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Microfabrication of semiconductors by means of excimer laser doping
Microelectronic Elsevier
Engineering
131
20 (1993) 131-143
Microfabrication of semiconductors by means of excimer laser doping Koichi Toyoda and Koji Sugioka The Institute of Physical and Chemical Research (RIKEN),
Wako Saitama 351-01, Japan
Abstract.
Direct formation of three-dimensional structures of Si in GaAs by excimer laser doping has been demonstrated. By using a thin solid Si dopant source, linewidths as narrow as 0.3 km are achieved. On the other hand, linewidths of 0.76 p.m are obtained by projection-patterned doping in SM., gas atmosphere. The linewidth of the doped regions are discussed in relation to the lateral profiles of the calculated transient temperature rise of the substrates. The patterned doping technique is applied to the selective metallization of GaAs. Au thin films are deposited selectively on the doped regions by the subsequent electroless plating process. Using the selective metallization process, non-alloyed ohmic contracts can be fabricated with a specific contact resistance as low as 4.95 x 10m6 Q cm*, which is l/l50 of that of the conventionally alloyed contacts.
Keywords. Excimer laser doping; Si-doped GaAs; Patterned doping; Submicron ing; Projection system; Selective metallization; Non-alloyed ohmic contact
pattem-
1. Introduction
With the progress of microelectronics, the development of sophisticated technology which can realize the formation of three-dimensional structures with very small dimensions in semiconductors is required. Excimer laser doping is one of the promising techniques for microfabrication, since the short wavelengths and the short pulse widths of excimer lasers make it possible to dope impurities with extremely high dopant concentration in excess of the solid solubility at very shallow depths from the surface [l-3]. In addition, by heating locally using a tightly focused laser beam or a mask, lateral confinement of the doped impurity to very small dimensions is achieved. In the direct-writing process using a scanned, tightly focused Ar-ion laser beam, high-resolution doped patterns of B with a linewidth of 0.3 km in Si substrates have been demonstrated [4]. Such a process is maskless and has great flexibility but the processing time is limited by the translation rate (C440 pm/s) of the wafer. Correspondence too:Koichi Toyoda, The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-01, Japan.
0167-9317/93/$06.00
@ 1993 Elsevier Science Publishers B.V. All rights reserved
132
K. Toyoda, K. Sugioka
I Excimer laser doping
For greater throughputs, large-area pattern transfer using masks is considered [5-71. In the present paper, two types of submicron patterned doping of Si into GaAs by excimer lasers using masks are described. One of them is the patterned doping using a thin solid Si dopant source and a contact mask. The advantage of the process is that the doping is completed by only a single pulse because of the presence of a sufficiently large amount of dopant source on the surface. Furthermore, Si thin film may act as an encapsulation. In fact, the doped regions of all samples showed a smooth surface. On the other hand, surface roughness was observed in the case of gas-phase doping at higher laser fluence and larger number of laser pulses, although the crystalline quality and the stoichiometry were maintained [8]. However, several processes are required, i.e., the deposition of the Si dopant source and its removal. The second method is the patterned doping using a projection system. In this system, a conventional mask projection system is used. A reactive gas is introduced underneath the lens and used as a dopant source. As a result, the projection-patterned doping simplifies the process. The doped regions had a smooth surface and a good crystalline quality at suitable experimental conditions, although surface roughness was observed at higher laser fluence and larger number of laser pulses [S]. Here, we discuss the linewidths of the regions doped by the two types of patterned doping with temperature profiles calculated by the transient heat conduction equation in the substrates. In addition, the patterned doping technique is applied to selective metallization of the doped samples [9-121. Au thin films were deposited selectively on the doped regions by a subsequent electroless plating process. Self-aligned microfabrication of non-alloyed ohmic contacts of GaAs by the simple technique is investigated. Koichi Toyoda received his BE in Electrical Engineering in 1959 and the Dr. Eng. degree in 1968, both from Osaka University, Japan. He joined RIKEN, a government-supported research institute, in 1968. In RIKEN, he has worked on the research of various high-power lasers such as carbon dioxide laser, TEA-CO2 laser, and HBr chemical laser. Since 1978, his main interest has been the photochemical microfabrication of solid surfaces employing excimer lasers and short-wavelength light sources. Dr. Toyoda is a member of the Institute of Electrical Engineers in Japan, Japan Society of Applied Physics, Physical Society of Japan and The Laser Society of Japan. Koji Sugioka received his BE in Electronics in 1984 and ME in 1986, both from Waseda university in Japan. He joined The Institute of Physical and Chemical Research (RIKEN) in 1986. In RIKEN, he is engaged in microfabrication of semiconductors and surface modification of metal surfaces using excimer lasers. He is a member of the Japan Society of Applied Physics and The Laser Society of Japan.
K. Toyoda, K. Sugioka
I Excimer laser doping
133
2. Depth profiles of Si in GaAs In order to investigate the vertical confinement of impurities by laser doping, the depth profiles of Si concentration in GaAs were measured by secondary ion mass spectroscopy (SIMS). The samples were prepared by irradiating GaAs substrates with a XeCl excimer laser beam with a wavelength of 308 nm and a pulse width of 8 ns in an ambient 10% SiH,-He gas. The gas pressure was 100Torr. As a substrate, non-doped semi-insulating (SI) (100) liquid encapsulated Czochralski (LEC) GaAs (resistivity >l x lo7 rR.cm) was used. In this case, the unpatterned doping was carried out without mask. The details are described elsewhere [3,8]. Figure 1 shows the depth profiles for various laser fluences for 100 pulses and a SiH, gas pressure of 100 Torr. The peak concentration of Si is as high as l-3 X lo*’ cmP3 at the surface in all cases, and the Si concentration decreases with increasing depth from the surface. The doped Si atoms extend deeper with increasing laser fluence. The doping depth, which is defined as the distance from the surface to the layer where the concentration is one-thousandth of the surface concentration, is less than about 110 nm even at the maximum laser fluence of 170 mJ/cm*. The minimum doping depth of 30 nm was obtained for a fluence of 56 mJ/cm*. Thus, such an ultra-shallow junction can be formed by laser doping.
lon
I
I
100 Torr 10% SiHL 100 Pulses -a56mJlcm2
_
lozc
p9 ”
5 i-
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5 c
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lo" Depth
Fig.
