Tubular oven

An improved tubular oven adapted for use in diffusion processing of semiconductors whose interior walls are comprised of polycrystalline silicon and whose exterior walls are comprised of phosphorous doped silicon. Block electrical contacts are located at opposed ends of such tubular oven and are comprised of conductive metal and graphite. A layer of thermal insulation circumscribes mid regions of such tubular oven.

BACKGROUND OF THE INVENTION 
German Offenlegungsschrift No. 1,933,128 shows an arrangement for diffusing 
doping materials into a semiconductor material wherein a tube of 
crystalline gas-tight semiconductor material is used as a diffusion 
container which can be heated by way of applying thereto a voltage 
directly or with the help of high-frequency energy. The tube serving as 
heating member may have its opposed ends connected with electrodes or may 
be circumferentially surrounded by an induction heating coil. To 
facilitate a heating of the tube by induction heating, a ring made of 
conductive material, such as graphite, is applied about the tube. When the 
tube is directly heated by an applied voltage, the voltage becomes 
dependent on the conductivity of the tube semiconductor material, 
independently of the tube dimensions in order to achieve diffusion 
temperatures. In order to use relatively low voltages for the initiation 
of the heating process, the above-mentioned reference suggests the use of 
highly doped semiconductor material for the diffusion tube, which can be 
produced relatively cheaply. When a predeterminable heating temperature is 
reached, the conductivity of the tube becomes independent of the doping of 
the semiconductor material and dependent upon the tube dimensions. 
Due to the gas-phase deposition technique employed for the production of a 
diffusion tube as taught, for instance, in German Pat. No. 1,805,870, 
wherein semiconductor material from a gaseous semiconductor compound is 
precipitated onto the outer surface of a carrier member such as graphite 
and the carrier member is removed without destroying the semiconductor 
material layer, the production of pure, gas-tight tubes made of 
semiconductor material, in particular silicon, have become possible. Such 
pure tubes are heatable by direct current passage only with preheating. 
When doped semiconductor material is used for the diffusion tube as 
mentioned in German Offenlegungsschrift No. 1,933,128, it is possible to 
eliminate such a pre-heating sequence and to heat such tube directly. 
However, this arrangement suffers from the fact that an undesired 
interaction between the high doping and a semiconductor component member 
can occur during a diffusion operation utilizing such tube. 
A directly heatable silicon tube is provided in German Offenlegungsschrift 
No. 2,253,411, which tube is produced in such a way that at least two 
layers are successively deposited on the circumferential surface portions 
of a carrier member provided for such deposition, and the outermost 
deposited layer is provided with a doping, while the innermost layer 
consists of highly pure silicon. In a tube of such construction, the tube 
outer doping does not influence a semiconductor member receiving a 
diffusion treatment therewithin. However, the problem of providing an 
electric contact for such a tube when such is used as a tubular oven for 
semiconductor diffusion processes is not solved in this 
Offenlegungsschrift. 
An electrical contact arrangement for a silicon tube which is to be heated 
directly is described in German Offenlegungsschrift No. 2,340,225. Here, 
the silicon tube which is being heated is provided with a doping in the 
areas where current contacts are to be applied in the assembled oven. A 
lacquer which contains phosphorus is applied to tube areas to be doped. 
Upon subsequent heating, the lacquer volatilizes without residue, and the 
doping material is diffused into the silicon tube. Heating collars made of 
aluminum or an aluminum alloy are used as current contacts, and they are 
placed onto a support of graphite felt. In this construction, the problem 
of achieving a satisfactory electric contacting of a directly heatable 
silicon tube has not been solved with respect to temperature constancy and 
heating duration. This construction interferes essentially with the 
reproducability of diffusion processes conducted in the assembled oven. 
BRIEF SUMMARY OF THE INVENTION 
It is thus a principle task of this invention to provide an improved 
tubular oven adapted for use in diffusion processing of semiconductor 
elements which oven employs a silicon tube. 
The tube employed in this oven is characterized by having interior walls 
comprised of substantially pure silicon (typically polycrystalline in 
composition) and by further having exterial walls comprised of phosphorous 
doped silicon. Such a tube is producable by gas phase deposition 
technology known to the prior art (as above indicated). 
In such tubular oven, this tube is provided with electrical contact means 
which are so constructed and so interrelated to this tube that there is 
obtained an oven wherein the oven pre-heating period is shortened to a 
minimum value and wherein a desired oven temperature can be maintained 
with great constancy. 
In this tubular oven, such results are achieved by a coaction between the 
various components. The phosphorous doping associated with the tube 
external walls facilitates in combination with the electrical contact 
means rapid tube heating by a directly applied voltage, while the interior 
wall regions are maintainable at precisely controlled temperatures. 
Various additional advantages, aims, purposes, objects and the like will be 
apparent to those skilled in the art for the present invention taken with 
the accompanying drawings.

