Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal
Reexamination Certificate
1996-07-18
2001-07-24
Bowers, Charles (Department: 2823)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
C438S029000
Reexamination Certificate
active
06265236
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for manufacturing a light emitting diode with a frosted surface.
Green luminous light-emitting diodes are made from (1 1 1)-oriented gallium phosphide. Since gallium phosphide is an indirect semiconductor, the efficiency is lower than in mixed crystal systems where, with a suitable composition, direct band-to-band transitions are also possible. The external quantum efficiency, which is determined by the material property of internal quantum efficiency and by the losses which occur when the radiation emerges from within the diode, is no more than 0.3% for GaP diodes.
One reason for the poor efficiency of the diodes is the high proportion of radiation that cannot escape from the body of the diode because of the total reflection on the surface of the semiconductor. This is due to the high optical refractive index of the semiconductor material. This is approximately 3.4 for gallium phosphide. This results in a critical angle of total reflection of 17.7° on emergence of the radiation to air. By direct means, only that proportion of the radiation is emitted that falls on the boundary layer at a smaller angle to the surface normal. The remainder is reflected back into the diode body. A large part of the radiation reflected back is lost due to absorption in the semiconductor body and at the metal contacts. The efficiency can therefore be increased considerably if the emission of the radiation is improved.
In principle, there are many means of improving radiation emission from the interior. In the patent publication DE 42 31 007 A1, a method is described for increasing the critical angle of total reflection by applying a &lgr;/4-thick antireflective coating.
From EP 404 565, a method for manufacturing a radiation-emitting diode made from the III-V compound-semiconductor material GaAIAs is known where the entire surface of the semiconductor chip is roughened in order to improve the external quantum efficiency. The roughening or frosting takes place after the diodes have been singled out. By frosting, the total reflection of the radiation generated at the boundary layer between the diode chip and the surrounding material is largely avoided, the light path in the semiconductor material is shortened and thus the probability of reabsorption is reduced. At the same time, the effective surface of the diode is increased in size thereby allowing more radiation to escape from the inside of the diode. A disadvantage of the known method is, however, that the diode chip is very difficult to bond at the time of assembly because of the etched contact layer structures. Also, etch-resistant materials must be used for the contact layer structures. Furthermore, etching of the surface in the region of the radiation-emitting junction results in a reduction of the life of the diodes.
From DE 43 05 296 A1, a method for manufacturing a radiation-emitting diode with frosted side edges is known. Frosting takes place after the substrate wafer has been diced. The diodes are held together in a group by a carrier sheet applied on the rear side. A layer of silicon dioxide protects the contact surfaces, the front side of the diode and the mesa edges with the pn junction that emerge at the surface against attack by etching. The silicon dioxide must be removed again after frosting. The application and removal of the protective layer and the bonding of the carrier sheet make the known method very elaborate.
SUMMARY OF THE INVENTION
The object of the invention is to provide a method for manufacturing a light-emitting diode with a frosted or roughened surface where no further process steps are needed after the diodes have been singled, i.e., individually separated from the wafer.
The method according to the invention for the manufacture of a light emitting diode comprises the following process steps. The layers including the pn transition or junction and generating the radiation are produced on a substrate. Contact layers are made both on the outer surface of the layers including the pn transition, or junction and generating the radiation, and also on the rear surface of the substrate. The contact layers are tempered in order to form ohmic contacts. The method is characterized by the surface of the layers including the pn junction and generating the radiation being or roughened before the contact layers are deposited. The contact layers are not attacked by the frosting. No further process steps are needed after the diodes have been singled or separator.
The substrate is made advantageously of (1 1 1)-oriented gallium phosphide. Equally the series of layers which include the pn transition, that generates the radiation.
Frosting or rough etching of the outer surface of the layers including the pn transition or junction and generating the radiation is performed by wet chemical means with a mixture of phosphoric acid, hydrochloric acid and acetic acid. The ratio of phosphoric acid:hydrochloric acid:acetic acid is 1:1:1. This results in a frosted or roughened surface with a roughness depth of less than 1 &mgr;m.
The contact layers on the surface of the layers including the pn transition and generating the radiation and on the rear side of the substrate are tempered in a single process step.
The method for frosting or rough-etching the surface of a (1 1 1)-oriented GaP semiconductor wafer includes the use of an acid mixture comprising concentrated phosphoric acid H
3
PO
4
, concentrated hydrochloric acid HCl and concentrated acetic acid CH
3
COOH in equal volumetric amounts. The temperature of the acid mixture is in the range of 25° C. to 40° C. The semiconductor wafer is frosted or etched for a period of 2-15 minutes in the acid mixture. It is advantageous that the frosting can take place in an open basin because the composition of the frosting substances and the low temperatures result in no vapors that could endanger the health or cause corrosion.
In a particularly advantageous embodiment of the method the temperature of the acid mixture is approximately 30° C. The semiconductor wafers are then frosted rough-etched for about 5 minutes in the acid mixture.
The advantages associated with the method are in particular that the surface of the light emitting diode is frosted or roughened before the front side contact is produced. Frosting takes place in the wafer pack. The contact layer of the front side can be produced on the frosted surface without disadvantages for the function of the diode. This therefore makes it possible to also use less etch-resistant contact metals such as aluminum.
Furthermore, the frosting process is so designed that it can be performed without any special preparations at a relatively low temperature under normal laboratory conditions. The composition of the etching solution has been established in a series of experiments such that the structures that arise when frosting the (1 1 1)-oriented gallium phosphide do not have the pyramid shape with a sharp tip as known from the conventional media. A fine frosted surface with a roughness depth of less than 1 &mgr;m is produced by the method. This is important because otherwise a wire bonding of the contact located above the frosted surface would result in bursts in the semiconductor crystal and in failure of the diode and there would be no guarantee for being able to assemble the diodes.
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patent
Bowers Charles
Hawranek Scott J.
Kunitz Norman N.
Spencer George H.
TEMIC Telefunken microelectronic GmbH
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