrometers are the magnetic type [1,2], the quadrupole type [3], the time-of-¯ight type [4], and the Fourier transform ion cyclotron resonance type [5]. The time-of-¯ight mass spectrometer (TOFMS) has some advantages over others [4]. A unique advantage is the speed with which a spectrum can be obtained conveniently. With practical parameter values, a complete spectrum can be obtained every few microseconds. A second feature of the TOFMS is that the entire mass spectrum can be recorded for each accelerating pulse. The TOFMS is also mechanically simple and easy to construct and operate. * Corresponding author. Tel.: 82-31-219-2488; fax: 82-31-212-9531. E-mail address: [email protected] (H.J. Yoon).
Conventional mass spectrometers that are operated with high voltage in high vacuum are bulky and expensive. The micro-mass spectrometer, however, can analyze some samples in low vacuum and can be made cheap by batchfabrication. Recent reports on mass spectrometers fabricated by micro-machining technology show the possibility of the miniature mass spectrometer. Feustel et al. ®rst proposed the structure of a micro-mass spectrometer by using micromachining [6]. Siebert et al. fabricated a miniature mass spectrometer by anisotropic etching and electroplating [7]. But the mass spectrometer of Siebert has a dif®culty in generating plasma. Kornienko et al. fabricated a miniature ion trap mass spectrometer [8]. Tunstall et al. fabricated the micro quadrupole mass ®lter by using micro-machining [9]. Tunstall used a bulky conventional ion source. The micromass spectrometer of this paper differs from others with respect to the ionization and the ion separation method. In this paper, the bulk of the proposed mass spectrometer that is small compared with those mentioned earlier is fabricated by micro-machining and the fabricated micro-ion source and the ion separator are tested, respectively. 2. Principle of separation Fig. 1 shows the principle of the micro time-of-¯ight mass spectrometer (TOFMS). The separation principle of the micro-TOFMS is to separate the ions by the acceleration depending on the mass. That is to say, the heavy ions will
0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 2 ) 0 0 0 3 2 - 8
442
H.J. Yoon et al. / Sensors and Actuators A 97±98 (2002) 441±447
calculated from the time taken for the ion to reach the detector. Wiley and McLaren proposed the two-step acceleration time-of-¯ight mass spectrometer in order to improve the resolution of a non-magnetic TOFMS [4]. The ions are accelerated by two electric ®elds, Es and Ed, through the ionization and the acceleration regions. If it is assumed that ions are generated at the middle of the ionization region with zero initial energy, the ®nal ion energy, Ut is given as Ut qs0 Es qdEd
Fig. 1. The principle of the micro time-of-flight mass spectrometer.
travel more slowly than the light ones. Electrons produced from the heated ®lament are accelerated by the pulsed electric ®eld. The accelerated electrons collide with some sample molecules and ionize them. Ions generated are then accelerated toward the detector by the applied electric ®elds, Es and Ed, which are chosen to optimize the space focusing condition. At all times, the acceleration region has an electric ®eld, and the drift region is a free-®eld region. Then, ions reach the ion detector in the order of mass increase. There is no need for a magnetic ®eld. The relationship between the arrival time and the mass-to-charge ratio of the ion is obtained by considering that the applied electrostatic ®elds determine the velocity of the ion. For the given length of ¯ight and the acceleration voltages, the mass can be
(1)
where q is the ion charge, and s0 the half of the ionization region gap, and d is the acceleration region gap. According to [4,10], the arrival time of an ion, T, is given as ! 1=2 1=2 m 2k0 1=2 dD (2) T 2k0 s0 1=2 2Ut k0 1 where m is the mass of the ion and D is the drift region gap, and k0
s0 Es dEd s 0 Es
(3)
where s0 0:1 cm, d 0:2 cm, D 4:5 cm, Es 200 V/ cm, Ed 250 V/cm, and q corresponds to one electron charge, Ut 70 eVand k0 3:5. Consequently, T can easily be calculated as T 0:44m1=2
(4)
where the units of T and m are ms and amu, respectively. The arrival time of ion represents the linear dependence of ¯ight time on the square root of the mass. Fig. 2 shows the calculated time-of-¯ight versus mass. When the mass is
Fig. 2. The calculated time-of-flight vs. mass.
