Method for forming an ohmic contact to a germanium photodiode

Abstract

FIELD: optoelectronic equipment.SUBSTANCE: invention can be used to form an ohmic contact in germanium photodetector devices with low operating currents, operating in low-light conditions, to which requirements are imposed on minimum values of dark currents. Ohmic contacts to germanium are formed based on a two-layer Ti / Au metallization model, including vacuum deposition of a 0.05 mcm thick titanium adhesion layer, vacuum deposition of a 0.45 mcm thick conductive gold layer, creation of a contact pattern using photolithography, and rapid thermal annealing in a nitrogen atmosphere at a temperature of 240°C for 10 sec.EFFECT: reduction in the specific resistance of the contact, ensuring the required value of series resistance, leading to minimal values of dark currents in germanium photodiodes.1 cl, 16 dwg

Description

[0001] The invention relates to the field of production of optoelectronic equipment and can be used to form an ohmic contact in germanium photodetector devices with low operating currents and operating in low-light conditions, which are subject to requirements for minimum values ​​of dark currents.

[0002] The main mechanisms that influence the deterioration of dark currents in semiconductor photodiodes during the formation of an ohmic contact are: charge carrier injection due to imperfect ohmic contacts (high resistance or the formation of a Schottky barrier), recombination and generation at the contact interface (defects and surface states at the metal / semiconductor interface), current leakage through contact areas (poorly formed ohmic contacts can create parasitic current leakage paths), and contact heating (high resistance of ohmic contacts leads to local heating, which increases the thermal generation of charge carriers and, consequently, the dark current). Thus, ohmic contacts can significantly affect the dark current, and their quality is important for the operation of photodetector devices.

[0003] The series resistance in the p-n junction is the sum of the ohmic resistances of the p and n regions and the metallization contacts to these regions [AM Filachev, I.I. Taubkin, M.A. Trishenkov Solid-State Photoelectronics. Photodiodes - Moscow: Fizmatkniga, 2011]. The resistances of the p and n regions depend on their thickness and the concentration of impurities embedded in them, which makes it possible to reduce the resistance of these regions to a minimum by selecting optimal thicknesses and concentrations. The ohmicity of the contact depends on the selected materials and the method of its formation.

[0004] High-reliability devices primarily use multilayer contact pads made of various materials. Chromium, titanium, nickel, or molybdenum can be used as the adhesive (contact) sublayer. Titanium is a chemically resistant material and is often used as an adhesive sublayer in electronics manufacturing. It has good adhesion to the surface of semiconductors, particularly germanium, and to dielectric coatings such as silicon oxides and nitrides.

[0005] The conductive layer applied over the adhesive layer must have low resistivity and allow for easy mounting of external leads, typically gold wire. Gold (Au) is such a material for the conductive layer [L.A. Koledov, Technology and Design of Microcircuits, Microprocessors, and Microassemblies - Moscow: Radio and Communications, 1989, p. 78].

[0006] Taking into account the above, to form an ohmic contact we use a two-layer metallization model: conductive gold with an adhesive titanium sublayer (Au / Ti) with layer thicknesses of 0.45 μm and 0.05 μm, respectively.

[0007] A patent [RU 2790272C1] is known, which involves the formation of ohmic contacts to silicon based on a two-layer Ti / Au metallization system, including vacuum deposition of a two-layer metallization system consisting of 0.02 μm Ti and 0.5 μm Au films, patterning using photolithography, rapid thermal annealing in an H2 hydrogen atmosphere at 340°C for 20 seconds, which improved the electrical characteristics of devices. This invention has been selected as a prototype.

[0008] A drawback of the prototype is that the rapid thermal annealing mode adopted in it is not suitable for forming ohmic contacts to the germanium structures of photodiodes. At the prototype's annealing temperature and time, the contacts bulge (peel off).

[0009] The objective of the claimed invention is to form ohmic contacts with a low value of specific contact resistance, providing the required value of series resistance, leading to minimal values ​​of dark currents in germanium photodiodes.

