Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate
Reexamination Certificate
2001-08-30
2003-04-15
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
C438S558000, C438S795000
Reexamination Certificate
active
06548378
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for boron doping wafers using a vertical oven system. The boron doping of wafers, in particular silicon wafers, plays an important role in semiconductor technology. The present method can be used in particular for the production of semiconductor products such as power MOSFETs (metal oxide semiconductor field effect transistors) in the DMOS technology (double diffused metal oxide semiconductor technology) or for bipolar transistors.
STATE OF THE ART
Two different, technologically relevant techniques have been used so far for the boron doping of silicon wafers. One technique concerns the direct boron implantation into the silicon wafer, whereas the other technique uses source layers for diffusing the boron into the silicon wafer.
The first-mentioned boron-implanting method, for which boron ions are accelerated and impact at high speed with the silicon wafer, however, results in extremely high processing costs due to the high implantation dose required for specific applications. This method can furthermore be realized only as a single-wafer process, which increases the time expenditure and thus also the process costs. Another disadvantage of this technique is that the boron-concentration profile generated through implantation into the silicon wafer is not box-shaped, but has a Gaussian
1
shape. A second implantation is required to achieve an erfc
2
profile with this technique. However, box-shaped doping profiles in particular are needed for the aforementioned power MOSFETs and for bipolar transistors.
Thus, the second doping method mentioned herein, which makes use of source layers, is generally used for the production of doping profiles of this type at a reasonable cost. With this method, the boron doping occurs from a solid layer that is deposited on the wafer. The doping method requires a two-stage process. A thin, highly concentrated boron layer is generated in a first stage through low temperature depositing on the wafer surface. With the aid of a high-temperature diffusion process, the boron then diffuses from this thin boron layer during a second stage into the surface of the wafer, up to the desired depth.
The problem of doping uniformity and reproducibility of this doping between individual processing cycles comes becomes important as a result of the constantly increasing wafer size and the requirement for doping the highest possible number of wafers during a processing cycle. On the one hand, it must therefore be ensured that the desired doping profile on the individual wafer has the highest possible uniformity. On the other hand, the deviation in the doping profile or the doping concentration between individual wafers of a processing cycle, as well as between wafers from different processing cycles, must be negligibly small.
One variant of the method for boron doping silicon wafers from a solid layer uses boron nitride wafers as boron source for generating the source layers on the silicon wafers. A method of this type is known, for example, from J. Monkowski et al., “Solid State Technology,” November 1976, pages 38 to 42. This method uses a horizontal oven system, in which the individual wafers are arranged one behind the other inside the so-called quartz boot. The boron nitride wafers are arranged for this between the individual silicon wafers.
The disadvantage of this method, however, is that the oven capacity for the silicon wafers that must actually be doped is reduced by 50% due to the required arrangement of the boron-nitride wafers. Furthermore, the danger exists that the quartz component(s) of the processing chamber is (are) contaminated or damaged because the boron-nitride wafer adheres to the quartz boot. Another disadvantage is the involved storage and conditioning of the boron-nitride wafers, which additionally are very costly and have only a limited durability.
Another method for boron doping silicon wafers from a solid layer is disclosed in the Reference P. C. Parekh et al., “Proceedings of the IEEE,” Volume 57, Number 9, from Sep. 9, 1969, pages 1507 to 1512. With this method, liquid BBr
3
(boron tribromide) is used as a source. Oxygen and BBr
3
are fed jointly with nitrogen as carrier gas into the reaction room containing the wafers. Inside the reaction room, the BBr
3
together with the oxygen forms the so-called reactive gas, which reacts as follows:
2BBr
3
(
g
)+3/2O
2
(
g
)→B
2
O
3
+3Br
2
(
g
)
2B
2
O
3
+3Si→4B
+
+3SiO
2
(borosilicate glass)
The borosilicate glass is thus deposited on the surface of the wafer. The borosilicate glass created in this way functions as source layer, from which boron is diffused during the subsequent diffusion phase (drive in) into the wafer substrate underneath. A horizontal oven system with an expanded, constant temperature zone was used for this method. The borosilicate glass is deposited at a temperature range between 860 and 950° C., the diffusion occurs at 1220° C. The problem of a uniform doping was again of the utmost importance.
One disadvantage of the method shown herein is that the doping uniformity again could not be maintained, in particular over the length of the horizontal oven used.
Furthermore, larger wafer diameters cannot necessarily be processed automatically when using a horizontal oven system. Thus, maximum 5-inch wafer diameters can presently be doped inside horizontal oven systems. A change from the 5-inch wafer to the 6-inch wafer is possible only with great difficulties because of the required change in the processing specifications with respect to the doping uniformity of the silicon wafer.
Starting with this state of the technology, it is the object of the invention to specify a method for boron doping wafers, which makes it possible to achieve a high doping uniformity without requiring structural changes in the existing oven systems when changing from smaller to larger wafer diameters. In addition, a cost-effective realization of this method should be possible.
ILLUSTRATION OF THE INVENTION
This object is solved with the method according to claim 1. Advantageous embodiments of the method are the subject matter of the dependent claims.
A vertical diffusion oven is used with the method according to the invention for boron doping wafers. This oven is provided with a vertical reaction chamber, extending from an upper end toward a lower end, which comprises several independently heated temperature zones. A gas intake for a boron-containing reactive gas is located at the upper end of the reaction chamber. The individual temperature zones extend successively from the upper end toward the lower end of the reaction chamber. With the method according to the invention, the boron-containing reactive gas flows over the wafers arranged inside the reaction chamber to deposit a layer of boron, in particular a layer of borosilicate glass. Subsequently, the boron from the boron layer is diffused into the surface of the wafer. According to the invention, the temperature in the independently heated temperature zones is adjusted such that between the zone following the top temperature zone and the lowest temperature zone, a temperature increase is maintained during the deposit of the boron layer and a temperature drop is maintained during the subsequent diffusion.
Inside the reaction chamber of the vertical oven, these additional temperature zones extend over the region filled with wafers. The upper zone covers the region of the gas intake. The temperature increase or the temperature drop toward the lower end of the reaction chamber is initiated through a stage-by-stage increase or reduction in the temperature from zone to zone. Excellent results can be obtained with a vertical diffusion oven that is divided into five temperature zones, wherein the middle temperature zone extends over approximately half the height of the reaction chamber. The boron-containing reactive gas can be provided through different, liquid or gas-containing boron sources. Examples for these are BBr
3
, BCl
3
or B
2
H
6
sources.
The method ac
Boness Henning
Press Patrick
Kinberg Robert
Mulpuri Savitri
Venable LLP
Vishay Semiconductor Itzehoe GmbH
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