H.J. Yoon et al. / Sensors and Actuators A 97±98 (2002) 441±447
44 amu, the arrival time is 2.92 ms. This result means a complete mass spectrum of any ion can be obtained in a few microseconds. 3. Structure The micro-TOFMS consists of an ion source, an ion separator and an ion detector. Fig. 3 illustrates the structure of the ion source and separator for the micro-TOFMS. The ion source and ion separator consist of a tungsten ®lament, tungsten electrodes and four pairs of nickel electrodes on a silicon substrate. The tungsten ®lament is to emit hot electron. Nickel electrodes are to focus and accelerate ions. There is a cavity under the tungsten ®lament. The cavity reduces the heat loss by conduction heat transfer. The repeller electrode is used in the test of the electron emission. The width and height of the nickel electrodes are 80 and 20 mm, respectively. The total size of the ion source and separator of the micro-TOFMS is 10 mm 10 mm 1 mm. 4. Fabrication Fig. 4 shows the fabrication process of the ion source and separator of the micro-TOFMS. The starting material of the micro-mass spectrometer is 525 10 mm thick 4 in. n-type h1 0 0i silicon wafer. First, a 0.7 mm thick thermal oxide layer is grown as an etch mask. To make a step between the tungsten ®lament and nickel electrodes, the front side of the silicon wafer is etched by 10 mm with tetramethyl ammonium hydroxide (TMAH) solution. The maximum (1 0 0) silicon etch rate of TMAH is 1.0 mm/min at 90 8C with 22 wt.% concentration [11]. This step is to locate the
443
tungsten ®lament at the center height of nickel electrodes. After the residual oxide is etched off, a 0.7 mm thick thermal oxide is grown again. Ê ) seed layer for electroplating is The Cr/Au (300/3000 A deposited by thermal evaporation process. To leave electroplating parts, this layer is patterned by photoresist (AZ1512). Then, a 0.3 mm thick tungsten layer is sputtered with Ar 12 sccm at a vacuum of 10 mTorr, and rf power of 300 W. Ê /min. And the The deposition rate of tungsten is 200 A tungsten ®lament is fabricated by means of photolithography and etching with tungsten etch solution (34 g KH2PO4, 33 g K3Fe3(CN)6H2O, 13.4 g KOH to 1 l) [12]. The etch Ê /min. rate of tungsten is 1600 A To make the grid electrodes and acceleration electrodes, a thick photoresist (AZ-4620) mold is fabricated. Then, nickel electroplating is performed. The thickness of nickel electrode is about 20 mm. After removals of the thick photoresist and Cr/Au, the Cr etch mask layer is deposited by sputtering and patterned. In this study, chrome instead of silicon nitride has been used as a mask because RIE does harm to the tungsten ®lament when removing the nitride and also Cr etch rate in EPW is low compared to silicon etch rate. For the photolithography of the Cr mask pattern, the spin coating velocity of photoresist is decreased and the UV exposure time is increased. To make a cavity below the tungsten ®lament, the silicon substrate is etched by 30 mm with ethylenediamine:pyrocatechol:water (EPW) (250 ml:40 g:80 ml) solution [13]. The etch rate is 1.2 mm/min. Finally, the Cr layer is removed by using hydrochloric acid. The photograph of the fabricated ion source and separator of the microTOFMS compared with a coin is shown in Fig. 5. Fig. 6 shows the SEM image of the fabricated tungsten ®lament. The width and thickness of the tungsten ®lament are 20 and 0.3 mm, respectively.
Fig. 3. The structure of the ion source and separator.
444
H.J. Yoon et al. / Sensors and Actuators A 97±98 (2002) 441±447
Fig. 4. The fabrication process of the ion source and separator.
5. Experimental results Fig. 7 illustrates the measurement setup for the tungsten ®lament characteristic test of the fabricated ion source that is
Fig. 5. The photograph of the fabricated ion source and separator.
a part of the micro-TOFMS. To protect oxidation of the tungsten ®lament, the fabricated micro-TOFMS is installed in a vacuum chamber equipped with a turbo molecular pump and a mechanical pump. At the present experiments, the pressure of a vacuum chamber is typically less than 1:0 10 6 Torr. If the voltage is applied to both electrodes of the tungsten ®lament, heat is generated at the tungsten ®lament and hot electrons are emitted. To detect emitted
Fig. 6. The SEM image of the fabricated tungsten filament.
