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Recent advance of focused ion beam technology in maskless deposition and patterning
190
Nuclear Instruments and Methods in Physics Research B59/60
(1991) 190-196 North-Holland
Recent advance of focused ion beam technology in maskless deposition and patterning Kenji Gamo and Susumu Namba Research Center for Extreme Materials,
and Faculty of Engineering
Science, Osaka University,
Toyonaka,
Osaka 560, Japan
The present article will review recent advances in focused ion beam (FIB) technology. With increasing demands for scale of integration, microfabrication technology is becoming more important and various new microfabrication tools and processing techniques are desired. FIB is one of the promising tools for future microfabrication technology. This provides maskless patterning capability, which is of importance for process simplification, nanofabrication and in the development of in situ vacuum processing. In situ vacuum processing systems are being developed by combining FIB and a molecular beam epitaxy system. Radiation damage may limit applications of FIB. However, it was demonstrated that low energy FIB ( < 1 keV) with very high brightness was reached and promising results for low damage processing have been obtained
1. Introduction Focused ion beam techniques have many advantages and have attracted much interest as microelectronic device fabrication techniques. A focused ion beam yields a very simple processing. All process steps can be performed without using the lithography process. It enables us to control lateral profiles with deep submicron resolution. This makes it possible to fabricate devices with unique performance [1,2]. Moreover, it is compatible with the UHV system so that a totally dry process in a clean UHV ambient is possible. Typical focused ion beam systems are capable of minimum beam diameters of the order of 50 nm with a current density of 1 A/cm’ or larger. This high current density is also attractive as a research tool for material synthesis and modification. Major disadvantages are low current intensity and radiation damage. Current intensity will be improved by new high brightness ion sources and optics systems. Recently, an achromatic lens system which generates focused beams with a current that is an order of magnitude higher is proposed [3]. Also, to obtain high throughput for low current intensity, various processing techniques have been developed. To reduce radiation damage, the use of a low energy beam is important. Recently, such low energy, fine focused systems have been developed [4,5] and useful means as damage-free processing have been evaluated. Applications of focused ion beam cover a wide range of processing techniques. Presently, nanofabrication [6,7] and in situ processing, which are important for various quantum effect devices, and local modification and 0168-583X/91/$03.50
testing such as X-ray lithography mask and VLSI circuit repair or testing [8,9] are the most attractive applications. The present article will review the recent advance of focused ion beam techniques with emphasis on the applications of low energy FIB to minimization of radiation damage, deposition and in situ processing.
2. Ion beam deposition Thin film formation using energetic ions has been attracting much interest [lo-121 because it yields unique characteristics both in the deposition process and deposited film properties, which are difficult to obtain by conventional deposition techniques. One can expect films with isotopically high purity by using mass analyzed ion beam [ll] and the deposition process can be precisely controlled. It has been observed that the epitaxy temperature for Si and Ge is much lowered by ion beam deposition. Low temperature processing is very important for VLSI fabrication. Use of fine focused ion beams is very attractive as means for selective deposition which is getting increasing interest in microelectronic device fabrication. For direct ion beam deposition, low energy (< 1 keV) beams are required to reduce sputtering. However, formation of a low energy beam is difficult using conventional ion optics systems because of increasing chromatic aberration at the low energy. Recently, however, low energy focused ion beam systems which generate submicron sized beams with a beam energy of 100 eV or lower have been developed using retarding potential techniques [4,5].
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
K. Game, S. Namba / Focused
ion beam technologv
191
Liquid metal ion sources for Si, Ge and Au have already been developed and maskless formation of these films may also be possible by the similar procedures.
3. Ion beam assisted deposition
I
o-1’
10-10
10-9
10-8
I,,, kw-4
Fig. 1. Simulated probe diameter as a function of the beam current for landing energies of 50.1000 and 50000 eV.
Fig. 1 shows an example of focusing performance of low energy focused ion beam systems [4]. As shown in fig. 1, we can expect that 50 eV focused ion beams with a beam size much smaller than 0.2 urn are formed and that 0.2 pm wide and thick lines can be deposited with a speed of a few pm/s. Fig. 2 shows scanning electron micrographs of gallium lines deposited using a 900 pA GaC beam, at a final landing energy of 50 eV, on a 100 nm gold film over silicon (fig. 2a) and bare Si (fig. 2b) 1131.
Although the direct ion beam deposition yields films with high purity and is useful for applications such as circuit modification and lithography mask repair, the deposition rate is low. A more than an order of magnitude larger deposition rate is obtained by utilizing ion beam assisted deposition techniques. Ion beam assisted deposition is done by irradiating focused ion beams in a gas ambient. The irradiation induces decomposition in the adsorbates, volatile fragments are vaporized and nonvolatile fragments pile up at the surface, resulting in film deposition. As source gas, various metal organics like trimethyl aluminum [14], trimethylamine alane [15] and penta-ethoxy tantalum [16], or metal halides like WF, [17,18] or carbonyl [19] are used. The deposition rate is determined by the balance between deposition and sputtering. Therefore, the deposition process depends on parameters such as the gas flwr rate, current intensity, beam energy and ion mass. T’he typical deposition rate is l-30 atoms/ion. Table 1 shows a summary of the film characteristics deposited by ion beam assisted deposition techniques.