[0010] The solution to the stated problem of the invention consists in forming ohmic contacts to germanium based on a two-layer Ti / Au metallization model, including vacuum deposition of an adhesive titanium layer with a thickness of 0.05 μm, vacuum deposition of a conductive gold layer with a thickness of 0.45 μm, creation of a contact pattern using photolithography, and rapid thermal annealing in a nitrogen atmosphere at a temperature of 240°C for 10 s.

[0011] Titanium has good adhesion to the surface of semiconductors, particularly germanium, and to dielectric coatings such as silicon oxides and nitrides.

[0012] Gold has low resistivity and allows for easy mounting of external terminals, typically with gold wire.

[0013] Rapid thermal annealing in a nitrogen atmosphere at a temperature of 240°C for 10 seconds ensures the fusion of the adhesive sublayer with the germanium surface.

[0014] Using nitrogen as a medium for rapid thermal annealing gave better results (determined experimentally) in dark current characteristics.

[0015] The difference in thickness between the adhesive and conductive layers is due to the material to which the ohmic contact is formed. Germanium requires a thicker adhesive sublayer; in the prototype, the sublayer thickness is 0.02 µm, while for germanium it is 0.05 µm.

[0016] The technical result of the formation is ohmic contacts with a low value of specific contact resistance, providing the required value of series resistance, leading to minimal values ​​of dark currents in germanium photodiodes.

[0017] To control the quality of the method of the claimed invention, test samples were made on which, after the deposition of metallization and the isolation of contacts by photolithography, the operation of rapid thermal annealing in a nitrogen atmosphere at a temperature of 240°C for 10 seconds was not carried out, and test samples on which, after the deposition of metallization and the isolation of contacts by photolithography, the operation of rapid thermal annealing in a nitrogen atmosphere at a temperature of 240°C for 10 seconds was carried out.

[0018] Comparison of the volt-ampere characteristics of the variants - with thermal annealing and without annealing - is illustrated by the figures in Figs. 1-16.

[0019] The figures in Figs. 1-8 show that the dark current on the test samples without rapid thermal annealing is high and increases with increasing reverse voltage. Measurements were conducted on eight photosensitive areas of the samples; therefore, the eight figures show the results for eight areas.

[0020] On the test samples (Fig. 9-16), which were subjected to rapid thermal annealing, the dark current has small values ​​and remains constant as the reverse voltage increases.

[0021] Measurements on annealed samples were also performed on eight photosensitive pads. Across all eight pads, the dark current has comparable low values ​​and remains constant as the reverse voltage increases.

[0022] The implementation of the invention, using an example, is as follows.

[0023] Gold with a titanium sublayer was vacuum-deposited onto germanium wafers used to create photodiode structures. Titanium was deposited using cathodic sputtering, while gold was deposited resistively using a Varicoat A 430 vacuum deposition system from Leybold Heraeus. Contacts were selected photolithographically to match the photodiode structure. The wafers were then placed in an RSO-650-200 pulse annealer for rapid thermal annealing in the selected mode (in a nitrogen atmosphere at 240°C for 10 s), during which titanium fuses with germanium, which increases the contact ohmicity. The results were monitored using a Hewlett Packard 4145 V I / V Characteristic Analyzer.

Claims

A method for forming ohmic contacts to a germanium photodiode based on a two-layer Ti / Au metallization system, including vacuum deposition of an adhesive titanium sublayer, vacuum deposition of a conductive gold layer, creation of a contact pattern using a photolithographic method, rapid thermal annealing, characterized in that the vacuum deposition of the adhesive titanium sublayer is carried out with a thickness of 0.05 μm, vacuum deposition of the conductive gold layer is carried out with a thickness of 0.45 μm, and rapid thermal annealing is carried out in a nitrogen atmosphere at a temperature of 240°C for 10 seconds.

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