H.J. Yoon et al. / Sensors and Actuators A 97±98 (2002) 441±447
445
Fig. 7. The measurement setup for the tungsten filament characteristic test of the fabricated ion source of the micro-TOFMS.
current, a pico-ammeter is connected to the repeller electrode behind the ®lament electrode. Fig. 8 shows the photograph of the incandescent light of the working tungsten ®lament in vacuum. The resistance of the tungsten ®lament is 1.6 kOe at room temperature. Fig. 9 is the plot of measured electron current versus the applied voltage. As the applied voltage increases from 2.5 to 35 V, the electron current increases from 0.7 to 7 nA. To perform the test of the gas molecule ionization by the hot electron emission, sample gas is injected and the vol-
Fig. 8. The photograph of the incandescent light of the working tungsten filament.
tages are applied to each grid electrodes and tungsten electrodes. The tungsten ®lament is dif®cult to make ions, because the thermo-ionization energy is low. Also, it is dif®cult to ®nd out the optimal voltage that must be applied to each electrode with low ion energy. This is important to make ions and distinguish between signal and noise. Therefore, to obtain the ion separation performance, we adopted laser ionization instead of thermo-ionization and redesigned the micro-TOFMS. The redesigned one has a laser hole and is slightly larger than the ®rst one, but is fabricated through the same process except for the hole etching. Fig. 10 shows the photograph of the fabricated micro-TOFMS with a hole. The laser hole is fabricated by the backside silicon etching. The size of the laser hole is 1:1 mm 1:1 mm. The overall
Fig. 9. The plot of measured electron current vs. the applied voltage.
446
H.J. Yoon et al. / Sensors and Actuators A 97±98 (2002) 441±447
Fig. 10. The photograph of the fabricated micro-TOFMS with a hole.
size of the micro-TOFMS with a hole is 15 mm 11 mm 0:5 mm. The photograph of the device assembled for the test is shown in Fig. 11. The fabricated microTOFMS is mounted over channel electron multipliers for the ion detection. To test the ion separation of the microTOFMS, the sample gas is injected to the vacuum chamber and the voltage is applied to each electrode. At this time, the pressure of the vacuum chamber is 2:5 10 5 Torr. The voltage applied to acceleration electrodes (Gs in Fig. 1.) next to the laser hole is 90 V and the voltage applied to acceleration electrodes (Gd in Fig. 1) is 50 V. The injected sample is acetone. Neutral molecules are ionized by the fourth harmonic output of a Q-switched Nd-YAG laser. The pulse width and the wavelength of the laser output are 10 ns and 266 nm, respectively. A UV-grade lens of focal length 35 cm was used to focus the laser light into the ionization region of the mass spectrometer. Fig. 12 is a mass spectrum of acetone using the fabricated micro-TOFMS. Laser pulse energy is 40 mJ. We could get a good result like the conventional mass spectrum of acetone. The peaks in Fig. 12 correspond to CH3, CH3OH, CH3COCH3 ions, respectively. The lower line is noise and the higher line is the signal presenting the mass spectrum of acetone. This result shows that proposed mass spectrometer is promising. 6. Conclusions
Fig. 11. The photograph of the assembled device.
In this paper, the ion source and separator of the microTOFMS have been fabricated by micro-machining. The size and the power of the TOFMS have been reduced compared to the conventional TOFMS. Through the characteristic test
Fig. 12. The mass spectrum of acetone using the fabricated TOFMS.
H.J. Yoon et al. / Sensors and Actuators A 97±98 (2002) 441±447
of a tungsten ®lament, the emission of the hot electrons from the tungsten ®lament has been identi®ed. The test of the gas molecule ionization by the hot electron emission has been performed. To ®nd out the optimal voltage, the microTOFMS with a laser hole for ionization has been fabricated and tested. The electrode voltages adequate for the ion separation have been determined. It has been con®rmed that the fabricated ion separator is feasible for the microTOFMS and the mass spectrum analysis with the microTOFMS can be accomplished in several seconds. In the near future, the power of the tungsten ®lament will be increased and the test of the gas molecule ionization by the hot electron emission will be performed. Also, the research about the miniature of the detector will be performed.
447
time-of-flight mass spectrometer using a simple high-voltage square pulse generator, Rev. Sci. Instrum. 62 (1991) 2125±2130. [11] O. Tabata, R. Asahi, H. Funabashi, K. Shimaoka, S. Sugiyama, Anisotropic etching of silicon in TMAH solutions, Sens. Actuators A 34 (1992) 51±57. [12] Michael Kohler, Etching in Microsystem Technology, Wiley, New York, 1999, p. 334. [13] A. Reisman, M. Berkenblit, S.A. Chan, F.B. Kaufman, D.C. Green, The controlled etching of silicon in catalyzed ethylenediamine± pyrocatechol±water solutions, J. Electrochem. Soc. 126 (8) (1979) 1406±1414.