Fig. 2. Scanning electron micrographs of gallium lines deposited using a 50 eV, !XKlpA Ga + beam on (a) a 100 nm thick gold film over silicon at a warming rate of 0.1 pm/s (line dose of - 6 x 10i4/cm) and (b) bare silicon at a scanning rate of 1 pm/s (line dose of - 6 x 10’3/cm). III. ION-ENHANCED
DEPOSITION
K Gamo, S. Narnba / Focused ion betm~technology
192
The deposited film contains oxygen or carbon which come from the background atmosphere or an insufficient decomposition. Resistivity which is comparable with or only a few times higher than the bulk value has been obtained for deposited metal films. Low resistive W films which contain low concentration of oxygen were formed by depositing in a clean vacuum ambient [If% In addition to the purity, reduction of damage in the underlying substrate and deposited film is also desirable. To reduce damage, use of low energy FIB is promising. Ch~acte~tion of damage induced by ion beam assisted etching and deposition has been performed by deep level transient spectroscopy (DLTS) measurements. Fig. 3 shows distribution profiles of the L-2 trap center which is produced in C&As by 100 eV focused Ga ion irradiation and is the major trap observed by DLTS in ion implanted GaAs samples (261. At a beam energy of 100 eV, the ion range is only about 1 nm but the trap center was observed much deeper than the ion range.ODefects are observed even at a depth deeper than 2000 A and the defect density increases with increasing the dose. It is likely that these defects are formed by diffusion of mobile defects such as divacancies or interstitials. Although defects are formed at this low energy of irradiation, the defect density is much less compared with high energy ion irradiation. At a dose of 1 x 10” cm*, for example, Schottky contacts could not be formed because of heavy damage for samples implanted at an
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AS-RRADIATED 0 5x10” cm” A lxld cm-’ 0 5x10y cm*.
n ?
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0 5x16 A lxld i 5x10”
._
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“‘8
’ 500
1
“‘I
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*
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1 * 1500
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n
a 1% 2000
(A)
energy of 150 keV, while at 100 eV, good Schottky
contacts were formed. Also, trap centers formed by the low energy focused ion beam irradiation are easily annealed out compared with high energy ~pl~~ti~. At a dose below 1 x 10i5/cm2, the trap center was significantly removed by annealing at 400°C, and was completely removed after annealing at 6OO*C, although defects still remained for the high dose at 5 x 10’6/cm2. It was observed that a very low energy beam is necessary to significantly reduce defect formation. Fig. 4 shows DLTS spectra of MBE-grown n-type GaAs
Ion
C/c,&
42 keV Ga
C/sty&e Al/TMA b Al,MMAA, TEAA= Ta/PMTA d W/WP,
20 keV Ga 50 keV Au 20 keV Ga 50 keV He, Ar, Ne, Xe 50 keV He, Ar, Ne, Xe 0.5.2 keV H, Ar Ga 15 keV Ga 750 eV Ar 50 keV Si 40 keV Ga 60 keV Si
Ga 25, C Al<42,C>33,0<24 AI, C, N Ta 56, C 17,0 25 W 750 25 W93,F5,02 W5O,C30,02,Ga20 Au75,Cc5,0<59,Ga20 Au 98 Au 75, C 140 10 Au 80, C 10, Ga 10
1 4-5 1 8-16 3
50 keV Ga 50 keV Ga
Al85,C10,05 Fe 85, C 10,O 5
>lOOO >lOOO
At’WZ,H,), Pe/Pe(CG),
’ Deposition rate in atoms/ion. b Trimethyl aluminum. ’ Trimethylamine alane, triethylamine alane. d Penta-ethoxy tantalum. ’ Dimethyl gold hexafluoro acetylacetonate.
1
Fig. 3. Distribution profiles of the trap centers produced by 100 eV focused Ga irradiation in n-GaAs and annealing effect.
Film/Source
SiO,/Si(OCH,),
-
‘\\,,
Table 1 Ion beam assisted deposition
W/W(CG), Au/DMGFA =
._
Film composition [at.%]
DPa 4.6 33 nm/lO” ions 10-20 10-50 10-50
R [Q ml
10-t-10* 9x10-4 lo-‘-lo* 1.5x10-5 2~10-~-4xlO-~ 5-13x10-4 2x10-5
Ref. 20 21 14 15 16 17 18 19 22
23 24 25 25
193
K Gamo, S. Nan&a / Focused ion beam technolow Ar IBE (EBEP)
I
100
I
1
300 200 TEMPERATURE (K)
I
400
Fig. 4. DLTS spectra of MBE-grown GaAs. As-grown sample (upper trace), sample irradiated by 60 eV Ar (middle trace) and sample irradiated by 10 eV Ar (bottom trace).
