Finely focused ion beam technology in III-V compound semiconductors

Nuclear Instruments and Methods in Physics Research B19/20 (1987) 381-387 North-Holland, Amsterdam

FINELY F O C U S E D H. H A S H I M O T O

I O N BEAM T E C H N O L O G Y IN III-V C O M P O U N D

381

SEMICONDUCTORS

and E. M I Y A U C H I

Optoelectronics Joint Research Laboratory, Kamikodanaka 1333, Nakahara-ku, Kawasaki 211, Japan

An advantage of focused ion beam (FIB) implantation has been greatly enhanced by computer software of varying accelerating voltage and interchanging ion species, making it possible to produce doping profiles of any kind without a mask. This implantation technique was also applied to a crystal growth process. The newly constructed facility composed of an MBE growth chamber and an FIB implanter could grow pattern-doping GaAs/AIGaAs multilayered crystals with good quality because of an enclosed process in an ultrahigh vacuum. FIB based technology with superior abilities offers a powerful device fabrication tool and opens up a new field of fabrication process.

1. Introduction Recently, focused ion beams (FIBs) have received much attention for use in microfabrication process of semiconductor devices, such as ion implantation [1-3], etching [4], lithography [5] and mask repair [6]. We have studied focused ion beam implantation (FIBI) in GaAs and A1GaAs and its application to crystal growth for an optoelectronic integrated circuit (OEIC) for the last few years. This implantation technology has various advantages over conventional implantation using broad ion beams. Fine pattern doping in crystal can be performed without a mask. The photolithography process making a mask and subsequent cleaning steps are reduced. Implantation process is simplified significantly. Using alloyed fiquid metal (LM) ion sources, successive interchanging doping with ions for p- and n-type doping and for isolation is done by changing an electric field of an E × B mass separator. Arbitrary lateral and depth doping profiles are formed by controlling dwell time and accelerating voltage of ion beams. This implantation technique with these superior abilities is also suitable for combination processes owing to the maskless method. To fabricate multilayered crystal with threedimensional, pattern-doped structure, we constructed a novel crystal growth facility equipped with an FIB implanter [8]. In this paper, the above crystal growth system using FIBI doping and its possibility of forming finely pattern-doping geometries are described. The results of characterizing grown crystal using photoluminescence and SIMS show that good-quality GaAs and A1GaAs can be grown by this facility. From the experimental results of fabricating a planar GaAs/AIGaAs D H laser and selectively polycrystalline growth for semi-insulat0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

ing electrical isolation, feasibility of this technique for a semiconductor device fabrication process is discussed.

2. Equipment An FIB-based processing facility (called FIBI-MBE) consisting of a maskless ion implanter and molecular beam epitaxial (MBE) growth chamber is illustrated in fig. 1 [8,9]. The maskless implanter is connected with the MBE growth chamber via a sample transfer chamber in an ultrahigh vacuum, better than 5 × 10-10 Torr. To produce pattern-doped multilayer crystals with complex structure required for OEICs, MBE growth and ion implantation doping are performed alternately. Transfer in U H V between processing steps avoids contamination of crystal surfaces caused by atmospheric exposure, thus making it possible to grow a high-quality epitaxial layer on ion implanted GaAs and AIGaAs. Fig. 2 shows a schematic diagram of the implanter which is the main part of the FIBI-MBE system [9]. In this implanter, a Au-Si-Be [2] or P d - N i - S i - B e - B [10] LM ion source is loaded. These ion sources emit Be and Si ions for p- and n-type doping, and B ions for semi-insulating isolation from a single emitter chip. The desired ion species is selected by an E × B mass separator and scanned over a target crystal to implant into designated regions. Ion species are interchangeable without the cumbersome source changing procedure accompanied by breaking the vacuum and readjusting a focusing column. The maximum accelerating voltage is 100 kV, and the minimum spot size of ion beams is less than 0.1/~m. For positional alJEnment, secondary electron signal responses produced by FIB scanning over the benchmark etched on the substrate are used. Alignment accuracy is within 0.1 #m before growth on the III. SEMICONDUCTORS

382

14. Hashimoto, E. Miyauchi / FIB technology in l l I - V compound semiconductors

FMBEgrowth Masklession \\ Sampleloading\chamber im 5pl~anter Anal ~chomber Y ~sis ~

Fig. 1. Schematic structure of the FIBI-MBE system.

mark. After the overgrowth, however, the anisotropic growth of crystallographic orientation distorts the benchmark features and affects alignment. It is enough to detect the mark and implant FIBs into the designated regions within an alignment accuracy of 0.3 /~m even when 2-/~m-thick GaAs or A1GaAs is grown. Ion beam selection, beam scanning, positional alignment, and so on are controlled by a computer via interfaces.