Biographies
Acknowledgements
Hyeun Joong Yoon was born in Korea in 1976. He received his BS degrees in mechanical engineering and electronics engineering in 1999 and his MS degree in electronics engineering in 2001 from Ajou University. He is currently a PhD candidate in Ajou University. His research interests include micro-mass spectrometer, micro-pump and micro-flow sensor.
This work was performed as a part of Leading Technology Development Project, and was jointly sponsored by Ministry Commerce, Industry & Energy and Ministry of Science & Technology of Republic of Korea.
Jung Hoon Kim was born in Korea in 1973. He received his BS degree in chemistry from Wonkwang University in 1999. He is currently an MS candidate in Wonkwang University. His current research is concentrated on chemical reactions of metal clusters using laser ablation and supersonic molecular beam.
References
Eun Soo Choi was born in Korea in 1972. He received his BS degree in mechanical engineering from Myoungji University in 2001. He is currently an MS candidate in electronics engineering in Ajou University. His research interests include micro-FACS and micro-mass spectrometer.
[1] A.J. Dempster, A new method of positive ray analysis, Phys. Rev. 11 (1918) 316±325. [2] A.O. Nier, T.R. Roberts, The determination of atomic mass doublets of a mass spectrometer, Phys. Rev. 81 (1951) 507±510. [3] J.H. Batey, Quadrupole gas analyzers, Vacuum 37 (1987) 659±668. [4] W.C. Wiley, I.H. McLaren, Time-of-flight mass spectrometer with improved resolution, Rev. Sci. Instrum. 26 (1955) 1150±1157. [5] F.A. White, G.M. Wood, Mass Spectrometry, Wiley, 1986, pp. 73±79. [6] A. Feustel, V. Relling, J. Schroder, J. Muller, A micro-mass spectrometer, in: Proceedings of the SENSOR'95, Kongressband, Vol. B09.4, 1995, pp. 465±470. [7] P. Siebert, G. Petzold, A. Hellenbart, J. Muller, Surface microstructure/miniature mass spectrometer: processing and applications, Appl. Phys. A 67 (1998) 155±160. [8] O. Kornienko, P.T.A. Reilly, W.B. Whitten, J.M. Ramsey, Micro-ion trap mass spectrometry, Rapid Commun. Mass Spectrom. 13 (1999) 50±53. [9] J.J. Tunstall, S. Taylor, R.R.A. Syms, T. Tate, M.M. Ahmad, Silicon micro-machined mass filter for a low power, low cost quadrupole mass spectrometer, in: Proceedings of the IEEE MEMS Workshop, Heidelberg, Germany, January 1998, pp. 438±442. [10] K.W. Jung, S.S. Choi, K.H. Jung, An electron-impact ionization
Sang Sik Yang was born in Korea in 1958. He received his BS and MS degree in mechanical engineering from Seoul National University in 1980 and 1983, respectively. In 1988, he received his PhD degree in mechanical engineering from University of California, Berkeley. He was then a research assistant professor at New Jersey Institute of Technology. Since 1988, he has been a professor in the School of Electronics Engineering at Ajou University. His research interests include the mechanism and actuation of micro-electromechanical devices, motion control and nonlinear control. Kwang Woo Jung was born in Korea in 1960. He received his BS degree in educational chemistry from Seoul National University in 1983, and his MS degree in physical chemistry from the Korea Advanced Institute of Science and Technology (KAIST). In 1992, he received his PhD degree in physical chemistry from the KAIST. Then he spent the following year working as a research scholar at the University of California, Los Angeles and the Georgia Institute of Technology, Atlanta. Since 1993, he has been a professor in the Department of Chemistry at Wonkwang University. His current researches include development of time-of-flight mass spectrometer and its application to the reaction dynamics in laser-ablated cluster chemistry.