Ll""'1 39
37 35
BINDING
[27]. The as-grown sample shows no defect signals as shown in the upper trace. After the bombardment at 60 eV, three kinds of defect peak were observed as shown in the middle trace. At 10 eV, however, no defect peaks were observed as shown in the bottom trace. This result suggests that a very low energy (below 60 eV) is necessary for damage-free etching or deposition. However, several results have been observed which suggest that defects induced by low energy irradiation induce only a small effect on electrical characteristics. Tungsten films deposited on the GaAs surface using 500 eV H and Ar show Schottky characteristics before annealing [28], which indicates that a low energy beam is useful to minimize defects in the substrates. In addition to defect reduction, it is also important that enough decomposition should occur to obtain high purity. Fihn purity was investigated for deposition of tungsten by measuring XPS spectra. Fig. 5 shows XPS spectra of deposited tungsten [18]. The spectra of bulk tungsten are also shown for comparison. The deposition was performed using 500 eV Ar ions and WF, gas. A spectrum for WOs is also shown in the bottom for comparison. It is clear that there is no significant difference between deposited tungsten and bulk tungsten, suggesting that metal tungsten can be formed by the decomposition of WF, using low energy ion beam irradiation. The deposited tungsten includes only a small amount of W-O bond. The composition ratio estimated from the XPS intensity is 93% W, 4.4% F and 2.3% 0 and the resistivity is only 3 times larger than the bulk value.
33 31 29
ENERGY
W).
Fig. 5. XPS spectra of tungsten deposited by ion beam assisted deposition technique using 500 eV Ar.
4. In
situ procesdng
Many quantum effect devices are fabricated using heterostructure crystals grown by molecular beam epitaxy and have a buried three-dimensional structure. TO fabricate these devices, successive layer growth and patterning techniques are required and the interface of successively grown layers should be clean and contamination-free because defects and contaminations at the interface severely degrade device properties such as luminescence efficiency and carrier mobility. Therefore, in situ processing in clean UHV ambient is very attrao tive. Use of focused ion beams enables one to pattern in situ without using conventional lithography processing and is compatible with the UHV MBE system. However, the processing rate is generaBy low and the substrate receives ion irradiation. Therefore, it is crucial to develop maskless patterning techniques which produce less damage and yield a high processing rate. Applications of photochemical etching and vacuum lithography are of potential importance as such in situ patterning techniques. The surface carrier plays an important role in the photochemical etching process of GaAs using HCl gas. When the surface is irradiated by a focused ion beam, the resultant damage creates recombination centers and consequently, the photo-generated electron-hole pairs III. ION-ENHANCED DEPOSITION
K. Gamo, S. Namba / Focused ion beam technology
194
Fig. 6. Microphotograph of GaAs patterned by the photochemical etching technique. For the etching, samples were irradiated by focused (a) Be at a dose of 3 x 10’2/cm2, and (b) Si and (c) Au at a dose of 1 x 10’3/cm2. The HCl pressure was 10 Torr.
supply of either electrons or holes at the surface can suppress the etching reaction and results in different etching rates between ion implanted and unimplanted regions. Fig. 6 shows typical results of the photochemical etching of a GaAs surface [29]. In fig. 6a, GaAs was first implanted by a 200 keV Be focused ion beam and successively etched in situ by photochemical dry etching. The etched depth was 20 nm. Fig. 6b shows SEM images of GaAs line patterns formed by Si and Au ion implantation. The pattern width of the top regions for recombine
immediately.
An insufficient
OXIDE GROWTH Ga ION BEAM
OVERGROWTH 4
DESORB OXIDE %
IoN BEAM WRIUNG
_
ETCH
Fig. 7. Schematic diagram of in situ fabrication procedures utilizing vacuum lithography.
Au implantation is about 0.2 urn. Lines formed by Si implantation have gently sloped side walls. This may be due to the lateral spread of incident ions or the diffusion of defects created in the substrates. The dose required to obtain an etching selectivity of 5 is only 1 x 10’3/cm2 for Si and Au ions. This low dose makes the present technique very attractive to realize in situ processing techniques which yield high throughput and minimize damage. Fig. 7 shows another example of in situ film pattern formation technique on patterned substrates or vacuum lithography technique [30,31]. In this technique, very thin natural or intentional oxide plays an important role for in situ patterning. First, the thin oxide layer is subjected to sputter removal by Ga focused ion beams and to form an oxide mask for the successive etching. Then, the sample is gas-etched using chlorine gas. The etching is performed at 200°C with and without low energy Ar irradiation. The etching proceeds by desorption of reactant chloride and is isotropic. The ion irradiation gives anisotropy but removes the mask oxide and limits the selective etching depth. After the etching, the mask oxide is thermally desorbed in a phosphorus ambient to grow the overlayer. This process can be repeated by forming oxide in situ. This patterning technique is also very attractive. The oxide mask formation process is performed effectively because the dose required to remove the thin natural oxide layer is only 1 X 10’5/cmz. The damaged layer formed by the oxide removal is etched off by the post-gas dry etching.
