Method of making ohmic contact to gaas

FIELD: physics.

SUBSTANCE: in the method of making an ohmic contact to GaAs, a mask is formed on the surface of an n-GaAs plate, having a doped layer, in order to carry out a lift-off lithography process. To clean the surface in the windows of the mask, the n-GaAs plate is treated in aqueous H2SO4 or HCl solution and then washed in deionised water and dried. Further, via electron-beam and/or thermal evaporation in a vacuum at residual pressure lower than 5x10-6 torr, Ge and Cu are deposited with total thickness of 100-500 nm and weight content of germanium in the double-layer composition equal to 20-45%. Further, in a single vacuum cycle, the n-GaAs plate undergoes first thermal treatment at temperature T1=150-460°C in an atmosphere of atomic hydrogen with hydrogen atom flux density on the surface of the plate equal to 1013-1016 at.cm2 s-1. The n-GaAs plate is removed form the vacuum chamber, and after removing the mask, undergoes second thermal treatment in an atmosphere of an inert gas or in a vacuum at temperature T2=280-460°C for t=0.5-30 min.

EFFECT: lower value of reduced contact resistance.

2 cl, 1 dwg

 

The invention relates to the technology of microelectronics, in particular to a technology for discrete devices and integrated circuits based on semiconductor compounds AIIIBVin particular to the creation of ohmic contacts (OK) for drain-source field-effect transistors with Schottky barrier, as well as heterostructure transistors with high electron mobility.

Ohmic contacts must have a low ohmic resistance, smooth surface morphology pads, high thermal stability parameters, small depth of interaction metallization contact with the semiconductor, and low cost (Raskin A.A., Shalimov S. Foreign electronic equipment. 1990, No. 12, p.32 47).

Known methods of forming low resistance ohmic contacts to n-GaAs using a metallization system comprising the following components: Au, Ge and Ni. Gold and germanium are deposited on the semiconductor in the form of a film eutectic AuGe alloy (88% Au and 12% Ge) and a layer of Ni (Pietrovska A., Gulvatch A., Peloua G. // Solid St. Electron, 1983, v.26, p.179), or as three separate films of these elements (Bruce R. A. Piercy G.R. // Solid St. Electron. 1987, v.30, No.7, p.729). Moreover, in the latter case, the ratio of film thickness of Ge and Au is chosen such as to correspond to the eutectic composition AuGe. Thus the total thickness of the deposited films Au/Ge is in the range of 10-150 nm, and the thickness of the Nickel layer in the range of 10-50 nm. After deposition, the contact is subjected to heat treatment. In the annealing process is the formation of low-melting alloy and liquid-phase mixing of Au, Ge, Ni and GaAs.

The disadvantages of these methods is low enough given the contact resistance, the depth of recrystallized region OK (~0.1 ám), low thermal stability of electrical parameters of contact, due to the presence of the contact fusible phase AuGa with a melting point of 370°C, the developed relief of the surface of the pad, as well as its uneven edge, the high cost of OK, which is due to the use of gold.

A known method of making low resistance contact to GaAs (Jim-Tsuen Lai, Joseph Ya-Min Lee/Pd/Ge ohmic contacts to n-type GaAs formed by rapid thermal annealing Appl. Phys. Lett. 64(2), 1994, pp.229-237), which use the metallization system, consisting of a series of sputtered films of Pd and Ge. Nespravnou Pd/Ge ohmic contact is formed by solid-state diffusion of atoms Germany in GaAs through a layer of palladium with education signalisierung n+layer and shows the best thermal stability of electrical parameters and smoother the surface morphology of the contact compared with Au/Ge/Ni equivalent.

The disadvantages of this contact can be attributed to a higher contact resistance than the structure with Au/Ge/Ni, and the complexity of its formation in a typical route for the fabrication of integrated circuits due to the presence on the surface of the ohmic contact reactive and oxidize film Germany.

Known methods in which to obtain better electrical contact to the surface of the ohmic contact Pd/Ge deposited gold film (Rnno // On the low resistance of the Au/Ge/Pd ohmic contact to n-GaAs // J.Appl. Phys., 79(8), 1996) or copper (US patent No. 7368822, IPC H01L 23/48, publ. 06.05.2008). The advantages of these methods include low value of contact resistance. The disadvantages of the method include low thermal stability of electrical parameters OK due to the penetration of the rapidly diffusing atoms of gold and copper in GaAs.

A method of obtaining thermally stable ohmic contact to GaAs (Aboelfotoh MO / Microstructure characterization of Cu3Ge/n-type GaAs ohmic contacts // J.Appl. Phys., 76(10), 1994), whereby to obtain a OK system is used for metallization on the basis of the two-layer film Ge/Cu (thickness are chosen so as to form a composition of Cu3Ge), which forms ohmic contact with both n-and p-type GaAs. These contacts are characterized by high thermal stability of electrical parameters, as well as low production costs, due to lack of precious metals in the composition is OK.