Fabrication of a novel micro time-of-¯ight mass spectrometer Hyeun Joong Yoona,*, Jung Hoon Kimb, Eun Soo Choia, Sang Sik Yanga, Kwang Woo Jungb a
School of Electronics Engineering, Ajou University, San 5 Wonchun-Dong, Paldal-Gu, Suwon 442-749, South Korea b Department of Chemistry, Wonkwang University, 344-2 Shinyong-Dong, Iksan 570-749, South Korea Received 11 June 2001; received in revised form 4 December 2001; accepted 5 December 2001
Abstract This paper presents the fabrication and the tests of a novel micro-mass spectrometer fabricated by micro-machining and emission test of hot electron for ionization. A micro-mass spectrometer consists of a micro-ion source and a micro-ion separator. The micro-ion source consists of a hot ®lament and grid electrodes. Electrons emitted from a hot ®lament are to ionize some sample molecules. The ions are accelerated to an ion detector by an electric ®eld of the ion separator. Mass can be analyzed by using the time-of-®ght depending on the mass-to-charge ratio. The current of hot electron emission from the hot ®lament is measured for various input voltages and the ion separation is tested for acetone. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Micro time-of-¯ight mass spectrometer; Micro-ion source; Hot ®lament
1. Introduction The mass spectrometer is a powerful instrument that serves for establishment of the molecular weight and structure of organic compounds, and the identi®cation and determination of the components of inorganic substances. It is used as a de®nitive method to monitor environmental pollutants, to analyze biological samples and drugs, to detect residual gas, and to determine chemical abuse in athletic competitions. The mass spectrometer uses the difference in mass-to-charge ratio (m/e) of ionized atoms or molecules to separate them from each other. It consists of an ion source that generates ions from the sample, an analyzer that separates the ions according to their m/e values, and a detector that gives the intensity of the ion current for each species. The major types of mass spectrometers are the magnetic type [1,2], the quadrupole type [3], the time-of-¯ight type [4], and the Fourier transform ion cyclotron resonance type [5]. The time-of-¯ight mass spectrometer (TOFMS) has some advantages over others [4]. A unique advantage is the speed with which a spectrum can be obtained conveniently. With practical parameter values, a complete spectrum can be obtained every few microseconds. A second feature of the TOFMS is that the entire mass spectrum can be recorded for each accelerating pulse. The TOFMS is also mechanically simple and easy to construct and operate. * Corresponding author. Tel.: 82-31-219-2488; fax: 82-31-212-9531. E-mail address: [email protected] (H.J. Yoon).
Conventional mass spectrometers that are operated with high voltage in high vacuum are bulky and expensive. The micro-mass spectrometer, however, can analyze some samples in low vacuum and can be made cheap by batchfabrication. Recent reports on mass spectrometers fabricated by micro-machining technology show the possibility of the miniature mass spectrometer. Feustel et al. ®rst proposed the structure of a micro-mass spectrometer by using micromachining [6]. Siebert et al. fabricated a miniature mass spectrometer by anisotropic etching and electroplating [7]. But the mass spectrometer of Siebert has a dif®culty in generating plasma. Kornienko et al. fabricated a miniature ion trap mass spectrometer [8]. Tunstall et al. fabricated the micro quadrupole mass ®lter by using micro-machining [9]. Tunstall used a bulky conventional ion source. The micromass spectrometer of this paper differs from others with respect to the ionization and the ion separation method. In this paper, the bulk of the proposed mass spectrometer that is small compared with those mentioned earlier is fabricated by micro-machining and the fabricated micro-ion source and the ion separator are tested, respectively. 2. Principle of separation Fig. 1 shows the principle of the micro time-of-¯ight mass spectrometer (TOFMS). The separation principle of the micro-TOFMS is to separate the ions by the acceleration depending on the mass. That is to say, the heavy ions will
0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 2 ) 0 0 0 3 2 - 8
442
H.J. Yoon et al. / Sensors and Actuators A 97±98 (2002) 441±447
calculated from the time taken for the ion to reach the detector. Wiley and McLaren proposed the two-step acceleration time-of-¯ight mass spectrometer in order to improve the resolution of a non-magnetic TOFMS [4]. The ions are accelerated by two electric ®elds, Es and Ed, through the ionization and the acceleration regions. If it is assumed that ions are generated at the middle of the ionization region with zero initial energy, the ®nal ion energy, Ut is given as Ut qs0 Es qdEd
Fig. 1. The principle of the micro time-of-flight mass spectrometer.
travel more slowly than the light ones. Electrons produced from the heated ®lament are accelerated by the pulsed electric ®eld. The accelerated electrons collide with some sample molecules and ionize them. Ions generated are then accelerated toward the detector by the applied electric ®elds, Es and Ed, which are chosen to optimize the space focusing condition. At all times, the acceleration region has an electric ®eld, and the drift region is a free-®eld region. Then, ions reach the ion detector in the order of mass increase. There is no need for a magnetic ®eld. The relationship between the arrival time and the mass-to-charge ratio of the ion is obtained by considering that the applied electrostatic ®elds determine the velocity of the ion. For the given length of ¯ight and the acceleration voltages, the mass can be
(1)
where q is the ion charge, and s0 the half of the ionization region gap, and d is the acceleration region gap. According to [4,10], the arrival time of an ion, T, is given as ! 1=2 1=2 m 2k0 1=2 dD (2) T 2k0 s0 1=2 2Ut k0 1 where m is the mass of the ion and D is the drift region gap, and k0
s0 Es dEd s 0 Es
(3)
where s0 0:1 cm, d 0:2 cm, D 4:5 cm, Es 200 V/ cm, Ed 250 V/cm, and q corresponds to one electron charge, Ut 70 eVand k0 3:5. Consequently, T can easily be calculated as T 0:44m1=2
(4)
where the units of T and m are ms and amu, respectively. The arrival time of ion represents the linear dependence of ¯ight time on the square root of the mass. Fig. 2 shows the calculated time-of-¯ight versus mass. When the mass is
Fig. 2. The calculated time-of-flight vs. mass.
