SEM alloyed AuGeNi ohmic contacts to GaAs

Applications of Surface Science 22/23 (1985) 1019-1026 North-Holland, Amsterdam

SEM A L L O Y E D A u - G e - - N i O H M I C A.G. NASSIBIAN

1019

C O N T A C T S T O GaAs

a n d T.S. K A L K U R

Department of Electrical and Electronic Engineering, University of Western Australia, Nedlands, WA 6009, Australia

Received 27 August 1984; accepted for publication 31 October 1984

A scanned electron beam from a commercial SEM is used for the localized alloying of vacuum evaporated Au-Ge-Ni contacts on n-type GaAs. The contact quality is studied for metallization thickness, 450-1350.~, for furnace and electron beam alloyed contacts. For the electron beam alloyed method, the contact resistivity decreases with increasing metallization thickness and remains constant for thicknesses above 750 A. Scanning electron microscopy and electron microprobe analysis shows that electron beam alloyed contacts undergo less redistribution of contact constituents than furnace alloyed contacts. The stability of the contacts is determined by high temperature ageing. o

1. Introduction T h e p e r f o r m a n c e of m i c r o w a v e field effect transistors, o p t o e l e c t r o n i c d e v i c e s a n d high s p e e d digital i n t e g r a t e d circuits d e p e n d s on the quality of o h m i c contacts. A u - G e with a small a m o u n t of Ni is the m o s t c o m m o n m e t a l l i z a t i o n u s e d for t h e f a b r i c a t i o n of o h m i c contacts. T h e g e n e r a l u n d e r s t a n d i n g of c o n t a c t f o r m a t i o n involves the i n c o r p o r a t i o n of d o p a n t G e into G a sites, l e a d i n g to a N + surface r e g i o n so that t h e s p a c e c h a r g e l a y e r t h e r e d u e to F e r m i level p i n n i n g at the i n t e r f a c e b e c o m e s sufficiently thin for t u n n e l l i n g to t a k e p l a c e [1-4]. T h e d e p e n d e n c e of c o n t a c t quality on n - t y p e G a A s in t e r m s of c o n t a c t resistivity, s u r f a c e m o r p h o l o g y a n d d e p t h of p e n e t r a t i o n of c o n s t i t u t i n g m a t e r i a l s on p r o c e s s p a r a m e t e r s such as t i m e a n d t e m p e r a t u r e of a l l o y i n g has b e e n widely i n v e s t i g a t e d for p a r t i c u l a r t h i c k n e s s e s of m e t a l l i z a t i o n . Since the a l l o y i n g p r o c e s s is r e d i s t r i b u t i o n a n d diffusion of G a , A s , Ni, A u a n d G e , t h e m e t a l l i z a t i o n thickness p l a y s an i m p o r t a n t role in the quality of o h m i c contacts. T h e c o n t a c t resistivity i n c r e a s e s with i n c r e a s i n g t h i c k n e s s of t h e A u l a y e r o v e r p r e a l l o y e d A u - G e film [5]. O t s u b o et al. p r o p o s e d a p r o c e s s rule for t h e t h i c k n e s s of t h e Ni l a y e r with r e s p e c t to t h e t h i c k n e s s of t h e A u - G e l a y e r o n the basis of t h e i r studies on t h e solubility of A u - G e in G a A s [6]. T h e d e p e n d e n c e of c o n t a c t quality on A u - G e m e t a l l i z a t i o n 0378-5963/85/$03.30 © E l s e v i e r Science P u b l i s h e r s B.V. ( N o r t h - H o l l a n d Physics Publishing D i v i s i o n )

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A.G. Nassibian, T.S. Kalkur / S U M alloyed A u - G e - N i

thickness for furnace annealed contacts has been reported recently [7]. Pulsed laser and electron beams are used for the alloying of ohmic contacts to reduce the penetration of some contact constituents into GaAs, to improve surface morphology and to minimize the out diffusion of As [8,9]. Ar-ion lasers have also been used for the alloying of ohmic contacts [10]. The main problem associated with laser beam alloying is the high reflectivity of the metal. In this paper we are reporting on the quality of ohmic contacts on G a A s obtained by localized alloying using a scanned electron beam in a commercial SEM. Localized alloying of ohmic contacts gives an opportunity to conduct alloying operations during any stage of device fabrication.