LiquidMetal Ion . . .Source . . e)

For computer control, software as well as hardware is very important to utilize the powerful ability of FIB technology. Implantation is carried out by following procedure: Prior to implantation, basic data for controlling the implanter are collected. The optimum bias voltages of the focusing column lens (steering, stigmatar, objective lens) and E x B mass separator are obtained by sweeping each controlling voltage. The beam diameters in x and y direction and the currents of individual ion species corresponding to the bias voltages are measured automatically through computer software. Such pre-adjustments and data collection are performed at a series of accelerating voltages and stored in the memory. Desired ion species are implanted in succession at the selected energy and dose by adjusting the focusing column with these basic data. One example of Be implantation by continuously varying the accelerating voltage is demonstrated in fig. 3.0.2-/~m-Be FIBs were implanted in lines into the (100) undoped GaAs epitaxial layer with a dose of 5 x 1013/cm 2. Four Be doping lines implanted at different voltages from 40 to 100 kV at intervals of 20 kV were overlapped with separation of 0.5-/~m distance. After annealing at 850 ° C for 20 rain with SiO2 en* 0.2-urn - Be

FIB

'0kW'00kVt:t

undoped GaAs epi.

e-B)

GaAs sub. l a 3rotor

RE( &

DR~

Fig. 2. Detailed diagram of the FIB implanter incorporated in the FIBI-MBE system.

2 ~m |

i

Fig. 3. Cross-section of Be doping GaAs implanted by varying accelerating voltage of Be FIBs in lateral direction from 40 to 100 kV; (a) the sample structure; (b) the SEM photograph.

383

H. Hashimoto, E. Miyauchi / FIB technology in III- V compound semiconductors

2)Jm !

I

Be + : 8 0 k e V n-GaAs

Si+÷ : 1 6 0

keV

n+-GaAs

Fig. 4. SEM photograph of a cross-section of the grown GaAs multilayer crystal doped with Be and Si FIB. capsulation, a SEM photograph of the cleaved and stained sample was taken. It can be seen that doping depth profiles vary continuously with accelerating voltage. The function of varying accelerating voltage together with that of changing dwell time and ion species makes it possible to form any dopant concentration profiles in both lateral and depth directions, which are not achieved by conventional implantation using a broad beam. The photograph shown in fig. 4 is a SEM image of a cross-section of the grown GaAs multilayer crystal doped with Be and Si FIBI. First, 160 keV of 0.2-/~mdiameter Si FIBs were implanted in lines into a 1-/tinthick p-type GaAs epitaxial layer (1 X 1016//cm 3) at an interval of 3 /zm with a dose of I × 1014/cm2. After the same p-type 1-/~m-thick GaAs overgrowth at 600 ° C, 80 keV Be FIBs were implanted into the upper epitaxial layer with positional alignment the same as in Si FIBI. Then, a 0.5-#m capped GaAs layer was grown, followed by annealing at 850 ° C. Fine pattern doping in the upper and lower epitaxial layer with precise alignment is possible.

nealed in the same way as the previous experiment of Be implantation doping. PL intensity depth profiles at room temperature were measured in conjunction with a differential stripping technique at the unimplanted and Be implanted regions indicated by arrows. As shown in fig. 5(b), there is no decrease in PL intensity at the interface where the MBE growth was interrupted at 5 × 10-10 Torr for 3 h during Be implantation. This data shows that a good quality GaAs layer is grown by the F I B I - M B E system, regardless of Be implantation doping. For the same profiles of the compared sample shown in fig. 5(c), which was purposely exposed to the air before Be implantation, intensity near the regrown interface decreased remark-

