Coating apparatus – Gas or vapor deposition – With treating means
Reexamination Certificate
2001-02-02
2003-02-11
Mills, Gregory (Department: 1763)
Coating apparatus
Gas or vapor deposition
With treating means
C438S566000, C118S719000, C118S720000
Reexamination Certificate
active
06516743
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and an apparatus for diffusing zinc (Zn) into groups III-V compound semiconductors. The groups III-V compound semiconductor means a semiconductor of a pair of a group III element gallium(Ga), indium(In) or aluminum(Al), and a group V element arsenic(As), phosphorus(P) or antimony(Sb). Bulk single crystal wafers are available for GaAs, InP and GaP. GaAs wafers, InP wafers and GaP wafers are useful as substrates of laser diodes (LDs), light emitting diodes (LEDs), photodiodes (PDs) or other semiconductor devices. Though this invention can be applied to any III-V compound semiconductor wafers, explanation will be done by only citing GaAs and InP.
The III-V compound semiconductor wafers are inherently n-type in many cases. Fabrication of a pn-junction requires epitaxial growth of p-type thin films on the n-type wafer, ion implantation of a p-type impurity, or thermal diffusion of a p-type impurity into the n-type wafer. The epitaxial growth of the p-type films is improper for making localized p-regions through a mask. The ion implantation is not the most suitable, since it requires a large apparatus, a plenty of steps and annealing of the ion implanted wafer, which raise the cost of producing the pn-junction. The thermal diffusion is the most suitable way for making pn-junctions in an n-type wafer. Zinc (Zn) acts as a p-type impurity in GaAs or InP crystals. Magnesium (Mg) and cadmium (Cd) are also p-type impurities in GaAs or InP, but Zn is the most feasible p-impurity for InP or GaAs. Zn-diffusion is one of the most important techniques of fabricating LEDs, LDs, PDs and other semiconductor devices having the group III-V semiconductor substrates. The purpose of the Zn-diffusion is to make localized p-regions on a crystal by diffusion.
Here, the crystal includes substrate crystals and film crystals grown on substrate crystals. A purpose of the present invention is to provide a new Zn-diffusion method and apparatus applicable to a wide wafer. Another purpose of the present invention is to provide a Zn-diffusion method and apparatus of high controllability. A further purpose of the present invention is to provide a Zn-diffusion method and apparatus immune from the use of poisonous materials.
This invention is a version of vapor phase diffusion methods but is different from conventional vapor phase diffusion methods. This invention is rather akin to liquid phase epitaxy (LPE). This invention rather diverts the manner and the device from the liquid phase epitaxy to the Zn-diffusion. Though this invention resembles the liquid phase epitaxy, this invention is essentially a vapor phase diffusion of Zn. Instead of material liquid, a vapor of Zn is filled in a sliding jig. This invention is not epitaxy but diffusion. This invention must be clearly discriminated from the liquid phase epitaxy.
This application claims the priority of Japanese Patent Application No.10-213954(10-213954) filed on Jul. 29, 1998 which is incorporated herein by reference.
2. Description of Prior Art
Impurity diffusion is classified into two categories of vapor phase diffusion and solid phase diffusion by the distinction whether the impurity is supplied from solid phase or vapor phase. There is no concept of “liquid phase diffusion”. The solid phase diffusion is a new technology recently proposed by the present Inventors for the first time (Japanese Patent Application No.5-177233, Japanese Patent Laying Open No.7-14791). The solid phase diffusion method has steps of growing a Zn-containing InGaAsP film epitaxially on an n-InP crystal substrate and diffusing Zn from the InGaAsP film into the InP substrate by heat. Since the object InP is protected by the InGaAsP film, P atoms do not escape from the bottom InP substrate in spite of heating in the solid phase diffusion. However, an excess number of steps have been prohibiting the practical use of the solid phase diffusion.
The Zn-diffusion is still actually done exclusively by vapor phase diffusion. The vapor phase diffusion is further classified into two methods. One of the vapor phase diffusion methods is a closed tube method. The other is an open tube method. Both two methods are well known. But only the closed tube method is put into practice on a large scale in the semiconductor industry at present. The open tube method is poorly employed on a small scale in some laboratories, because the open tube method is still suffering from unsolved difficulties. Two methods are explained in detail for clarifying the present state of the art of impurity diffusion.
[A. Closed Tube Method]
FIG. 14
shows a closed tube method for diffusing Zn into a group III-V semiconductor wafer. A long quartz tube
61
having an open end and a closed end is prepared. An InP wafer (or GaAs wafer)
62
is put on an inner spot near an end
60
of the quartz tube
61
. A diffusion source
66
is put on an inner point near the other end
65
of the quartz tube
61
. The quartz tube
61
is vacuumed and the open end is sealed by an oxygen-hydrogen flame burner. Sometimes the quartz tube
61
necks in a part
63
containing the solid diffusion source
66
. The Zn diffusion source
66
is either a sublimable compound of Zn and As or a sublimable compound of Zn and P. For example, zinc phosphide (ZnP
2
), zinc arsenide (ZnAs
2
) or so is selected as a material of the Zn-diffusion source, because they satisfy the conditions of inclusion of Zn, sublimability from solid phase to vapor phase and immunity from foreign materials except Zn and the substrate material. This method is called a closed tube method, because the quartz tube is fully closed.
The sealed quartz tube
61
is put into a horizontal furnace having heaters
67
and
68
. The furnace heaters
67
and
68
heat the whole of the quartz tube
61
and maintain the Zn-diffusion source
66
at a higher temperature than the wafer
62
. The Zn-diffusion source
66
sublimes into vapor at the higher temperature. The vapor flies in the quartz tube to the wafer
62
of GaAs or InP and adheres to the wafer at the lower temperature. The Zn atoms diffuse deeply in the wafer by heat. The diffusion depth in the wafer can be controlled by the temperature and the time. After the determined time has passed, the temperature of the furnace is reduced. When the furnace is cooled to a pertinent temperature, the quartz tube
61
is pulled out of the furnace. The object GaAs wafer (or InP wafer) is taken out by breaking the quartz tube
61
. The wafer is provided with pn-junctions by the Zn-diffusion.
FIG. 15
shows an improvement of the closed tube method. A long quartz tube
70
is prepared. A diffusion source
76
is put in at an end
75
of the quartz tube
70
. A GaAs wafer (or an InP wafer)
74
is placed in a half-closed short tube
73
. The half-closed tube
73
is put in at a middle of the quartz tube
70
. A vacuum is formed in the quartz tube
70
and the tube end is sealed by the oxygen-hydrogen flame burner. The closed tube is inserted into a furnace having heaters
79
,
80
and
82
. The diffusion source
76
is heated to the highest temperature by the heater
79
for subliming the source material. The middle part of the tube is heated at the lowest temperature by the heater
80
for converting the diffusion material vapor into powder and once depositing the powder
78
on the inner wall. Then, the powder
78
is heated for flying to the GaAs wafer
74
for depositing on the wafer. There are some new proposals of the close tube methods other than the method of FIG.
15
.
Why the tube must be sealed in the closed tube method? The sealing is required for the necessity of controlling the vapor pressure of the group V element (As or P). The closed space enables the dissolving speed of the diffusion source to uniquely determine the vapor pressure of the group V element. The dissolving speed is determined only by the temperature T of the diffusion source. Namely, in the closed tube, the vapor pressure is controlled only by the temperature T of the diff
Iguchi Yasuhiro
Sowa Sosuke
Kackar Ram N
Mills Gregory
Smith , Gambrell & Russell, LLP
Sumitomo Electric Industries Ltd.
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