2. Sample preparation G a A s epitaxial wafers with n = 9 x 1()~6cm 3 and a buffer layer of n = 10~4cm 3 on semi-insulating substrates were systematically cleaned in the usual manner by boiling in acetone, trichloroethylene and methanol. Organic contamination was avoided by rinses in isopropyl alchol and drying in a stream of N 2 gas. Devices for contact resistivity measurements were mesa isolated by etching in a solution of H2SO a : H20: : H20 with the ratio 1 : 10 : 10. Patterns for contact resistivity measurements by the TLM method [11] were defined by positive shiply A Z 1400 photoresist. Before metallization was applied in an evaporator at about 10 6Torr, a short etch was performed on the wafer in the following solution for 5 s - p a r t 1: 2% NaOh in distilled deionized (dd) H20, part 2: 4% H202 in dd H:O, both mixed in the ratio 1 : 1.

3. SEM alloying and thermal alloying A J E O L JSM-U3 microscope was used for the SEM alloying of ohmic contacts. This SEM is equipped with a TV monitor system which has fast scanning (15.75 kHz horizontal, 60 Hz vertical). This allowed the dynamical observation of surface changes during heating. Any area to be alloyed can be brought to the field of view and the magnification is adjusted depending on the area to be alloyed and the input power. The input power can be varied by changing the filament current, accelerating voltage, and the voltage applied to the objective lens. The specimen is mounted on the specimen holder with silver epoxy. The specimen holder is provided with a temperature controlled heating stage so as to vary the specimen temperature. The beam voltage and the current are varied so as to observe the melting

1021

A.G. Nassibian, T.S. Kalkur / SEM alloyed A u - G e - N i

of the contacts in the T V monitor. The minimum area that can be alloyed is 20 x 16/zm 2 and this corresponds to the m a x i m u m magnification in the T V monitor mode. Increasing the area of scanning beyond 100 x 80/zm 2 resulted in non-uniform alloying. The alloying is performed for various values of input power and time. Wafers for thermal alloying were placed in a graphite boat with forming gas flowing. The alloying time was 45 s at a temperature of 470°C with a rise time of 36s and a fall time 15s. A Au layer of 2000,~ thickness was evaporated onto wafers used for the m e a s u r e m e n t of contact resistivity. 4. Thermal simulation

Since fast scanning is used for the alloying of contacts, uniform heating of the scanned area is assumed for the calculation of temperature. The thermal transient analysis was undertaken by using the t e m p e r a t u r e - d e p e n d e n t thermal conductivity of G a A s given by K = ( 0 . 0 7 6 - 0 . 0 0 0 1 T ) m W / ~ m - K , where T is the temperature. The structure used for the thermal analysis is shown in fig. 1. The effect of silver epoxy is also taken into consideration in 500

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1022

A.G. Nassibian,

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the calculation of thermal resistance. The specific heat capacity for G a A s was taken from Grove to be of 1.862× 1 0 - g m J / # m 3 - K [12]. The power retention factor of 0.755 is used for the calculation which is the average value for G a A s and A u - G e for beam energy of 4 5 k V . The computer program was suitable for two-dimensional problems of thermal power flow spreading. At the scanned area the variation of temperature with time for a heat sink assumed to be at room temperature is shown in fig. 1.