Direction of PL measure 12 pm 2

4

{a)

3. Doping GaAs/AIGaAs growth The quality of crystals grown by this system is also very good since it is an enclosed process in UHV. For pattern-doping during layer-by-layer growth, it is important to overgrow good-quality crystals on the ion implanted layer. Crystal quality was characterized by measuring photoluminescence (PL), SIMS and fabricating a laser. Figs. 5(b) and (c) show PL intensity depth profiles of Ga.As [8]. The sample structure is ilhistrated in fig. 5(a). First, p-type GaAs was grown on a semi-insulating GaAs substrate to a thickness of 2.4/~m. Be was then implanted in a 500 × 500/~m area on the epitaxial layer at 160 keV with a dose of 1 × 1013/cm 2. Subsequently, about 1.2-/zm-thick, p-type GaAs was overgrown again on the Be implanted layer at 600 ° C without hightemperature post-implant annealing. After growth, the samples were taken out of the growth system and an-

Be-doped GoAs ( I x 1016/cm 3 )

regrowth intefoce pm ---~---~-P-7-'~-~'///-[-~MBE ~ ) ~-~ Be implanted region GoAs sub. Dose: I x lOiS/Gin 2

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Fig. 5. Depth profiles of photoluminescent intensity in Be-FIB doping GaAs; (a) the sample structure and the measured regions indicated by arrows; (b) profiles of the grown crystal in a UHV process; (c) profiles of the compared sample purposely exposed to the atmosphere before Be implantation. III. SEMICONDUCTORS

H. Hashimoto, E. Miyauchi / FIB technology in I l l - V compound semiconductors

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Fig. 6. Carbon, oxygen and implanted Si depth profiles in growth-interrupted GaAs (a) kept in 5 × 10 -t° Torr for 3 h during interruption; (b) exposed to the atmosphere.

ably. Non,radiative recombination centers increased at the interface. It has been speculated that such deterioration is caused by interface states related to adsorbed contaminants during growth interruption for ion implantation [11]. Figs. 6(a) and (b) show the SIMS profiles of carbon and oxygen in grown GaAs [12]. The sample structure was almost the same as illustrated in fig. 5(a) except for implanted species; Si was implanted instead of Be. The profiles in fig. 6(a) show the SIMS data of the sample grown by the through process in UHV. There is no increase in either carbon or oxygen at the regrown interface. It should be noted that carbon is not increased even on the implanted layer. For the exposed sample, a high concentration of carbon was detected at the interface. F o r GaAs, a preheat treatment at 630 ° C for 3 min prior to MBE growth did not remove the adsorbed carbon, though oxygen was removed. Complex defects involving carbon are probably formed at the regrown interface and act as nonradiative centers. This system is also able to grow multilayer crystals including A l G a A s which is more reactive than GaAs without deterioration of regrown interface properties. F o r the A1GaAs growth interruption during ion implantation in a vacuum of 5 × 10 - l ° Torr, the regrown interface properties are almost comparable to the standard MBE crystal without growth interruption. In this material, it is difficult to remove both carbon and oxygen adsorbed on AlGa.As during growth interruption in the atmosphere or in a lower grade vacuum. Crystal quality was also characterized by fabricating a planar A l G a A s / G a A s DH laser shown in fig. ?(a)

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CURRENT (mA) (b) Fig. 7. (a) Schematic structure of the GaAs/A1GaAs DH laser fabricated by using the FIBI-MBE system and (b) the light/current characteristics.

385

H. Hashimoto, E. Miyauchi / FIB technology in III-V compound semiconductors

[13]. The embedded conductive region for current confinement was formed in the undoped A1GaAs layer by Si implantation. Si was implanted at 200 keV with a dose of 5 × 1013/cm 2 at 5 × 10-1° Torr. Following implantation, the active layer was grown directly on the Si implanted layer without high-temperature postimplant annealing. After the crystal growth process was completely over, the sample was annealed at 850 o C to activate implanted Si. Fig. 7(b) shows the light output/current characteristics of the diode laser at room temperature. In spite of the severe growth conditions for the active layer, successful laser operation was observed. Based on this data, it was confirmed that this UHV system can grow pattern-doping GaAs/AIGaAs multilayers with crystal quality capable of fabricating devices.

polyerystalline region p-type GaAs epi. ~.r.-n-type GaAs epi.