1. The fluences
I nm 1
depth profiles of Si concentration in GaAs measured by SIMS at various laser of 56, 83, 100, 130, and 170 mJ/cm’ at 100 pulses in a SiH, gas of 100 Torr.
K. Toyoda, K. Sugioka
134
3. Patterned
I Excimer laser doping
doping using a thin solid dopant source
3.1. Experimental
procedure
Figure 2 shows the experimental procedure: (a) Silicon thin films of 10 nm thickness were deposited on the SI GaAs substrates with a deposition rate of 0.3 rim/s at room temperature by ultra-high vacuum (UHV) evaporation. The pressure during the deposition was less than 1 x lo-’ Torr. (b) A single pulse of the XeCl excimer laser beam irradiated the sample surface nearly perpendicular in air. A reticle with chromium patterns of 1 pm features on a synthetic quartz substrate, which was kept in contact with the sample surfaces, was used as a contact mask to define the doped region. The laser fluence was varied from 100 to 150 mJ/cm* at the sample surface. (c) After irradiation, the samples were etched in boiling ethylenediamine (7.5 ml) + H,O (2.4 ml) + pyrazine (0.24 g) solution for several minutes until the remaining Si on the surface was completely removed without etching the GaAs nor the Si-doped GaAs [13]. Then, the Si atoms were doped into the irradiated region only. 3.2. Results and discussion For quantitative measurement of the linewidth of the doped regions, relief structures of the doped regions were formed by etching the non-doped GaAs selectively in 40HF + lH,O + 4H,O, solution for 50 s with light illumination [14]. The linewidth of the remaining relief where the Si concentration was high was measured as the linewidth of the doped regions using a scanning electron microscope (SEM).
308 nm XeCl Excimer IpO - 150 mJ/cm* Single Pulse
Laser
IO nm Thick Oepsi ted
SI (100) (a)
LEC
Si
/-Cr
GaAs
Mask
1After
Etching)
Si Doped
Cb)
Region
(C)
Fig. 2. The experimental procedure of the patterned doping using a thin solid Si dopant source: (a) Deposition of Si thin film on GaAs; (b) irradiation of XeCl excimer laser beam to the sample surface through a reticle; and (c) removal of the remaining Si on the surface.
K. Toyoda, K. Sugioka
I Excimer laser doping
135
In order to investigate the dependence of the linewidth on the laser fluence, variation of the linewidth of the remaining relief as a function of the laser fluence is shown in Fig. 3. The linewidth decreases monotonously from 0.75 km at a fluence of 150 mJ/cm* to 0.3 km at 100 mJ/cm2. In addition, it should be noted that the linewidths are smaller than the mask’s feature size of 1 pm in all cases. In order to discuss the power-dependent narrowing of the linewidth, it is necessary to analyze the transient diffusion of dopant due to transient heating by a laser pulse. For simplicity, we assumed that the linewidth is determined by the lateral dimension over which the surface temperature exceeds the melting point, since the impurities distribute only in the molten region, as was modeled by Sigmon [15]. Here, the temperature distributions T(x, y, z, t) in the substrates produced by the laser irradiation were calculated by integrating the thermal conduction equation. The parameters used in the calculation are summarized in Table 1. Since the thermal diffusivity and the thermal conductivity are sensitive functions of temperature, the intermediate values between room temperature and the melting point of GaAs were used as effective values [16]. The reflectivity was a value for Si from Ref. [17], because the sample
Table 1 Parameters used in the calculation of surface temperature distribution Thermal diffusivity [ 1l] Thermal conductivity [1l] Reflectivity [ 121 Absorption coefficient [12] Beam size Heating duration
X
0.16 cm’/s 0.27 W/cm”C 59.7% 788,550 cm-’ 1 pm 8 ns
Ipml
Fig. 3. The variations of the linewidth of the doped regions by irradiation of single-pulse XeCl excimer laser beam as a function of laser fluences. The solid line and dashed line correspond to experimental and calculated results, respectively.
K. Toyoda, K. Sugioka
136
-+-Experiment
t
L
OO
I Excimer laser doping
I 100
-----Calculation 150
Fluence at Substrate
Fig. 4. The lateral
temperature distribution laser irradiation
-I
[
mJ/cm2]
T(x, y,), 0,8 ns) at the surface for various laser fluences.
immediately
after the
surface is covered with Si thin film. The absorption coefficient is a value at a wavelength of 308 nm [17]. Figure 4 shows the calculated lateral temperature distributions T(x, y,,, 0,8 ns) at the surface immediately after laser irradiation for various fluences, assuming that the laser beam profile has a box-like shape with 1 pm width. The heating duration by laser irradiation was 8 ns, which was the laser pulse width. The temperature after laser irradiation decreases rapidly. The horizontal axis, X, is the distance from the center of the beam, so that the edge of the beam is at x = 0.5 km. The temperature decreases abruptly away from the center of the beam. The assumption that the doping takes place only at the region where the surface temperature exceeds the melting point, T,, was used because of the extremely large diffusion coefficient in the liquid phase as compared with that in the solid phase. The diffusion coefficient of Si in molten GaAs is about 8 x lo-” cmP2/s, which is several orders larger than that in solid GaAs [18]. Then, the intersections of each temperature profile would give the resulting linewidths of the doped regions. The variation of linewidth obtained by the above considerations is shown in Fig, 3 by the dashed line. The power dependence of the linewidths shows good agreement with the experimental results. We conclude that the linewidth of the doped region can be estimated by the lateral profile of the maximum temperature.