5. Results and discussion The variation of contact resistivity with annealing time for 1 3 5 0 ~ of metallization thickness is shown in fig. 2. The contact resistivity is determined by the TLM method by considering the modified sheet resistivity under the contact. The contact resistivity decreases with increasing alloying time and shows a tendency towards saturation. The variation of sheet

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A.G. Nassibian, T.S. Kalkur / SUM alloyed A u - G e - N i

1023

resistivity under the contact is shown by the dashed line in fig. 2. The sheet resistivity under the contact decreases with increasing alloying time and it is less than the sheet resistivity of the epitaxial layer. This confirms the fact that alloying by electron beam dopes the GaAs layer under the contact. The variation of contact resistivity with metallization thickness for an alloying time of 3 s is shown in fig. 3. The contact resistivity decreases with increasing metallization thickness and remains constant beyond a thickness of 750 A. The variation of sheet resistivity under the contact with metallization thickness is shown by the dashed line in fig. 3. For furnace annealed contacts, the contact resistivity decreases with the increase in metallization thickness from 150 to 750 A, but increases beyond 750 A. This result is in agreement with the result obtained from Sharma et al. [13] and Kalkur et al. [7]. The increase in contact resistivity with the increase in contact thickness beyond 750 ,~ is attributed to increased dissolution of GaAs. Since the time involved in SEM annealing is short compared to furnace annealing, an increase in contact resistivity with the increase in metallization thickness is not observed. The contact resistivity measurements for electron beam alloyed samples have been performed with a post alloying metallization of Ag of thickness

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Fig. 3. V a r i a t i o n of c o n t a c t resistivity a n d s h e e t resistivity u n d e r the contact with m e t a l l i z a t i o n thickness. E l e c t r o n b e a m e n e r g y 600 m W a n d a 3 s a l l o y i n g time.

1024

A.G. Nassibian, T.S. Kalkur / S E M alloyed A u - G e - N i

1500/~ since they showed a poor adhesion of the Au overlayer. The surface morphology of contacts aliooyed at an input power of 600 raW for 3 s for a metallization thickness of 750 A is shown in fig. 4A. The contact morphology shows the formation of islands rich in Au and Ge, identified by microprobe M a and La lines. Figs. 4B and 4C show the contact morphology for electron beam alloyed and furnace alloyed contacts, respectively, for a

Fig. 4. Surface morphology of SEM alloyed contacts for mctallization thickness: (A) 7NI A, isolated A u - G e islands identified by microprobe M a and L~r lines: (B) 1350 A with no redistribution of contact constituents; (Co) back scattered electron image of furnace alloyed contact for metallization thickness 1350A, isolated dark areas Ge and Ni rich. Corresponding electron microprobe images: (D) Au (Moe line), (E) Ge (La line), (F) Ni ( K a line). Magnification - × 1500.

A.G. Nassibian, T.S. Kalkur / S E M alloyed A u - G e - N i

1025

thickness of 1350/~. When the contacts were analyzed by an electron microprobe, furnace alloyed contacts showed characteristic bright and dark areas and the dark areas were found to be Au deficient and G e and Ni rich as identified by La, K a and M a lines and as shown in figs. 4D, 4E and 4F. Such characteristic dark areas were not observed in electron beam annealed contacts, confirming the fact that these contacts undergo less redistribution of contact constituents during alloying. Samples were aged in an oven maintained at a t e m p e r a t u r e of 350-+ 1°C in nitrogen. The variation of contact resistivity with ageing time is shown in fig. 5. SEM alloyed contacts show less contact resistivity variation c o m p a r e d to the furnace alloyed contacts. This might be due to the lower redistribution of contact constituents during alloying [14].

6. Conclusion

The scanned electron b e a m in an SEM is used for the alloying of ohmic contacts. The contact resistivity attained is slightly higher than that achieved by furnace annealing. The contacts have undergone less redistribution during alloying and show higher reliability than furnace annealing. The process is cumbersome, however, since m a n y parameters have to be optimized. Furthermore it has a limited scanning area.

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A.G. Nassibian, T S . Kalkur / S E M alloyed A u - G e - N i

Acknowledgements The authors acknowledge the Radio Research Board and the Australian Research Grants Committee for their financial support for this project.

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