/ Be implanted region

(I00) S.I. GaAs

4. Selective polycrystalline growth In these doping experiments, FIB was implanted with a relatively low dose. For high-dose implantation, implant damage remains in crystals after preheat treatment at 600 to 700 o C, and a polycrystalline layer grows on the damaged regions. Using enhanced etching of polycrystalline regions, selective polycrystalline growth also leads to the idea of self-aligned etching. Fig. 8(b) and (c) show a cross-sectional view of GaAs with the polycrystalline region grown selectively on the Be implanted epitaxial layer, followed by etching in a solution of H F : H 2 0 2 : H 2 0 (1:1:10) [14]. The sample structure is schematically shown in fig. 8(a). 0.2/~m-diameter-Be FIBs were line-implanted into an n-type epitaxial layer grown on a (100) S.I. GaAs substrate along [110] at 160 keV with a dose of 2 × 101S/cm2. Following implantation, 2.5-/~m-thick, p-type GaAs was grown on the implanted layer at 600 o C. The surface of overgrown layer was almost flat. As seen in fig. 8(b), no more than a slight ridge of polycrystal on the surface manifests its planarity. When the sample was etched sufficiently, polycrystalline region dissolved selectively, making possible selfaligned etching of submicron structure. In this sample, a narrow groove of about 0.7/~m width and 2.5/zm depth corresponding nearly to the epitaxial layer thickness was created. Side walls of the vertical gap correspond to the (110) crystallographic plane. The resistance of polycrystal formed in a doped overlayer for planar isolation was measured [14]. Fig. 9 illustrates a schematic structure of the prepared sample and the resistivity measurement method. Single-line implantation in semi-insulating GaAs substrates with 0.3/~m-diameter Be ion beams was carried out at 160 keV with various doses. Next, 0.2-/~m-thick n-type GaAs layers with high concentrations (3 × 101S/cm3) were

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Fig. 8. A cross-sectional view of GaAs with the polycrystalline region grown selectively on the Be-FIB line-implanted layer, followed by etching in HF:H202:H20 (1:1:10); (a) the sample structure; (b) after etching for 7 s: (c) 23 s. grown on it. After growth followed by metallization of electrodes, the wafers were mesa-etched down to the substrates, leaving rectangles including the polycrystalline isolation region with 50 p,m width and the electrodes. Fig. 10 shows the resistance between the both electrodes as a function of implanted Be doses. The resistance changed to high resistivity above the dose of 9 × 1015/cm2. This selective polycrystalline growth using the FIBI-MBE system is also useful as a technique for

[mplanf energy ; 160 keY

Micro-

polycrystal/ Post -grown

Implant region

layer ( n*:3 x I0 la cm 3 )

;.I. GoAs substrate -

Fig. 9. Illustration of the sample structure for measuring resistivity of the polycrystalline isolation region.

III. SEMICONDUCTORS

386

H. Hashimoto, E. Miyauchi / FIB technology in I I I - V compound semiconductors

I0 6

this technology, the F I B I - M B E system has been reconstructed to add the functions of electron beam annealing and etching. The study is now under way. Use of the new enclosed facility based on FIB technology will afford the possibility of not only eliminating the need for time-consuming process steps of photolithography and subsequent cleaning, but also fabricating a new structural crystal which is unable to be grown by a conventional process.

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6. Summary

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planar isolation with submicron size and self-aligned etching.

5. Prospect of FIBI Having seen these experimental results of patterndoping GaAs/A1GaAs growth and selective polycrystalline growth using focused ion implantation doping, we feel this F I B I - M B E technique gives us a powerful fabrication tool for semiconductor devices and flexibility of device design, leading to novel devices with high performance. However, there are many issues still to be settled before practical application of this technique. The main issue is the significantly lower throughput of ion implantation doping at production level. Area throughput of FIBI decreases as decreasing FIB diameter. For practical implantation in device fabrication, it is not always necessary to scan over the whole wafer with a finely focused ion beam. Therefore, one way of settling throughput problem is to develop an implanter with variable shaped ion beams like an electron beam lithography system [15]. Technically it is possible to make this kind of implanter. As in the case of the FIBI-MBE, the masldess technology using FIB makes possible the realization of a single-enclosed semiconductor processing facility in UHV, having the potential for dramatic reduction in the number of process steps. We have recently proceeded with this study on F I B I - M B E to the next stage of developing such a single enclosed process. To study