K. Gamo, S. Namba / Focused ion beam technology 20 kV
195
FOCUSED
-5OcV
Ar BEAM OVERGROWTH PATTERNED SUBSTRATE
11111
5x
10-4
1orr
ON
Cl~
RESULT
ETCHING ‘i%FiF
Fig. 8. Schematic diagram of the experimental apparatus for the in situ fabrication technique utilizing vacuum lithography.
The same technique is also applied for GaAs by Akiyama et al. [32]. They used electron beam bombardment to form the thin oxide etch mask. Fig. 8 shows the block diagram of the vacuum lithography apparatus. The focused ion beam system, the Ar ion etching system, the oxidation chamber and the gas source MBE system are combined by the UHV vacuum tunnel and the in situ pattern deposition is performed by transferring samples between the four chambers. Fig. 9 shows Nomarski contrast micrograph of fabricated InP/GaInAs double heterostructure wires
[30]. Excellent morphology example.
5.
is obtained
in the present
summary
Recent advance of focused ion beam technology was reviewed. The present focused ion beam systems are capable of a minimum beam diameter of 50 nm with a current density of 1 A/cm2 and the capability of maskless microfabrication makes them promising for wide
IOOOA GalnAs
1 --I
/
5m
InP BUFFER InP SUBSTRATE
Fig. 9. InP/GaInAs
double heterostructure
overgrown on in situ patterned InP substrate by vacuum lithography. III. ION-ENHANCED
DEPOSITION
196
K. Gamo, S. Namba / Focused ion beam technology
applications including in situ fabrication and local modification. Focused ion beam has found important practical applications in mask and VLSI circuit modification and testing but is still at the stage of development and various new techniques have been proposed which yield high throughput and minimize damage introduction.
References [l] R.H. Reuss, D.V. Morgan, E.W. Greeneich, W.M. Clark and D.B. Rensch, J. Vat. Sci. Technol. B3 (1985) 62. [2] H.J. Lezec, K. Ismail, L.J. Mahoney, M.I. Shepard, D.A. Antoniadis and J. Melngaihs, IEEE Electron. Device Lett. EDL-9 (1988) 476. [3] L.R. Harriott, W.L. Brown and D.L. Barr, to be published in J. Vat. Sci. Technol. [4] D.H. Narwn and R.F.W. Pease, J. Vat. Sci. Technol. B6 (1988) 966. [5] H. Kasahara, H. Sawaragi, R. Aihara, K. Gamo and S. Namba, J. Vat. Sci. Technol. B6 (1988) 974. [6] R.L. Kubena, R.J. Joyce, J.W. Ward, H.L. Garvin, F.P. Stratton and RG. Brault, J. Vat. Sci. Technol. B6 (1988) 353. [7] F. Wakaya, T. Kakuta, Y. Takagaki, Y. Yuba, Sadao, Takaoka, K. Murase, T. Shiokawa, K. Gamo and S. Namba, to be published in J. Vat. Sci. Technol. [8] H. Ogawa, K. Tamura, K. Matsuyama, M. Fukumoto and H. Iwasaki, Extended abstr. 22nd Conf. on Solid State Devices and Materials (Jpn. Sot. Appl. Phys., 1990) p. [9] ?‘;shitani, 1990 Int. Microprocess Conf., to be published in Jpn. J. Appl. Phys. [lo] T. Tokuyama, K. Yagi, K. Miyake, M. Tamura, N. Natsuaki and S. Tachi, Nucl. Instr. and Meth. 182/183 (1981) 241. 1111 B.R. Appleton, S.J. Pennycook, R.A. Zuhr, N. Herbots and T.S. NoggIe, Nucl. Instr. and Meth. B19/20 (1987) 975. [12] P.C. Zahn and L.J. Beckers, Appl. Phys. Lett. 41 (1982) 167. [13] D.H. Narum and R.F.W. Pease, J. Vat. Sci. Technol. B6 (1988) 2115.
[14] K. Gamo, N. Takakura, N. Samoto, R. Shimizu and S. Namba, Jpn. J. Appl. Phys. 23 (1984) L293. [15] M.E. Gross, L. Harriott and R.L. Opiia, Jr., to be published in J. Appl. Phys. (161 K. Gamo, D. Takehara, Y. Hamamura, M. Tomita and S. Namba, Microelectron. Eng. 5 (1986) 163. [17] Z. Xu, K. Gamo and S. Namba, J. Vat. Sci. Technol. B7 (1989) 1959. [18] Z. Xu, T. Kosugi, K. Gamo and S. Namba, Jpn. J. Appl.