However the com of this method is the instability obtain a low value of contact resistance, due to uncontrolled oxidation of copper and/or Germany during the inter-operation of polerowanie in the air, resulting in non-repeatable processes in the formation of OK during annealing.

A known method of manufacturing a contact-based films Ge/Cu (..Aboelfotoh, S.Oktyabrsky, and J.Narayan / Electrical and microstructural characteristics of GeCu ohmic contacts to n-type GaAs // J.Mater. Res., Vol.12, No.9, 1997, pp.2325-2332), essentially the most close to the proposed technical solution chosen for the prototype. The method consists in the following. On the surface of the plate n-GaAs (100) with the concentration of electrons in the epitaxial layer is n=3×1017cm-3with the aim of implementing the process of the inverse lithography mask is formed. To clean the surface in the Windows of the mask, the GaAs wafer is processed in an aqueous solution of HCl (1:1) and then rinsed in deionized water and drying in a stream of nitrogen. Then by the method of electron beam evaporation in vacuum at a residual pressure atmosphere 10-7Torr is performed layer-by-layer films deposited on Ge and Cu with a total thickness of 0.2 μm and the thickness of the Ge film, which sets the mass content of Ge in the metallization equal to 40%. Thereafter, the GaAs wafer is subjected to first heat treatment in a single vacuum cycle at T1=100°C for t=60 minutes and Then the plate is removed from the vacuum chamber and after removal of the mask is exposed to second the second heat treatment at a temperature T 2=400°C for t=30 min in vacuum at a residual pressure atmosphere 10-7Torr. Conducting a first annealing in a single vacuum cycle allows you to start the formation of the contact conditions, when the surface of the deposited films has not yet oxidized.

The disadvantages of this method include low enough the value of a given contact resistance.

The main technical objective of the proposed method is to reduce the values shown in the contact resistance.

The main technical problem is achieved by a method of making ohmic contact to GaAs, including the creation on the surface of the plate n-GaAs mask for the implementation of the inverse process of lithography, deposition of Ge and Cu on the plate surface of n-GaAs, the first heat treatment in a single vacuum cycle with the deposition process, the extraction plate n-GaAs from the vacuum chamber, removing the mask and the second heat treatment, characterized in that the first heat treatment is carried out in an atmosphere of atomic hydrogen at a temperature of from 150 to 460°C and flux density of hydrogen atoms on the plate surface of the n-GaAs equal to 1013-1016at.·cm-2·with-1the deposition of Cu and Ge are performed simultaneously or alloy CuGexor from two independent sources of Cu and Ge to form a thin film CuGexwhere x=0,2-0,45.

In the particular case of osaid the Ge and Cu, and first heat treatment is carried out in a vacuum chamber at a pressure of residual atmosphere of less than 5×10 -6Torr.

Conducted by the applicant's analysis of the level of technology has allowed to establish that the analogs are characterized by the sets of characteristics is identical for all features of the proposed device are missing.

Search results known solutions in this and related areas of technology in order to identify characteristics that match the distinctive features of the prototype of the invention has shown that they do not follow explicitly from the prior art.

Of certain of applicant's prior art there have been no known effect provided essential features of the invention transformations on the achievement of the technical result. Therefore, the invention meets the condition of patentability "inventive step".

The figure shows the dependence of the given contact resistivity ρ of ohmic contacts to GaAs with 45%germanium concentration from the temperature of the first heat treatment T1obtained by the method-prototype (1) and proposed method (2).

The proposed method consists in the following. On the surface of the plate n-GaAs having a doped layer, with the aim of implementing the process of the inverse lithography mask is formed. To clean the surface in the Windows of the mask plate n-GaAs is processed in an aqueous solution of H2SO4or HCl followed the th its rinsed in deionized water and drying. Then using methods of electron-beam and/or thermal evaporation in vacuum at a residual pressure of less than 5×10-6Torr is the deposition of Ge and Cu total thickness of 100-500 nm with a mass content of germanium in a two-layer composition, equal 20-45%. Then, the plate n-GaAs in a single vacuum cycle is subjected to first heat treatment at a temperature T1=150-460°C in an atmosphere of atomic hydrogen at a flux density of hydrogen atoms on the surface of the plate equal to 1013-1016at.·cm-2·c-1. Then, the plate n-Ga-As extracted from the vacuum chamber and after removal of the mask is subjected to a second heat treatment in an atmosphere of inert gas or in vacuum in the temperature range T2=280-460°C for t=0.5 to 30 minutes

The minimum and maximum values of mass content of Ge in the two-layer composition the 20% and 45%, respectively, are determined by the fact that at smaller or larger values of contact resistance OK becomes unacceptable regardless of the method and its modes of the first and second heat treatment.