FIB-based technology for I I I - V compound semiconductors was studied. FIB implantation which has various advantages over conventional implantation with a broad ion beam was made more powerful by computer software. Fine pattern-doping geometries with any desired impurity concentration profiles (for p- and n-type, and for isolation) in both lateral and depth directions were formed by computer control. This maskless implantation technique has been applied to MBE growth. The F I B I - M B E system equipped with the FIB implanter could grow implantation doping GaAs/A1GaAs multilayered crystals with good quaiity because of an enclosed through process in UHV. Selective polycrystalline growth on the high-dose-implanted layer using the F I B I - M B E system appears to achieve planar isolation with submicron and self-aligned etching. FIBbased technology will offer a powerful device fabrication tool and open up a new field of fabrication processes in the near future. The authors would like to thank Y. Bamba, A. Takamori, H. Arimoto and T. Morita for carrying out this study. The authors also thank T. Iizuka, M. Hirano and I. Hayashi for their helpful advice and encouragement in this work. The present research is supported by the Agency of Industrial Science and Technology, Ministry of International Trade and Industry.

References

[1] R.L. Kubena, C.L. Anderson, R.L. Seliger, R.A. Jullens, E.H. Stevens and I. Lagnado, J. Vac. Sci. Technol. 19 (1981) 916. [2] E. Miyauchi, H. Arimoto, H. Hashimoto and T. Utsumi, J. Vac. Sci. Technol. B1 (1983) 1113. [3] Y. Bamba, E. Miyauchi, H. Arimoto, K. Kuramoto, A. Takamori and H. Hashimoto, Jpn. J. Appl. Phys. 22 (1983) L650. [4] Y. Ochiai, K. Gamo and S. Namba, J. Vac. Sci. Technol. B1 (1983) 1047. [5] H. Morimoto, H. Onoda, T. Kato, Y. Sassaki, K. Saitoh and T. Kato, J. Vac. Sci. Technol. B4 (1986) 205. [6] A. Wagner, Solid State Technol. (May 1983) 97.

H. Hashimoto, E. Miyaucm / FIB technology in III-V compound semiconductors [7] H. Hashimoto and E. Miyauchi, Abstract of 1984 Int. Conf. Solid State Devices and Materials, Kobe, Japan (1984) p. 121. [8] A. Takamori, E. Miyauchi, H. Arimoto, Y. Bamba and H. Hashimoto, Jpn. J. Appl. Phys. 23 (1984) L599. [9] E. Miyauclai, T. Morita, A. Takamori, H. Arimoto, Y. Bamba and H. Hashimoto, J. Vac. Sci. Technol. B4 (1986) 189. [10] H. Arimoto, A. Takamori, E. Miyauchi and H. Hashimoto, Jpn. J. Appl. Phys. 23 (1984) L165. [11] N.J. Kawai, C.E.C. Wood and L.F. Eastman, J. Appl. Phys. 53 (1982) 6208.

387

[12] A. Takamori, E. Miyauchi, H. Arimoto, Y. Bamba, T. Morita and H. Hashimoto, Jpn. J. Appl. Phys. 24 (1985) L414. [13] A. Takamori, E. Miyauchi, H. Arimoto, T. Morita, Y. Bamba, and H. Hashimoto, 12th Int. Symp. on GaAs and Related Compounds, Karuizawa, Japan, (1985) Inst. Phys. Conf. Ser. No. 79, p. 247. [14] Y. Bamba, E. Miyauclai, H. Arimoto, T. Morita, A. Takamori and H. Hashimoto, Abstract of SPIES's Santa Clara Symp. on Microlithography (1986) p. 80. [15] R.D. Moore, G.A. Caccoma, H.C. Pfeiffer, E.V. Weber and O.C. Woodard, J. Vac. Sci. Technol. 19 (1981) 950.

III. SEMICONDUCTORS