H.J. Yoon et al. / Sensors and Actuators A 97±98 (2002) 441±447
44 amu, the arrival time is 2.92 ms. This result means a complete mass spectrum of any ion can be obtained in a few microseconds. 3. Structure The micro-TOFMS consists of an ion source, an ion separator and an ion detector. Fig. 3 illustrates the structure of the ion source and separator for the micro-TOFMS. The ion source and ion separator consist of a tungsten ®lament, tungsten electrodes and four pairs of nickel electrodes on a silicon substrate. The tungsten ®lament is to emit hot electron. Nickel electrodes are to focus and accelerate ions. There is a cavity under the tungsten ®lament. The cavity reduces the heat loss by conduction heat transfer. The repeller electrode is used in the test of the electron emission. The width and height of the nickel electrodes are 80 and 20 mm, respectively. The total size of the ion source and separator of the micro-TOFMS is 10 mm 10 mm 1 mm. 4. Fabrication Fig. 4 shows the fabrication process of the ion source and separator of the micro-TOFMS. The starting material of the micro-mass spectrometer is 525 10 mm thick 4 in. n-type h1 0 0i silicon wafer. First, a 0.7 mm thick thermal oxide layer is grown as an etch mask. To make a step between the tungsten ®lament and nickel electrodes, the front side of the silicon wafer is etched by 10 mm with tetramethyl ammonium hydroxide (TMAH) solution. The maximum (1 0 0) silicon etch rate of TMAH is 1.0 mm/min at 90 8C with 22 wt.% concentration [11]. This step is to locate the
443
tungsten ®lament at the center height of nickel electrodes. After the residual oxide is etched off, a 0.7 mm thick thermal oxide is grown again. Ê ) seed layer for electroplating is The Cr/Au (300/3000 A deposited by thermal evaporation process. To leave electroplating parts, this layer is patterned by photoresist (AZ1512). Then, a 0.3 mm thick tungsten layer is sputtered with Ar 12 sccm at a vacuum of 10 mTorr, and rf power of 300 W. Ê /min. And the The deposition rate of tungsten is 200 A tungsten ®lament is fabricated by means of photolithography and etching with tungsten etch solution (34 g KH2PO4, 33 g K3Fe3(CN)6H2O, 13.4 g KOH to 1 l) [12]. The etch Ê /min. rate of tungsten is 1600 A To make the grid electrodes and acceleration electrodes, a thick photoresist (AZ-4620) mold is fabricated. Then, nickel electroplating is performed. The thickness of nickel electrode is about 20 mm. After removals of the thick photoresist and Cr/Au, the Cr etch mask layer is deposited by sputtering and patterned. In this study, chrome instead of silicon nitride has been used as a mask because RIE does harm to the tungsten ®lament when removing the nitride and also Cr etch rate in EPW is low compared to silicon etch rate. For the photolithography of the Cr mask pattern, the spin coating velocity of photoresist is decreased and the UV exposure time is increased. To make a cavity below the tungsten ®lament, the silicon substrate is etched by 30 mm with ethylenediamine:pyrocatechol:water (EPW) (250 ml:40 g:80 ml) solution [13]. The etch rate is 1.2 mm/min. Finally, the Cr layer is removed by using hydrochloric acid. The photograph of the fabricated ion source and separator of the microTOFMS compared with a coin is shown in Fig. 5. Fig. 6 shows the SEM image of the fabricated tungsten ®lament. The width and thickness of the tungsten ®lament are 20 and 0.3 mm, respectively.
Fig. 3. The structure of the ion source and separator.
444
H.J. Yoon et al. / Sensors and Actuators A 97±98 (2002) 441±447
Fig. 4. The fabrication process of the ion source and separator.