Phys. 29 (1990) L23. (191 Y. Mashiko, H. Morimoto, H. Koyama, S. Kawazu, T. Kaito and T. Ada&i, Proc. 25th Ann. Reliability Phys., San Diego (1987) p. 111. [20] P.J. Heard and P.D. Prewett, Microcircuit Eng. 11 (1990) 421. (211 L. Harriott and M.J. VasiIe, J. Vat. Sci. Technol. B6 (1988) 1035. [22] G.M. Shedd, H. Lexc, A.D. Dubner and J. Mehxaihs. 1 , Appl. Phys. Lett. 49 (1986) 1584. ]231 P.G. Blauner, Y. Butt, J.S. Ro, C.V. Thompson and J. Melngaihs, J. Vat. Sci. Technol. B7 (1989) 1816. t241 H. Komano, Y. Ogawa and T. Takigawa, Jpn. J. Appl. Phys. 28 (1989) 2372. 1251 R.L. Kubena, F.P. Stratton and T.M. Mayer, J. Vat. Sci. Technol. B6 (1988) 1865. WI T. Kosugi, K. Gamo and S. Namba, to be published in Jpn. J. Appl. Phys. ]271 J. Yu, N. Masui, Y. Yuba, T. Ham, M. Hamagaki, Y. Aoyagi, K. Gamo and S. Namba, Jpn. J. Appl. Phys. 28 (1989) 2391. WI Z. Xu, T. Kosugi, K. Gamo and S. Namba, Jpn. J. Appl. Phys. 29 (1990) L23. ~291 H. Arimoto, M. Kosugi, H. Kitada and E. Miyauchi, Microcircuit Eng. 9 (1989) 321. 1301L.R. Harriott, H. Temkin, R.A. Hamm, J. Weiner and M.B. Panish, J. Vat. Sci. Technol. B7 (1989) 1467. 1311J.S. Weiner, Y.L. Wang, H. Temkin, L.R. Harriott, R.A. Hamm and M.B. Panish, to be published in J. Vat. Sci. Technol. [32] M. Taneya, Y. Sugimoto, H. Hidaka and K. Akita, Jpn. J. Appl. Phys. 29 (1990) L182.
Nuclear Instruments and Methods in Physics Research B59/60
(1991) 190-196 North-Holland
Recent advance of focused ion beam technology in maskless deposition and patterning Kenji Gamo and Susumu Namba Research Center for Extreme Materials,
and Faculty of Engineering
Science, Osaka University,
Toyonaka,
Osaka 560, Japan
The present article will review recent advances in focused ion beam (FIB) technology. With increasing demands for scale of integration, microfabrication technology is becoming more important and various new microfabrication tools and processing techniques are desired. FIB is one of the promising tools for future microfabrication technology. This provides maskless patterning capability, which is of importance for process simplification, nanofabrication and in the development of in situ vacuum processing. In situ vacuum processing systems are being developed by combining FIB and a molecular beam epitaxy system. Radiation damage may limit applications of FIB. However, it was demonstrated that low energy FIB ( < 1 keV) with very high brightness was reached and promising results for low damage processing have been obtained
1. Introduction Focused ion beam techniques have many advantages and have attracted much interest as microelectronic device fabrication techniques. A focused ion beam yields a very simple processing. All process steps can be performed without using the lithography process. It enables us to control lateral profiles with deep submicron resolution. This makes it possible to fabricate devices with unique performance [1,2]. Moreover, it is compatible with the UHV system so that a totally dry process in a clean UHV ambient is possible. Typical focused ion beam systems are capable of minimum beam diameters of the order of 50 nm with a current density of 1 A/cm’ or larger. This high current density is also attractive as a research tool for material synthesis and modification. Major disadvantages are low current intensity and radiation damage. Current intensity will be improved by new high brightness ion sources and optics systems. Recently, an achromatic lens system which generates focused beams with a current that is an order of magnitude higher is proposed [3]. Also, to obtain high throughput for low current intensity, various processing techniques have been developed. To reduce radiation damage, the use of a low energy beam is important. Recently, such low energy, fine focused systems have been developed [4,5] and useful means as damage-free processing have been evaluated. Applications of focused ion beam cover a wide range of processing techniques. Presently, nanofabrication [6,7] and in situ processing, which are important for various quantum effect devices, and local modification and 0168-583X/91/$03.50
testing such as X-ray lithography mask and VLSI circuit repair or testing [8,9] are the most attractive applications. The present article will review the recent advance of focused ion beam techniques with emphasis on the applications of low energy FIB to minimization of radiation damage, deposition and in situ processing.