4. Projection-pattern 4.1. Experimental
doping
procedure
The alternative scheme for submicron patterned laser projection system. In this system, a conventional mask used. A reactive gas is introduced underneath the lens source. In this experiment, 10% SiH, gas diluted with He of the projection system for the patterned doping have
doping is the use of a projection system is and used as a dopant was used. The details been described else-
K. ‘Toyoda.
K. Sugioku
I Excimer
her
doping
137
where [6, 71. All lenses and windows used in the projection system are made of synthetic quartz. The numerical aperture (NA) and the reduction ratio of the projection lenses are 0.25 and 5: 1, respectively. A reticle with chromium patterns of 10 km features was used. Therefore, the projected beam patterns were reduced to 2 pm on the sample surface. The projected KrF excimer laser beam with a linewidth of 248 nm and pulse width of 23 ns irradiated the SI GaAs substrates in the small gas cell at a repetition rate of 1 Hz and a laser fluence of 370 mJ/cm’ through the illumination system. The cell was filled with 10%~ SiH, gas of 100 Torr after evacuation down to 3 x 10m3 Torr in vacuum, and was installed on a movable X-Y-Z stage. 4.2.
Results
and discussion
In order to measure the linewidth of the doped regions using SEM, a relief of the doped region was formed by selective etching of the non-doped GaAs. Figure 5 shows a SEM micrograph of the relief structure for a fluence of 370 mJ/cm’, 10 pulses, and a SiH, gas pressure of 100 Torr. The average linewidth of the relief is 0.76 km in spite of a linewidth of the projected beam of 2 pm. The smaller dimensions are due to the transient lateral diffusion of the heat produced by the laser irradiation as mentioned above. The larger linewidth compared with resolution expected from the wavelength used might arise from the relatively low NA of 0.25. In addition, chromatic aberration occurred, since the bandwidth of the excimer laser was not narrowed. By using a projection system with a large NA and a narrow band beam, the linewidth would be narrowed to quarter-micron.
Fig. 5. SEM
micrograph
of the doped
regions after selective 100 Tow).
etching
(370 mJ/cm’.
10 pulses.
138
K. Toyoda, K. Sugioka 2000
___“_.._____.___T
I
Fig. 6. The lateral
temperature
I Excimer laser doping
i-
Beam
370 mJ lcm2 Proflle
distribution T(x, y,,, 0,23 ns) at the surface laser irradiation of 370 mJ/cm’.
immediately
after
The linewidth was also estimated under the assumption used in the case of the solid dopant source (Section 3.2). Figure 6 shows the calculated lateral temperature distribution T(x, y,, 0,23 ns) at the surface immediately after laser irradiation. In this case, the absorption coefficient and the reflectivity used were 2,069,810 cm-’ and 66.8% for GaAs at 248 nm, respectively [17]. In the calculation, a beam size of 2 km, which was the reduced feature size of the patterns on the surface, was used. The heating duration by laser irradiation was 23 ns, which was the laser pulse width. The horizontal axis, x, is the scale of the distance from the center of the beam, so the edge of the beam is at x = 1 km. The beam profile was assumed to have a box-like shape but the calculated temperature profile seems to have a bell-like shape. In the experiment, although the number of laser pulses was 10, there was not an accumulation effect of heat by previous laser pulses because of the slow repetition rate of 1 Hz. Therefore, the temperature profiles after the irradiation of each pulse almost coincide with each other. Thus, the linewidth of the doped regions could be determined by the temperature profile of single-pulse irradiation. The linewidth obtained from the same consideration as used in the previous section is 0.78 km, which shows good agreement with the experimental results of 0.76 pm. The slight decreasing of the observed linewidth may accompany the process of selective etching of non-doped GaAs. That is, the outlines of the doped regions would be chemically etched because of the light doping. The lightly doped regions on both sides of the lines may be chemically reactive as compared with the heavily doped region. Another reason might be due to the simple calculation model. That is, in spite of the fact that both the thermal diffusivity and the thermal conductivity are sensitive functions of the tempera-
K. Toyoda, K. Sugioka
I Excimer laser doping
139
ture, in the calculation, the intermediate values between room temperature and the melting point of GaAs were adopted as effective values [15]. Furthermore, the latent heat was not considered. However, the fact that the calculation coincided well with the experimental results would suggest the effectiveness of the simple model used. Thus; it is concluded that the linewidth of the doped region is predicted from the calculation of the maximum temperature distribution. 5. Selective 5.1.
deposition
Experimental
of Au films on the doped regions
procedure
Figure 7 shows the experimental procedure of selective deposition of Au films on n-type GaAs. (a) The projection-patterned doping was performed using a Si-doped n-type (100) oriented horizontal Bridgeman (HB) GaAs substrate (carrier concentration 2.5 x 1017cm-‘) placed in a 10% SiH,-He gas atmosphere of 100 Torr. (b) Then the doped sample was immersed in a commercial Au-24s aqueous solution at 70°C for 5-10 min. (c) As a result, selective deposition of Au films on the doped regions was observed.
Gas Molecules(SiH,) issociated Atoms(Si) ubstrate( n-GaAs)
Molten Regiqn (a)
Au-24s (70 “C) Doped Region (n+-Layer)
(W
Deposited Metal
Fig. 7. Experimental projection-patterned
procedures of selective deposition laser doping; (b) electroless plating; prepared sample.
of Au films on n-type GaAs and (c) schematic cross-section
by (a) of the
140
5.2.
K. Toyoda, K. Sugioka
I Excirner laser doping
Results and discussion
Figure 8 shows a SEM micrograph of the Au films deposited selectively by electroless plating for 5 min at 70°C. The doping was carried out with a laser fluence of 365 mJ/cm* and 10 pulses. The Au thin films are also deposited selectively only on the doped regions (bright regions), and no deposition on the unirradiated regions is observed (dark regions). The selectivity might be attributed to the high electron density in the conduction band of the laserdoped region since the electroless plating results from a kind of reducing reaction. Therefore, the Au films are deposited selectively on the regions where the electron density in the conduction band is higher. The average linewidth and thickness of the deposited films are measured to be 1.57 pm and 39 nm, respectively. The linewidth of the doped regions of the sample prepared at 365 mJ / cm2 is estimated to be 1.47 p,rn by using the same assumption used in Sections 3.2 and 4.2. The increased width of 0.1 pm is 2.6 times as large as the film thickness of 39 nm. Therefore, the Au films might be deposited by nearly isotropic growth in the electroless plating. In this case, the doped regions were first covered with Au atoms by a reducing reaction since the conduction band of the doped region has high electron density as compared with that of the unirradiated region. Immediately after that, Au films were deposited isotropitally by a reducing reaction between the deposited Au films and Au ions in Au-24s aqueous solution.