The minimum value of the flux density of hydrogen atoms on the surface of the plate equal to 1013at.·cm-2·with-1defined by the fact that at lower values is not achieved technical result of the invention in connection with the competition between the processes of oxidized who I am and deposition OK gases, present in the residual atmosphere in the vacuum chamber, and its reduction with hydrogen atoms. The maximum value of the flux density of hydrogen atoms on the surface of the plate equal to 1016at.·cm-2·with-1the marginal technical capabilities available sources of atomic hydrogen.

The minimum temperature of the first heat treatment T1=150°C is determined experimentally established facts, according to which at a lower temperature is not observed differences in the value of the given contact resistance obtained by the prototype method and the proposed method. The maximum value of the temperature of the first heat treatment T1=460°C is determined by the maximum possible temperature, which is used in a GaAs microelectronics.

The temperature and time intervals of the second heat treatment (T2=280-460°C, t=0.5 to 30 min) sets the minimum and maximum temperature budgets required for the formation of OK with the minimum value of the given contact resistance.

Example

The example demonstrates the technical result achieved by the proposed method on how the prototype, and the ability to achieve a technical result in a wide range of temperatures first termoobrabotki and in atomic hydrogen.

Used ion-doped wafer of n-Guas (100) with the concentration of electrons in the layer thickness of 0.12 μm, equal to n=2×1017cm-3. On the surface of the plate n-GaAs was formed a two-layer dielectric mask, which was opened window with a negative angle of inclination of the walls. Before deposition of Ge and Cu to clean the surface and remove native oxides of arsenic and gallium plate n-GaAs were processed in an aqueous solution of H2SO4(1:10) for 3 minutes, subsequently rinsed in deionized water and drying in a stream of nitrogen. Then, the plate n-GaAs was divided into two parts and loaded into the vacuum chamber. On both parts of the plate using a sequential deposition of Ge and Cu was formed a two-layer composition with a mass content of germanium equal to 45%. The residual pressure of the atmosphere was 4×10-6Torr. Immediately after deposition, by analogy with the method prototype, the first part of the plate n-GaAs subjected to first heat treatment in vacuum at T1=75°C for t=60 minutes thereafter, in accordance with the proposed method, the second part of the plate n-GaAs subjected to first heat treatment in a similar way, but in an atmosphere of atomic hydrogen at a pressure of molecular hydrogen p=10-4Torr and flux density of the hydrogen atoms of 1015at.·cm-2·with-1. Steelstone n-GaAs is removed from the vacuum chamber, removed the mask, which led to the formation topology, OK. Then both parts of the plate n-GaAs simultaneously subjected to a second heat treatment in the setting of rapid thermal annealing at a temperature T2=400°C for t=60 seconds in an atmosphere of pure nitrogen.

This sequence was repeated 5 times for various temperatures of the first heat treatment (T1=75-400°C). The value of the given contact resistance was measured by the method of transmission lines 10 tests, and then were averaged.

From the figure, which shows the dependencies listed in the contact resistance ρ from the temperature of the first heat treatment T1for OK, obtained by the method prototype, and OK, obtained by the proposed method, it is seen that the formation of OK for the proposed method at temperatures above T1=150°C can reduce the value of a given contact resistance in 2-2,5 times, about how the prototype. This is achieved due to the impact of hydrogen atoms that are chemically active particles with reducing properties, reduce the rate of oxidative reactions on the surface OK during the first heat treatment in vacuum. In addition, the first heat treatment active recovery environment (atomic hydrogen), you will increase is more than an order of magnitude, the residual pressure of the atmosphere in the vacuum chamber relative to the method prototype which is heat treatment, which reduces the requirements on the vacuum system and reduces the duration of pumping.

When forming the ohmic contact to GaAs p-type conductivity similar results were obtained.

1. Method of making ohmic contact to GaAs, including the creation on the surface of the plate n-GaAs mask for the implementation of the inverse process of lithography, deposition of Ge and si on the plate surface of n-GaAs, the first heat treatment in a single vacuum cycle of deposition layers, removing the plate n-GaAs from the vacuum chamber, removing the mask and the second heat treatment, characterized in that the first heat treatment is produced in an atmosphere of atomic hydrogen at a temperature of from 150 to 460°C and flux density of hydrogen atoms on the plate surface of the n-GaAs equal to 1013-1016ATM-2with-1and the deposition of si and Ge are performed simultaneously or alloy CuGexor from two independent sources si and Ge to form a thin film CuGexwhere x=0,2-0,45.

2. Method of making ohmic contact to GaAs according to claim 1, characterized in that the deposition of Ge and si and the first heat treatment is carried out in a vacuum chamber at a pressure of residual atmosphere of less than 5×10-6Torr.



 

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