2. Ion beam deposition Thin film formation using energetic ions has been attracting much interest [lo-121 because it yields unique characteristics both in the deposition process and deposited film properties, which are difficult to obtain by conventional deposition techniques. One can expect films with isotopically high purity by using mass analyzed ion beam [ll] and the deposition process can be precisely controlled. It has been observed that the epitaxy temperature for Si and Ge is much lowered by ion beam deposition. Low temperature processing is very important for VLSI fabrication. Use of fine focused ion beams is very attractive as means for selective deposition which is getting increasing interest in microelectronic device fabrication. For direct ion beam deposition, low energy (< 1 keV) beams are required to reduce sputtering. However, formation of a low energy beam is difficult using conventional ion optics systems because of increasing chromatic aberration at the low energy. Recently, however, low energy focused ion beam systems which generate submicron sized beams with a beam energy of 100 eV or lower have been developed using retarding potential techniques [4,5].
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
K. Game, S. Namba / Focused
ion beam technologv
191
Liquid metal ion sources for Si, Ge and Au have already been developed and maskless formation of these films may also be possible by the similar procedures.
3. Ion beam assisted deposition
I
o-1’
10-10
10-9
10-8
I,,, kw-4
Fig. 1. Simulated probe diameter as a function of the beam current for landing energies of 50.1000 and 50000 eV.
Fig. 1 shows an example of focusing performance of low energy focused ion beam systems [4]. As shown in fig. 1, we can expect that 50 eV focused ion beams with a beam size much smaller than 0.2 urn are formed and that 0.2 pm wide and thick lines can be deposited with a speed of a few pm/s. Fig. 2 shows scanning electron micrographs of gallium lines deposited using a 900 pA GaC beam, at a final landing energy of 50 eV, on a 100 nm gold film over silicon (fig. 2a) and bare Si (fig. 2b) 1131.
Although the direct ion beam deposition yields films with high purity and is useful for applications such as circuit modification and lithography mask repair, the deposition rate is low. A more than an order of magnitude larger deposition rate is obtained by utilizing ion beam assisted deposition techniques. Ion beam assisted deposition is done by irradiating focused ion beams in a gas ambient. The irradiation induces decomposition in the adsorbates, volatile fragments are vaporized and nonvolatile fragments pile up at the surface, resulting in film deposition. As source gas, various metal organics like trimethyl aluminum [14], trimethylamine alane [15] and penta-ethoxy tantalum [16], or metal halides like WF, [17,18] or carbonyl [19] are used. The deposition rate is determined by the balance between deposition and sputtering. Therefore, the deposition process depends on parameters such as the gas flwr rate, current intensity, beam energy and ion mass. T’he typical deposition rate is l-30 atoms/ion. Table 1 shows a summary of the film characteristics deposited by ion beam assisted deposition techniques.
Fig. 2. Scanning electron micrographs of gallium lines deposited using a 50 eV, !XKlpA Ga + beam on (a) a 100 nm thick gold film over silicon at a warming rate of 0.1 pm/s (line dose of - 6 x 10i4/cm) and (b) bare silicon at a scanning rate of 1 pm/s (line dose of - 6 x 10’3/cm). III. ION-ENHANCED
DEPOSITION
K Gamo, S. Narnba / Focused ion betm~technology
192
The deposited film contains oxygen or carbon which come from the background atmosphere or an insufficient decomposition. Resistivity which is comparable with or only a few times higher than the bulk value has been obtained for deposited metal films. Low resistive W films which contain low concentration of oxygen were formed by depositing in a clean vacuum ambient [If% In addition to the purity, reduction of damage in the underlying substrate and deposited film is also desirable. To reduce damage, use of low energy FIB is promising. Ch~acte~tion of damage induced by ion beam assisted etching and deposition has been performed by deep level transient spectroscopy (DLTS) measurements. Fig. 3 shows distribution profiles of the L-2 trap center which is produced in C&As by 100 eV focused Ga ion irradiation and is the major trap observed by DLTS in ion implanted GaAs samples (261. At a beam energy of 100 eV, the ion range is only about 1 nm but the trap center was observed much deeper than the ion range.ODefects are observed even at a depth deeper than 2000 A and the defect density increases with increasing the dose. It is likely that these defects are formed by diffusion of mobile defects such as divacancies or interstitials. Although defects are formed at this low energy of irradiation, the defect density is much less compared with high energy ion irradiation. At a dose of 1 x 10” cm*, for example, Schottky contacts could not be formed because of heavy damage for samples implanted at an
1WeV
: SlO
Ga’ FIB
AS-RRADIATED 0 5x10” cm” A lxld cm-’ 0 5x10y cm*.
n ?
a i=
“•
*.
ANN;$Wz
0 5x16 A lxld i 5x10”
._
d 1
0
AT 600% cm”*
cm” cm*
-D-----Q-__ ’ . . .
‘. 0
lo*
ANNEALED
--‘oo-.___a
-
s
t;_$d’C
‘O-.._O
@‘O..