Fig. 8. SEM micrograph of the Au films deposited selectively by electroless plating for 5 min at 70°C on the regions doped with a laser tluence of 365 mJ/cm’, 10 pulses, and a SiH, gas pressure of 100Torr. The bright regions correspond to the deposited Au film.
K. Toyoda, K. Sugioka
I Excimer laser doping
141
Figure 9 shows a cross-sectional profile of the deposited film measured by alpha-step 200. The laser fluence was 385 mJ/cm’, and the plating time was 10 min. The film shows an almost flat top with a width of about 1 pm, and both sides are somewhat round with slopes. It is considered that the slope of the shoulders is due to the isotropic deposition. The full width at half maximum (FWHM) of the linewidth is estimated to be 1.59 urn, and the film thickness is measured to be 69 nm. The resistivity of the deposited films has been evaluated by a four-point probe method. In this case, large-area doping without a reticle has been performed at a laser fluence of 355 mJ/cm’. The resistivity was estimated to be 3.33 X lop6 R cm, which was about 1.5 times larger than the 2.2 x lop6 L! cm resistivity of bulk Au at 18°C. Accordingly, Au films with satisfactory resistivity can be obtained by this technique. Contacts of metals and the shallow surface layers of semiconductors having extremely high carrier concentration formed by laser doping show ideal ohmic characteristics. Dot contacts of Au were formed on n-type GaAs substrates by similar procedures. In this case, a reticle with 300 km diameter dot patterns spaced 900 pm apart was used in the projection system, so that the resulting diameter of the deposited Au films was about 60 km diameter, spaced 180 km apart. The projection doping was performed at a fluence of 355 mJ/cm’, 10 pulses, and a 10% SiH, gas pressure of 100Torr. The electroless plating was carried out by immersing the doped sample into an Au-24s aqueous solution for 10 min at 70°C. In the procedures, post-annealing was not used. The currentvoltage (Z-V) characteristic between each dot showed ideal ohmic contacts. Figure 10 shows a contact pair resistance as a function of contact spacing. The resistance extrapolated to zero contact spacing corresponds to the contact
Fig. 9. Cross-sectional profile of the Au film deposited by electroless plating for 10 min at 70°C measured by alpha-step 200. The laser fluence was 385 mJ/cm*.
K. Toyoda, K. Sugioka
142
6-
1
I Excimer laser doping
I
I
100 Torr 10% 355 mJ/cmZ
I
1
SiHG
5-
4-
3-
2-
+ Rc= 4.95~10~~ Rxm2
l-
(Au dots:60pmq,) 4 4 OO
resistance estimated
d,
of 0.175 LR. Therefore,
Contact Spacing
the
contact resistance per pair is specific contact resistance obtained contacts formed using this technique contacts. Hence, the selective microfabrication process of metal films on the doped regions presented advantage in achieving contacts with a low contact resistance.
Conclusion Direct formation of three-dimensional structures with submicron width of Si in GaAs by excimer laser doping has been demonstrated. The doping profile of Si in GaAs formed by this technique was limited in a very shallow region (cl10 nm) with extremely high concentration (>l x lo*” cm-j). Patterned doping of Si into GaAs substrates using a thin solid dopant source and a XeCl excimer laser has been shown to be a simple process to define a submicron feature size as narrow as 0.3 km. The linewidth of the doped region decreased monotonically with decreasing laser fluence. Projection-patterned doping of Si into GaAs substrate using a KrF excimer laser and a SiH, gas has achieved a linewidth of 0.76 km. The process is useful for single-step doping to define fine doped regions with high dopant concentration.
K. Toyoda, K. Sugioka
I Excimer laser doping
143
Furthermore, the projection-patterned laser doping technique has been applied to selective metallization of GaAs. Au films with a linewidth of 1.57 pm have been deposited selectively by electroless plating in an Au-24s aqueous solution. The metal films formed by the process have been deposited isotropically and showed comparable resistivity to that of the bulk. Self-aligned microfabrication of non-alloyed ohmic contacts with a specific contact resistance as low as 4.95 x 10e6 Cl cm* presented here has proven that the technique has a great advantage to fabricate non-alloyed ohmic contacts.
References [l] T.F. Deutsch, J.C.C. Fan, G.W. Turner, R.L. Chapman, D.J. Ehrlich and R.M. Osgood, Jr., Appl. Phys. Left. 38 (1981) 144. [2] P.G. Carey, T.W. Sigmon, R.L. Press and T.S. Fahlen, IEEE Electron Device Lett. EDL-6 (1985) 291. [3] K. Sugioka and K. Toyoda, Appl. Phys. A 45 (1988) 189. [4] D.J. Ehrlich and J.Y. Tsao, Appl. Phys. Lett. 41 (1982) 297. [5] K. Sugioka and K. Toyoda, J. Vat. Sci. Technol. B 6 (1988) 850. [6] K. Sugioka and K. Toyoda, J. Vat. Sci. Technol. B 6 (1988) 1694. [7] K. Sugioka and K. Toyoda, Jpn. J. Appl. Phys. 28 (1989) 2162. [8] K. Sugioka, K. Toyoda, K. Tachi and M. Otsuka, Appl. Phys. A 49 (1989) 723. [9] K. Sugioka, K. Toyoda, Y. Gomi and S. Tanaka, Appl. Phys. Lett. 55 (1989) 619. [lo] K. Sugioka and K. Toyoda, Vacuum 41 (1990) 1258. [ll] K. Sugioka and K. Toyoda, Jpn. J. Appl. Phys. 29 (1990) 2255. [12] K. Sugioka and K. Toyoda, Appl. Phys. A 54 (1992) 380. [13] T.E. Shim and T. Itoh, Appl. Phys. Lett. 48 (1986) 641. [14] T. Shiokawa, private communication. [15] T.W. Sigmon, Mater. Res. Sot. Symp. Proc. 75 (1987) 619. [16] J. Ralston, A.L. Moretti, R.K. Jain and F.A. Chambers, Appl. Phys. Lett. 50 (1987) 1817. [17] D.E. Aspnes and A.A. Studna, Phys. Rev. B 27 (1983) 985. [18] F. Sato, T. Sunada and J. Chikawa, Mater. Lett. 1 (1982) 111.