5. Experimental results Fig. 7 illustrates the measurement setup for the tungsten ®lament characteristic test of the fabricated ion source that is
Fig. 5. The photograph of the fabricated ion source and separator.
a part of the micro-TOFMS. To protect oxidation of the tungsten ®lament, the fabricated micro-TOFMS is installed in a vacuum chamber equipped with a turbo molecular pump and a mechanical pump. At the present experiments, the pressure of a vacuum chamber is typically less than 1:0 10 6 Torr. If the voltage is applied to both electrodes of the tungsten ®lament, heat is generated at the tungsten ®lament and hot electrons are emitted. To detect emitted
Fig. 6. The SEM image of the fabricated tungsten filament.
H.J. Yoon et al. / Sensors and Actuators A 97±98 (2002) 441±447
445
Fig. 7. The measurement setup for the tungsten filament characteristic test of the fabricated ion source of the micro-TOFMS.
current, a pico-ammeter is connected to the repeller electrode behind the ®lament electrode. Fig. 8 shows the photograph of the incandescent light of the working tungsten ®lament in vacuum. The resistance of the tungsten ®lament is 1.6 kOe at room temperature. Fig. 9 is the plot of measured electron current versus the applied voltage. As the applied voltage increases from 2.5 to 35 V, the electron current increases from 0.7 to 7 nA. To perform the test of the gas molecule ionization by the hot electron emission, sample gas is injected and the vol-
Fig. 8. The photograph of the incandescent light of the working tungsten filament.
tages are applied to each grid electrodes and tungsten electrodes. The tungsten ®lament is dif®cult to make ions, because the thermo-ionization energy is low. Also, it is dif®cult to ®nd out the optimal voltage that must be applied to each electrode with low ion energy. This is important to make ions and distinguish between signal and noise. Therefore, to obtain the ion separation performance, we adopted laser ionization instead of thermo-ionization and redesigned the micro-TOFMS. The redesigned one has a laser hole and is slightly larger than the ®rst one, but is fabricated through the same process except for the hole etching. Fig. 10 shows the photograph of the fabricated micro-TOFMS with a hole. The laser hole is fabricated by the backside silicon etching. The size of the laser hole is 1:1 mm 1:1 mm. The overall
Fig. 9. The plot of measured electron current vs. the applied voltage.
446
H.J. Yoon et al. / Sensors and Actuators A 97±98 (2002) 441±447
Fig. 10. The photograph of the fabricated micro-TOFMS with a hole.
size of the micro-TOFMS with a hole is 15 mm 11 mm 0:5 mm. The photograph of the device assembled for the test is shown in Fig. 11. The fabricated microTOFMS is mounted over channel electron multipliers for the ion detection. To test the ion separation of the microTOFMS, the sample gas is injected to the vacuum chamber and the voltage is applied to each electrode. At this time, the pressure of the vacuum chamber is 2:5 10 5 Torr. The voltage applied to acceleration electrodes (Gs in Fig. 1.) next to the laser hole is 90 V and the voltage applied to acceleration electrodes (Gd in Fig. 1) is 50 V. The injected sample is acetone. Neutral molecules are ionized by the fourth harmonic output of a Q-switched Nd-YAG laser. The pulse width and the wavelength of the laser output are 10 ns and 266 nm, respectively. A UV-grade lens of focal length 35 cm was used to focus the laser light into the ionization region of the mass spectrometer. Fig. 12 is a mass spectrum of acetone using the fabricated micro-TOFMS. Laser pulse energy is 40 mJ. We could get a good result like the conventional mass spectrum of acetone. The peaks in Fig. 12 correspond to CH3, CH3OH, CH3COCH3 ions, respectively. The lower line is noise and the higher line is the signal presenting the mass spectrum of acetone. This result shows that proposed mass spectrometer is promising. 6. Conclusions
Fig. 11. The photograph of the assembled device.
In this paper, the ion source and separator of the microTOFMS have been fabricated by micro-machining. The size and the power of the TOFMS have been reduced compared to the conventional TOFMS. Through the characteristic test
Fig. 12. The mass spectrum of acetone using the fabricated TOFMS.
H.J. Yoon et al. / Sensors and Actuators A 97±98 (2002) 441±447
of a tungsten ®lament, the emission of the hot electrons from the tungsten ®lament has been identi®ed. The test of the gas molecule ionization by the hot electron emission has been performed. To ®nd out the optimal voltage, the microTOFMS with a laser hole for ionization has been fabricated and tested. The electrode voltages adequate for the ion separation have been determined. It has been con®rmed that the fabricated ion separator is feasible for the microTOFMS and the mass spectrum analysis with the microTOFMS can be accomplished in several seconds. In the near future, the power of the tungsten ®lament will be increased and the test of the gas molecule ionization by the hot electron emission will be performed. Also, the research about the miniature of the detector will be performed.