“‘8
’ 500
1
“‘I
n
*
3
3
1000 DEPTH
1 * 1500
0
n
a 1% 2000
(A)
energy of 150 keV, while at 100 eV, good Schottky
contacts were formed. Also, trap centers formed by the low energy focused ion beam irradiation are easily annealed out compared with high energy ~pl~~ti~. At a dose below 1 x 10i5/cm2, the trap center was significantly removed by annealing at 400°C, and was completely removed after annealing at 6OO*C, although defects still remained for the high dose at 5 x 10’6/cm2. It was observed that a very low energy beam is necessary to significantly reduce defect formation. Fig. 4 shows DLTS spectra of MBE-grown n-type GaAs
Ion
C/c,&
42 keV Ga
C/sty&e Al/TMA b Al,MMAA, TEAA= Ta/PMTA d W/WP,
20 keV Ga 50 keV Au 20 keV Ga 50 keV He, Ar, Ne, Xe 50 keV He, Ar, Ne, Xe 0.5.2 keV H, Ar Ga 15 keV Ga 750 eV Ar 50 keV Si 40 keV Ga 60 keV Si
Ga 25, C Al<42,C>33,0<24 AI, C, N Ta 56, C 17,0 25 W 750 25 W93,F5,02 W5O,C30,02,Ga20 Au75,Cc5,0<59,Ga20 Au 98 Au 75, C 140 10 Au 80, C 10, Ga 10
1 4-5 1 8-16 3
50 keV Ga 50 keV Ga
Al85,C10,05 Fe 85, C 10,O 5
>lOOO >lOOO
At’WZ,H,), Pe/Pe(CG),
’ Deposition rate in atoms/ion. b Trimethyl aluminum. ’ Trimethylamine alane, triethylamine alane. d Penta-ethoxy tantalum. ’ Dimethyl gold hexafluoro acetylacetonate.
1
Fig. 3. Distribution profiles of the trap centers produced by 100 eV focused Ga irradiation in n-GaAs and annealing effect.
Film/Source
SiO,/Si(OCH,),
-
‘\\,,
Table 1 Ion beam assisted deposition
W/W(CG), Au/DMGFA =
._
Film composition [at.%]
DPa 4.6 33 nm/lO” ions 10-20 10-50 10-50
R [Q ml
10-t-10* 9x10-4 lo-‘-lo* 1.5x10-5 2~10-~-4xlO-~ 5-13x10-4 2x10-5
Ref. 20 21 14 15 16 17 18 19 22
23 24 25 25
193
K Gamo, S. Nan&a / Focused ion beam technolow Ar IBE (EBEP)
I
100
I
1
300 200 TEMPERATURE (K)
I
400
Fig. 4. DLTS spectra of MBE-grown GaAs. As-grown sample (upper trace), sample irradiated by 60 eV Ar (middle trace) and sample irradiated by 10 eV Ar (bottom trace).
Ll""'1 39
37 35
BINDING
[27]. The as-grown sample shows no defect signals as shown in the upper trace. After the bombardment at 60 eV, three kinds of defect peak were observed as shown in the middle trace. At 10 eV, however, no defect peaks were observed as shown in the bottom trace. This result suggests that a very low energy (below 60 eV) is necessary for damage-free etching or deposition. However, several results have been observed which suggest that defects induced by low energy irradiation induce only a small effect on electrical characteristics. Tungsten films deposited on the GaAs surface using 500 eV H and Ar show Schottky characteristics before annealing [28], which indicates that a low energy beam is useful to minimize defects in the substrates. In addition to defect reduction, it is also important that enough decomposition should occur to obtain high purity. Fihn purity was investigated for deposition of tungsten by measuring XPS spectra. Fig. 5 shows XPS spectra of deposited tungsten [18]. The spectra of bulk tungsten are also shown for comparison. The deposition was performed using 500 eV Ar ions and WF, gas. A spectrum for WOs is also shown in the bottom for comparison. It is clear that there is no significant difference between deposited tungsten and bulk tungsten, suggesting that metal tungsten can be formed by the decomposition of WF, using low energy ion beam irradiation. The deposited tungsten includes only a small amount of W-O bond. The composition ratio estimated from the XPS intensity is 93% W, 4.4% F and 2.3% 0 and the resistivity is only 3 times larger than the bulk value.
33 31 29
ENERGY
W).
Fig. 5. XPS spectra of tungsten deposited by ion beam assisted deposition technique using 500 eV Ar.