Engineering
131
20 (1993) 131-143
Microfabrication of semiconductors by means of excimer laser doping Koichi Toyoda and Koji Sugioka The Institute of Physical and Chemical Research (RIKEN),
Wako Saitama 351-01, Japan
Abstract.
Direct formation of three-dimensional structures of Si in GaAs by excimer laser doping has been demonstrated. By using a thin solid Si dopant source, linewidths as narrow as 0.3 km are achieved. On the other hand, linewidths of 0.76 p.m are obtained by projection-patterned doping in SM., gas atmosphere. The linewidth of the doped regions are discussed in relation to the lateral profiles of the calculated transient temperature rise of the substrates. The patterned doping technique is applied to the selective metallization of GaAs. Au thin films are deposited selectively on the doped regions by the subsequent electroless plating process. Using the selective metallization process, non-alloyed ohmic contracts can be fabricated with a specific contact resistance as low as 4.95 x 10m6 Q cm*, which is l/l50 of that of the conventionally alloyed contacts.
Keywords. Excimer laser doping; Si-doped GaAs; Patterned doping; Submicron ing; Projection system; Selective metallization; Non-alloyed ohmic contact
pattem-
1. Introduction
With the progress of microelectronics, the development of sophisticated technology which can realize the formation of three-dimensional structures with very small dimensions in semiconductors is required. Excimer laser doping is one of the promising techniques for microfabrication, since the short wavelengths and the short pulse widths of excimer lasers make it possible to dope impurities with extremely high dopant concentration in excess of the solid solubility at very shallow depths from the surface [l-3]. In addition, by heating locally using a tightly focused laser beam or a mask, lateral confinement of the doped impurity to very small dimensions is achieved. In the direct-writing process using a scanned, tightly focused Ar-ion laser beam, high-resolution doped patterns of B with a linewidth of 0.3 km in Si substrates have been demonstrated [4]. Such a process is maskless and has great flexibility but the processing time is limited by the translation rate (C440 pm/s) of the wafer. Correspondence too:Koichi Toyoda, The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-01, Japan.
0167-9317/93/$06.00
@ 1993 Elsevier Science Publishers B.V. All rights reserved
132
K. Toyoda, K. Sugioka
I Excimer laser doping
For greater throughputs, large-area pattern transfer using masks is considered [5-71. In the present paper, two types of submicron patterned doping of Si into GaAs by excimer lasers using masks are described. One of them is the patterned doping using a thin solid Si dopant source and a contact mask. The advantage of the process is that the doping is completed by only a single pulse because of the presence of a sufficiently large amount of dopant source on the surface. Furthermore, Si thin film may act as an encapsulation. In fact, the doped regions of all samples showed a smooth surface. On the other hand, surface roughness was observed in the case of gas-phase doping at higher laser fluence and larger number of laser pulses, although the crystalline quality and the stoichiometry were maintained [8]. However, several processes are required, i.e., the deposition of the Si dopant source and its removal. The second method is the patterned doping using a projection system. In this system, a conventional mask projection system is used. A reactive gas is introduced underneath the lens and used as a dopant source. As a result, the projection-patterned doping simplifies the process. The doped regions had a smooth surface and a good crystalline quality at suitable experimental conditions, although surface roughness was observed at higher laser fluence and larger number of laser pulses [S]. Here, we discuss the linewidths of the regions doped by the two types of patterned doping with temperature profiles calculated by the transient heat conduction equation in the substrates. In addition, the patterned doping technique is applied to selective metallization of the doped samples [9-121. Au thin films were deposited selectively on the doped regions by a subsequent electroless plating process. Self-aligned microfabrication of non-alloyed ohmic contacts of GaAs by the simple technique is investigated. Koichi Toyoda received his BE in Electrical Engineering in 1959 and the Dr. Eng. degree in 1968, both from Osaka University, Japan. He joined RIKEN, a government-supported research institute, in 1968. In RIKEN, he has worked on the research of various high-power lasers such as carbon dioxide laser, TEA-CO2 laser, and HBr chemical laser. Since 1978, his main interest has been the photochemical microfabrication of solid surfaces employing excimer lasers and short-wavelength light sources. Dr. Toyoda is a member of the Institute of Electrical Engineers in Japan, Japan Society of Applied Physics, Physical Society of Japan and The Laser Society of Japan. Koji Sugioka received his BE in Electronics in 1984 and ME in 1986, both from Waseda university in Japan. He joined The Institute of Physical and Chemical Research (RIKEN) in 1986. In RIKEN, he is engaged in microfabrication of semiconductors and surface modification of metal surfaces using excimer lasers. He is a member of the Japan Society of Applied Physics and The Laser Society of Japan.
K. Toyoda, K. Sugioka
I Excimer laser doping
133
2. Depth profiles of Si in GaAs In order to investigate the vertical confinement of impurities by laser doping, the depth profiles of Si concentration in GaAs were measured by secondary ion mass spectroscopy (SIMS). The samples were prepared by irradiating GaAs substrates with a XeCl excimer laser beam with a wavelength of 308 nm and a pulse width of 8 ns in an ambient 10% SiH,-He gas. The gas pressure was 100Torr. As a substrate, non-doped semi-insulating (SI) (100) liquid encapsulated Czochralski (LEC) GaAs (resistivity >l x lo7 rR.cm) was used. In this case, the unpatterned doping was carried out without mask. The details are described elsewhere [3,8]. Figure 1 shows the depth profiles for various laser fluences for 100 pulses and a SiH, gas pressure of 100 Torr. The peak concentration of Si is as high as l-3 X lo*’ cmP3 at the surface in all cases, and the Si concentration decreases with increasing depth from the surface. The doped Si atoms extend deeper with increasing laser fluence. The doping depth, which is defined as the distance from the surface to the layer where the concentration is one-thousandth of the surface concentration, is less than about 110 nm even at the maximum laser fluence of 170 mJ/cm*. The minimum doping depth of 30 nm was obtained for a fluence of 56 mJ/cm*. Thus, such an ultra-shallow junction can be formed by laser doping.
lon
I
I
100 Torr 10% SiHL 100 Pulses -a56mJlcm2
_
lozc
p9 ”
5 i-
$Ada c
5 c
s .*vY
lo" Depth
Fig.