447
time-of-flight mass spectrometer using a simple high-voltage square pulse generator, Rev. Sci. Instrum. 62 (1991) 2125±2130. [11] O. Tabata, R. Asahi, H. Funabashi, K. Shimaoka, S. Sugiyama, Anisotropic etching of silicon in TMAH solutions, Sens. Actuators A 34 (1992) 51±57. [12] Michael Kohler, Etching in Microsystem Technology, Wiley, New York, 1999, p. 334. [13] A. Reisman, M. Berkenblit, S.A. Chan, F.B. Kaufman, D.C. Green, The controlled etching of silicon in catalyzed ethylenediamine± pyrocatechol±water solutions, J. Electrochem. Soc. 126 (8) (1979) 1406±1414.
Biographies
Acknowledgements
Hyeun Joong Yoon was born in Korea in 1976. He received his BS degrees in mechanical engineering and electronics engineering in 1999 and his MS degree in electronics engineering in 2001 from Ajou University. He is currently a PhD candidate in Ajou University. His research interests include micro-mass spectrometer, micro-pump and micro-flow sensor.
This work was performed as a part of Leading Technology Development Project, and was jointly sponsored by Ministry Commerce, Industry & Energy and Ministry of Science & Technology of Republic of Korea.
Jung Hoon Kim was born in Korea in 1973. He received his BS degree in chemistry from Wonkwang University in 1999. He is currently an MS candidate in Wonkwang University. His current research is concentrated on chemical reactions of metal clusters using laser ablation and supersonic molecular beam.
References
Eun Soo Choi was born in Korea in 1972. He received his BS degree in mechanical engineering from Myoungji University in 2001. He is currently an MS candidate in electronics engineering in Ajou University. His research interests include micro-FACS and micro-mass spectrometer.
[1] A.J. Dempster, A new method of positive ray analysis, Phys. Rev. 11 (1918) 316±325. [2] A.O. Nier, T.R. Roberts, The determination of atomic mass doublets of a mass spectrometer, Phys. Rev. 81 (1951) 507±510. [3] J.H. Batey, Quadrupole gas analyzers, Vacuum 37 (1987) 659±668. [4] W.C. Wiley, I.H. McLaren, Time-of-flight mass spectrometer with improved resolution, Rev. Sci. Instrum. 26 (1955) 1150±1157. [5] F.A. White, G.M. Wood, Mass Spectrometry, Wiley, 1986, pp. 73±79. [6] A. Feustel, V. Relling, J. Schroder, J. Muller, A micro-mass spectrometer, in: Proceedings of the SENSOR'95, Kongressband, Vol. B09.4, 1995, pp. 465±470. [7] P. Siebert, G. Petzold, A. Hellenbart, J. Muller, Surface microstructure/miniature mass spectrometer: processing and applications, Appl. Phys. A 67 (1998) 155±160. [8] O. Kornienko, P.T.A. Reilly, W.B. Whitten, J.M. Ramsey, Micro-ion trap mass spectrometry, Rapid Commun. Mass Spectrom. 13 (1999) 50±53. [9] J.J. Tunstall, S. Taylor, R.R.A. Syms, T. Tate, M.M. Ahmad, Silicon micro-machined mass filter for a low power, low cost quadrupole mass spectrometer, in: Proceedings of the IEEE MEMS Workshop, Heidelberg, Germany, January 1998, pp. 438±442. [10] K.W. Jung, S.S. Choi, K.H. Jung, An electron-impact ionization
Sang Sik Yang was born in Korea in 1958. He received his BS and MS degree in mechanical engineering from Seoul National University in 1980 and 1983, respectively. In 1988, he received his PhD degree in mechanical engineering from University of California, Berkeley. He was then a research assistant professor at New Jersey Institute of Technology. Since 1988, he has been a professor in the School of Electronics Engineering at Ajou University. His research interests include the mechanism and actuation of micro-electromechanical devices, motion control and nonlinear control. Kwang Woo Jung was born in Korea in 1960. He received his BS degree in educational chemistry from Seoul National University in 1983, and his MS degree in physical chemistry from the Korea Advanced Institute of Science and Technology (KAIST). In 1992, he received his PhD degree in physical chemistry from the KAIST. Then he spent the following year working as a research scholar at the University of California, Los Angeles and the Georgia Institute of Technology, Atlanta. Since 1993, he has been a professor in the Department of Chemistry at Wonkwang University. His current researches include development of time-of-flight mass spectrometer and its application to the reaction dynamics in laser-ablated cluster chemistry.