4. In
situ procesdng
Many quantum effect devices are fabricated using heterostructure crystals grown by molecular beam epitaxy and have a buried three-dimensional structure. TO fabricate these devices, successive layer growth and patterning techniques are required and the interface of successively grown layers should be clean and contamination-free because defects and contaminations at the interface severely degrade device properties such as luminescence efficiency and carrier mobility. Therefore, in situ processing in clean UHV ambient is very attrao tive. Use of focused ion beams enables one to pattern in situ without using conventional lithography processing and is compatible with the UHV MBE system. However, the processing rate is generaBy low and the substrate receives ion irradiation. Therefore, it is crucial to develop maskless patterning techniques which produce less damage and yield a high processing rate. Applications of photochemical etching and vacuum lithography are of potential importance as such in situ patterning techniques. The surface carrier plays an important role in the photochemical etching process of GaAs using HCl gas. When the surface is irradiated by a focused ion beam, the resultant damage creates recombination centers and consequently, the photo-generated electron-hole pairs III. ION-ENHANCED DEPOSITION
K. Gamo, S. Namba / Focused ion beam technology
194
Fig. 6. Microphotograph of GaAs patterned by the photochemical etching technique. For the etching, samples were irradiated by focused (a) Be at a dose of 3 x 10’2/cm2, and (b) Si and (c) Au at a dose of 1 x 10’3/cm2. The HCl pressure was 10 Torr.
supply of either electrons or holes at the surface can suppress the etching reaction and results in different etching rates between ion implanted and unimplanted regions. Fig. 6 shows typical results of the photochemical etching of a GaAs surface [29]. In fig. 6a, GaAs was first implanted by a 200 keV Be focused ion beam and successively etched in situ by photochemical dry etching. The etched depth was 20 nm. Fig. 6b shows SEM images of GaAs line patterns formed by Si and Au ion implantation. The pattern width of the top regions for recombine
immediately.
An insufficient
OXIDE GROWTH Ga ION BEAM
OVERGROWTH 4
DESORB OXIDE %
IoN BEAM WRIUNG
_
ETCH
Fig. 7. Schematic diagram of in situ fabrication procedures utilizing vacuum lithography.
Au implantation is about 0.2 urn. Lines formed by Si implantation have gently sloped side walls. This may be due to the lateral spread of incident ions or the diffusion of defects created in the substrates. The dose required to obtain an etching selectivity of 5 is only 1 x 10’3/cm2 for Si and Au ions. This low dose makes the present technique very attractive to realize in situ processing techniques which yield high throughput and minimize damage. Fig. 7 shows another example of in situ film pattern formation technique on patterned substrates or vacuum lithography technique [30,31]. In this technique, very thin natural or intentional oxide plays an important role for in situ patterning. First, the thin oxide layer is subjected to sputter removal by Ga focused ion beams and to form an oxide mask for the successive etching. Then, the sample is gas-etched using chlorine gas. The etching is performed at 200°C with and without low energy Ar irradiation. The etching proceeds by desorption of reactant chloride and is isotropic. The ion irradiation gives anisotropy but removes the mask oxide and limits the selective etching depth. After the etching, the mask oxide is thermally desorbed in a phosphorus ambient to grow the overlayer. This process can be repeated by forming oxide in situ. This patterning technique is also very attractive. The oxide mask formation process is performed effectively because the dose required to remove the thin natural oxide layer is only 1 X 10’5/cmz. The damaged layer formed by the oxide removal is etched off by the post-gas dry etching.
K. Gamo, S. Namba / Focused ion beam technology 20 kV
195
FOCUSED
-5OcV
Ar BEAM OVERGROWTH PATTERNED SUBSTRATE
11111
5x
10-4
1orr
ON
Cl~
RESULT
ETCHING ‘i%FiF
Fig. 8. Schematic diagram of the experimental apparatus for the in situ fabrication technique utilizing vacuum lithography.
The same technique is also applied for GaAs by Akiyama et al. [32]. They used electron beam bombardment to form the thin oxide etch mask. Fig. 8 shows the block diagram of the vacuum lithography apparatus. The focused ion beam system, the Ar ion etching system, the oxidation chamber and the gas source MBE system are combined by the UHV vacuum tunnel and the in situ pattern deposition is performed by transferring samples between the four chambers. Fig. 9 shows Nomarski contrast micrograph of fabricated InP/GaInAs double heterostructure wires
[30]. Excellent morphology example.
5.
is obtained
in the present
summary
Recent advance of focused ion beam technology was reviewed. The present focused ion beam systems are capable of a minimum beam diameter of 50 nm with a current density of 1 A/cm2 and the capability of maskless microfabrication makes them promising for wide
IOOOA GalnAs
1 --I
/
5m
InP BUFFER InP SUBSTRATE
Fig. 9. InP/GaInAs
double heterostructure
overgrown on in situ patterned InP substrate by vacuum lithography. III. ION-ENHANCED
DEPOSITION
196
K. Gamo, S. Namba / Focused ion beam technology
applications including in situ fabrication and local modification. Focused ion beam has found important practical applications in mask and VLSI circuit modification and testing but is still at the stage of development and various new techniques have been proposed which yield high throughput and minimize damage introduction.
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