1. The fluences
I nm 1
depth profiles of Si concentration in GaAs measured by SIMS at various laser of 56, 83, 100, 130, and 170 mJ/cm’ at 100 pulses in a SiH, gas of 100 Torr.
K. Toyoda, K. Sugioka
134
3. Patterned
I Excimer laser doping
doping using a thin solid dopant source
3.1. Experimental
procedure
Figure 2 shows the experimental procedure: (a) Silicon thin films of 10 nm thickness were deposited on the SI GaAs substrates with a deposition rate of 0.3 rim/s at room temperature by ultra-high vacuum (UHV) evaporation. The pressure during the deposition was less than 1 x lo-’ Torr. (b) A single pulse of the XeCl excimer laser beam irradiated the sample surface nearly perpendicular in air. A reticle with chromium patterns of 1 pm features on a synthetic quartz substrate, which was kept in contact with the sample surfaces, was used as a contact mask to define the doped region. The laser fluence was varied from 100 to 150 mJ/cm* at the sample surface. (c) After irradiation, the samples were etched in boiling ethylenediamine (7.5 ml) + H,O (2.4 ml) + pyrazine (0.24 g) solution for several minutes until the remaining Si on the surface was completely removed without etching the GaAs nor the Si-doped GaAs [13]. Then, the Si atoms were doped into the irradiated region only. 3.2. Results and discussion For quantitative measurement of the linewidth of the doped regions, relief structures of the doped regions were formed by etching the non-doped GaAs selectively in 40HF + lH,O + 4H,O, solution for 50 s with light illumination [14]. The linewidth of the remaining relief where the Si concentration was high was measured as the linewidth of the doped regions using a scanning electron microscope (SEM).
308 nm XeCl Excimer IpO - 150 mJ/cm* Single Pulse
Laser
IO nm Thick Oepsi ted
SI (100) (a)
LEC
Si
/-Cr
GaAs
Mask
1After
Etching)
Si Doped
Cb)
Region
(C)
Fig. 2. The experimental procedure of the patterned doping using a thin solid Si dopant source: (a) Deposition of Si thin film on GaAs; (b) irradiation of XeCl excimer laser beam to the sample surface through a reticle; and (c) removal of the remaining Si on the surface.
K. Toyoda, K. Sugioka
I Excimer laser doping
135
In order to investigate the dependence of the linewidth on the laser fluence, variation of the linewidth of the remaining relief as a function of the laser fluence is shown in Fig. 3. The linewidth decreases monotonously from 0.75 km at a fluence of 150 mJ/cm* to 0.3 km at 100 mJ/cm2. In addition, it should be noted that the linewidths are smaller than the mask’s feature size of 1 pm in all cases. In order to discuss the power-dependent narrowing of the linewidth, it is necessary to analyze the transient diffusion of dopant due to transient heating by a laser pulse. For simplicity, we assumed that the linewidth is determined by the lateral dimension over which the surface temperature exceeds the melting point, since the impurities distribute only in the molten region, as was modeled by Sigmon [15]. Here, the temperature distributions T(x, y, z, t) in the substrates produced by the laser irradiation were calculated by integrating the thermal conduction equation. The parameters used in the calculation are summarized in Table 1. Since the thermal diffusivity and the thermal conductivity are sensitive functions of temperature, the intermediate values between room temperature and the melting point of GaAs were used as effective values [16]. The reflectivity was a value for Si from Ref. [17], because the sample
Table 1 Parameters used in the calculation of surface temperature distribution Thermal diffusivity [ 1l] Thermal conductivity [1l] Reflectivity [ 121 Absorption coefficient [12] Beam size Heating duration
X
0.16 cm’/s 0.27 W/cm”C 59.7% 788,550 cm-’ 1 pm 8 ns
Ipml
Fig. 3. The variations of the linewidth of the doped regions by irradiation of single-pulse XeCl excimer laser beam as a function of laser fluences. The solid line and dashed line correspond to experimental and calculated results, respectively.
K. Toyoda, K. Sugioka
136
-+-Experiment
t
L
OO
I Excimer laser doping
I 100
-----Calculation 150
Fluence at Substrate
Fig. 4. The lateral
temperature distribution laser irradiation
-I
[
mJ/cm2]
T(x, y,), 0,8 ns) at the surface for various laser fluences.
immediately
after the
surface is covered with Si thin film. The absorption coefficient is a value at a wavelength of 308 nm [17]. Figure 4 shows the calculated lateral temperature distributions T(x, y,,, 0,8 ns) at the surface immediately after laser irradiation for various fluences, assuming that the laser beam profile has a box-like shape with 1 pm width. The heating duration by laser irradiation was 8 ns, which was the laser pulse width. The temperature after laser irradiation decreases rapidly. The horizontal axis, X, is the distance from the center of the beam, so that the edge of the beam is at x = 0.5 km. The temperature decreases abruptly away from the center of the beam. The assumption that the doping takes place only at the region where the surface temperature exceeds the melting point, T,, was used because of the extremely large diffusion coefficient in the liquid phase as compared with that in the solid phase. The diffusion coefficient of Si in molten GaAs is about 8 x lo-” cmP2/s, which is several orders larger than that in solid GaAs [18]. Then, the intersections of each temperature profile would give the resulting linewidths of the doped regions. The variation of linewidth obtained by the above considerations is shown in Fig, 3 by the dashed line. The power dependence of the linewidths shows good agreement with the experimental results. We conclude that the linewidth of the doped region can be estimated by the lateral profile of the maximum temperature.
4. Projection-pattern 4.1. Experimental
doping
procedure
The alternative scheme for submicron patterned laser projection system. In this system, a conventional mask used. A reactive gas is introduced underneath the lens source. In this experiment, 10% SiH, gas diluted with He of the projection system for the patterned doping have
doping is the use of a projection system is and used as a dopant was used. The details been described else-
K. ‘Toyoda.
K. Sugioku
I Excimer
her
doping
137
where [6, 71. All lenses and windows used in the projection system are made of synthetic quartz. The numerical aperture (NA) and the reduction ratio of the projection lenses are 0.25 and 5: 1, respectively. A reticle with chromium patterns of 10 km features was used. Therefore, the projected beam patterns were reduced to 2 pm on the sample surface. The projected KrF excimer laser beam with a linewidth of 248 nm and pulse width of 23 ns irradiated the SI GaAs substrates in the small gas cell at a repetition rate of 1 Hz and a laser fluence of 370 mJ/cm’ through the illumination system. The cell was filled with 10%~ SiH, gas of 100 Torr after evacuation down to 3 x 10m3 Torr in vacuum, and was installed on a movable X-Y-Z stage. 4.2.
Results
and discussion
In order to measure the linewidth of the doped regions using SEM, a relief of the doped region was formed by selective etching of the non-doped GaAs. Figure 5 shows a SEM micrograph of the relief structure for a fluence of 370 mJ/cm’, 10 pulses, and a SiH, gas pressure of 100 Torr. The average linewidth of the relief is 0.76 km in spite of a linewidth of the projected beam of 2 pm. The smaller dimensions are due to the transient lateral diffusion of the heat produced by the laser irradiation as mentioned above. The larger linewidth compared with resolution expected from the wavelength used might arise from the relatively low NA of 0.25. In addition, chromatic aberration occurred, since the bandwidth of the excimer laser was not narrowed. By using a projection system with a large NA and a narrow band beam, the linewidth would be narrowed to quarter-micron.
Fig. 5. SEM
micrograph
of the doped
regions after selective 100 Tow).
etching
(370 mJ/cm’.
10 pulses.
138
K. Toyoda, K. Sugioka 2000
___“_.._____.___T
I
Fig. 6. The lateral
temperature
I Excimer laser doping
i-
Beam
370 mJ lcm2 Proflle
distribution T(x, y,,, 0,23 ns) at the surface laser irradiation of 370 mJ/cm’.
immediately
after
The linewidth was also estimated under the assumption used in the case of the solid dopant source (Section 3.2). Figure 6 shows the calculated lateral temperature distribution T(x, y,, 0,23 ns) at the surface immediately after laser irradiation. In this case, the absorption coefficient and the reflectivity used were 2,069,810 cm-’ and 66.8% for GaAs at 248 nm, respectively [17]. In the calculation, a beam size of 2 km, which was the reduced feature size of the patterns on the surface, was used. The heating duration by laser irradiation was 23 ns, which was the laser pulse width. The horizontal axis, x, is the scale of the distance from the center of the beam, so the edge of the beam is at x = 1 km. The beam profile was assumed to have a box-like shape but the calculated temperature profile seems to have a bell-like shape. In the experiment, although the number of laser pulses was 10, there was not an accumulation effect of heat by previous laser pulses because of the slow repetition rate of 1 Hz. Therefore, the temperature profiles after the irradiation of each pulse almost coincide with each other. Thus, the linewidth of the doped regions could be determined by the temperature profile of single-pulse irradiation. The linewidth obtained from the same consideration as used in the previous section is 0.78 km, which shows good agreement with the experimental results of 0.76 pm. The slight decreasing of the observed linewidth may accompany the process of selective etching of non-doped GaAs. That is, the outlines of the doped regions would be chemically etched because of the light doping. The lightly doped regions on both sides of the lines may be chemically reactive as compared with the heavily doped region. Another reason might be due to the simple calculation model. That is, in spite of the fact that both the thermal diffusivity and the thermal conductivity are sensitive functions of the tempera-
K. Toyoda, K. Sugioka
I Excimer laser doping
139
ture, in the calculation, the intermediate values between room temperature and the melting point of GaAs were adopted as effective values [15]. Furthermore, the latent heat was not considered. However, the fact that the calculation coincided well with the experimental results would suggest the effectiveness of the simple model used. Thus; it is concluded that the linewidth of the doped region is predicted from the calculation of the maximum temperature distribution. 5. Selective 5.1.
deposition
Experimental
of Au films on the doped regions
procedure
Figure 7 shows the experimental procedure of selective deposition of Au films on n-type GaAs. (a) The projection-patterned doping was performed using a Si-doped n-type (100) oriented horizontal Bridgeman (HB) GaAs substrate (carrier concentration 2.5 x 1017cm-‘) placed in a 10% SiH,-He gas atmosphere of 100 Torr. (b) Then the doped sample was immersed in a commercial Au-24s aqueous solution at 70°C for 5-10 min. (c) As a result, selective deposition of Au films on the doped regions was observed.
Gas Molecules(SiH,) issociated Atoms(Si) ubstrate( n-GaAs)
Molten Regiqn (a)
Au-24s (70 “C) Doped Region (n+-Layer)
(W
Deposited Metal
Fig. 7. Experimental projection-patterned
procedures of selective deposition laser doping; (b) electroless plating; prepared sample.
of Au films on n-type GaAs and (c) schematic cross-section
by (a) of the
140
5.2.
K. Toyoda, K. Sugioka
I Excirner laser doping
Results and discussion
Figure 8 shows a SEM micrograph of the Au films deposited selectively by electroless plating for 5 min at 70°C. The doping was carried out with a laser fluence of 365 mJ/cm* and 10 pulses. The Au thin films are also deposited selectively only on the doped regions (bright regions), and no deposition on the unirradiated regions is observed (dark regions). The selectivity might be attributed to the high electron density in the conduction band of the laserdoped region since the electroless plating results from a kind of reducing reaction. Therefore, the Au films are deposited selectively on the regions where the electron density in the conduction band is higher. The average linewidth and thickness of the deposited films are measured to be 1.57 pm and 39 nm, respectively. The linewidth of the doped regions of the sample prepared at 365 mJ / cm2 is estimated to be 1.47 p,rn by using the same assumption used in Sections 3.2 and 4.2. The increased width of 0.1 pm is 2.6 times as large as the film thickness of 39 nm. Therefore, the Au films might be deposited by nearly isotropic growth in the electroless plating. In this case, the doped regions were first covered with Au atoms by a reducing reaction since the conduction band of the doped region has high electron density as compared with that of the unirradiated region. Immediately after that, Au films were deposited isotropitally by a reducing reaction between the deposited Au films and Au ions in Au-24s aqueous solution.
Fig. 8. SEM micrograph of the Au films deposited selectively by electroless plating for 5 min at 70°C on the regions doped with a laser tluence of 365 mJ/cm’, 10 pulses, and a SiH, gas pressure of 100Torr. The bright regions correspond to the deposited Au film.
K. Toyoda, K. Sugioka
I Excimer laser doping
141
Figure 9 shows a cross-sectional profile of the deposited film measured by alpha-step 200. The laser fluence was 385 mJ/cm’, and the plating time was 10 min. The film shows an almost flat top with a width of about 1 pm, and both sides are somewhat round with slopes. It is considered that the slope of the shoulders is due to the isotropic deposition. The full width at half maximum (FWHM) of the linewidth is estimated to be 1.59 urn, and the film thickness is measured to be 69 nm. The resistivity of the deposited films has been evaluated by a four-point probe method. In this case, large-area doping without a reticle has been performed at a laser fluence of 355 mJ/cm’. The resistivity was estimated to be 3.33 X lop6 R cm, which was about 1.5 times larger than the 2.2 x lop6 L! cm resistivity of bulk Au at 18°C. Accordingly, Au films with satisfactory resistivity can be obtained by this technique. Contacts of metals and the shallow surface layers of semiconductors having extremely high carrier concentration formed by laser doping show ideal ohmic characteristics. Dot contacts of Au were formed on n-type GaAs substrates by similar procedures. In this case, a reticle with 300 km diameter dot patterns spaced 900 pm apart was used in the projection system, so that the resulting diameter of the deposited Au films was about 60 km diameter, spaced 180 km apart. The projection doping was performed at a fluence of 355 mJ/cm’, 10 pulses, and a 10% SiH, gas pressure of 100Torr. The electroless plating was carried out by immersing the doped sample into an Au-24s aqueous solution for 10 min at 70°C. In the procedures, post-annealing was not used. The currentvoltage (Z-V) characteristic between each dot showed ideal ohmic contacts. Figure 10 shows a contact pair resistance as a function of contact spacing. The resistance extrapolated to zero contact spacing corresponds to the contact
Fig. 9. Cross-sectional profile of the Au film deposited by electroless plating for 10 min at 70°C measured by alpha-step 200. The laser fluence was 385 mJ/cm*.
K. Toyoda, K. Sugioka
142
6-
1
I Excimer laser doping
I
I
100 Torr 10% 355 mJ/cmZ
I
1
SiHG
5-
4-
3-
2-
+ Rc= 4.95~10~~ Rxm2
l-
(Au dots:60pmq,) 4 4 OO
resistance estimated
d,
of 0.175 LR. Therefore,
Contact Spacing
the
contact resistance per pair is specific contact resistance obtained contacts formed using this technique contacts. Hence, the selective microfabrication process of metal films on the doped regions presented advantage in achieving contacts with a low contact resistance.
Conclusion Direct formation of three-dimensional structures with submicron width of Si in GaAs by excimer laser doping has been demonstrated. The doping profile of Si in GaAs formed by this technique was limited in a very shallow region (cl10 nm) with extremely high concentration (>l x lo*” cm-j). Patterned doping of Si into GaAs substrates using a thin solid dopant source and a XeCl excimer laser has been shown to be a simple process to define a submicron feature size as narrow as 0.3 km. The linewidth of the doped region decreased monotonically with decreasing laser fluence. Projection-patterned doping of Si into GaAs substrate using a KrF excimer laser and a SiH, gas has achieved a linewidth of 0.76 km. The process is useful for single-step doping to define fine doped regions with high dopant concentration.
K. Toyoda, K. Sugioka
I Excimer laser doping
143
Furthermore, the projection-patterned laser doping technique has been applied to selective metallization of GaAs. Au films with a linewidth of 1.57 pm have been deposited selectively by electroless plating in an Au-24s aqueous solution. The metal films formed by the process have been deposited isotropically and showed comparable resistivity to that of the bulk. Self-aligned microfabrication of non-alloyed ohmic contacts with a specific contact resistance as low as 4.95 x 10e6 Cl cm* presented here has proven that the technique has a great advantage to fabricate non-alloyed ohmic contacts.
References [l] T.F. Deutsch, J.C.C. Fan, G.W. Turner, R.L. Chapman, D.J. Ehrlich and R.M. Osgood, Jr., Appl. Phys. Left. 38 (1981) 144. [2] P.G. Carey, T.W. Sigmon, R.L. Press and T.S. Fahlen, IEEE Electron Device Lett. EDL-6 (1985) 291. [3] K. Sugioka and K. Toyoda, Appl. Phys. A 45 (1988) 189. [4] D.J. Ehrlich and J.Y. Tsao, Appl. Phys. Lett. 41 (1982) 297. [5] K. Sugioka and K. Toyoda, J. Vat. Sci. Technol. B 6 (1988) 850. [6] K. Sugioka and K. Toyoda, J. Vat. Sci. Technol. B 6 (1988) 1694. [7] K. Sugioka and K. Toyoda, Jpn. J. Appl. Phys. 28 (1989) 2162. [8] K. Sugioka, K. Toyoda, K. Tachi and M. Otsuka, Appl. Phys. A 49 (1989) 723. [9] K. Sugioka, K. Toyoda, Y. Gomi and S. Tanaka, Appl. Phys. Lett. 55 (1989) 619. [lo] K. Sugioka and K. Toyoda, Vacuum 41 (1990) 1258. [ll] K. Sugioka and K. Toyoda, Jpn. J. Appl. Phys. 29 (1990) 2255. [12] K. Sugioka and K. Toyoda, Appl. Phys. A 54 (1992) 380. [13] T.E. Shim and T. Itoh, Appl. Phys. Lett. 48 (1986) 641. [14] T. Shiokawa, private communication. [15] T.W. Sigmon, Mater. Res. Sot. Symp. Proc. 75 (1987) 619. [16] J. Ralston, A.L. Moretti, R.K. Jain and F.A. Chambers, Appl. Phys. Lett. 50 (1987) 1817. [17] D.E. Aspnes and A.A. Studna, Phys. Rev. B 27 (1983) 985. [18] F. Sato, T. Sunada and J. Chikawa, Mater. Lett. 1 (1982) 111.