DETAILED DESCRIPTION 
In this invention, the silicon tube employed in an oven can be prepared, 
for example, by the teachings of German Offenlegungsschrift No. 2,253,411. 
Preferably, this tube comprises a highly pure silicon layer adjacent its 
interior circumferentially extending wall surfaces, and a phosphorous 
doped silicon layer adjacent its exterior circumferentially extending wall 
surfaces, the doping thereof being sufficient to produce a specific 
electric resistance ranging from about 2 through 200 microohm-centimeters 
(measured at ambient temperatures). 
In an oven of this invention, opposed end portions of the silicon tube with 
its integral exterior phosphorous-doped layer are mounted in adjacent 
contacting relationship to conductive graphite support means. In turn, 
radially (relative to such tube) outer surface portions of such graphite 
support means are contacted and encased by highly conductive metal block 
means (which are preferably comprised of copper). The combination of metal 
block means plus associate graphite support means comprises electrical 
contact means at each opposed end of the silicon tube. 
In a preferred embodiment, the silicon tube may have a bandshaped region of 
strong doping extending circumferentially over the entire length of the 
outer surface portions of the tube. 
In order to obtain generally optimum conditions for dimensioning the 
graphite supports with respect to the achievement of a constant 
temperature profile in a tubular oven, the graphite supports are 
dimensioned in such a way that the ratio of the outer diameter of the 
silicon tube to the total axial contacting length of the graphite supports 
at each opposed tube end is not greater than about 10:1. In a particularly 
preferred embodiment in accordance with the teachings of the invention, 
this ratio is in the area of about 2:1. With such a ratio, as good a heat 
removal as possible is obtained at each tube end, and also a good 
contacting area between graphite supports and tube ends results. 
The dimensioning of the graphite surface area which contacts external 
circumferential surface portions of the tube is important, i.e., the area 
on the tube circumference which is to be contacted in relation to the tube 
diameter employed is important. The length of the tube may be randomly 
selected and may be independent of contact areas. The preferred contact 
area on the tube circumference at each tube end may be obtained by 
computation from the ratio of the tube diameter (d) to the tube length 
which is to be contacted. The contact area at each respective tube end for 
graphite contact(s) at that end thus typically falls in the range from 
about 1.times.d.sup.2 and 2.times.d.sup.2 [cm..sup.2 ]. The external 
electric contact achieved with the conductive metal block means at each 
tube end may be randomly dimensioned. Preferred proportions for the entire 
arrangement may be learned from the sample embodiment herein described. 
It has also proven to be particularly advantageous in the practice of this 
invention to have tube ends with exterior respective radii which closely 
match the respective radii of the graphite supports employed. For this 
purpose, the end regions of a silicon tube can be ground round, if 
desired. Similarly, the metal blocks preferably have surfaces which make 
close, face-to-face contact with the graphite supports adjacent thereto. 
Copper as a construction material for the metal blocks has advantages on 
account of improved contacting and electric conductivity. Aluminum may 
also be used, however, it cannot be as readily contacted. A copper block 
has advantages also as regards ease of water cooling thereof and as 
regards mechanical compression strength. Segmented graphite supports are 
preferred on account of improved compacting pressure performance 
characteristics relative to the tube. The better the pressure, the better 
the contact resistance (up to the point of tube collapse). 
In accordance with a particularly preferred embodiment in accordance with 
the teachings of the invention, the graphite supports and/or the copper 
blocks are each formed of a plurality of segments which are adapted to be 
in adjacent but circumferentially spaced relationship to one another about 
and in relation to the silicon tube. Any convenient mechanical mounting 
means may be employed to clamp and maintain the silicon tube, the graphite 
supports and the metal blocks in a desired interrelationship relative to 
one another. For example, screws interconnecting the metal blocks with one 
another may be employed. 
The metal blocks are preferably provided with cooling means which prevents 
localized overheating and aids in maintaining a desired constant oven 
temperature during operation thereof. 
Preferably the apparatus of the present invention is provided with a 
thermal insulation layer or blanket located about circumferentially 
exterior surfaces of the tube between the pair of electrical contact means 
(preferably in axially spaced, adjacent relationship to the latter). This 
blanket can be of any desired construction. For example, and preferably, 
such can be comprised of a thermally stable material, such as a layer of 
aluminum silicate fibers. For another example, this insulative layer can 
comprise a heat reflective metal sleeve. This layer aids in maintaining 
constant and desired high temperatures inside the silicon tube during 
operation of the oven of this invention. 
Referring to the illustrative embodiment depicted in the drawings, there is 
seen an oven of this invention herein designated in its entirety by the 
numeral 10. Oven 10 has a silicon tube 11 which here is about 32 cm in 
length, with an inner diameter 12 of about 26 mm and an outer diameter 13 
of about 31 mm. Tube 11 is open on its opposed ends 14 and 15. Tube 11 has 
an interior layer 1 comprised of substantially pure polycrystalline 
silicon extending over the entire interior circumferential regions of tube 
11. The tube 11 also has an exterior layer 2 (of 0.5 mm depth) consisting 
of silicon highly doped with phosphorous and having a specific resistance 
of about 3 microohm-centimeters which extends over the entire outer 
circumferential regions of tube 11. The silicon tube 11 is supported by 
and contacted with at the opposed ends graphite support sets 3 and 4, 
respectively. Each set 3 and 4 is comprised of four members arranged in 
circumferentially substantially equally spaced relationship to one 
another, each set 3 and 4 being at a different end of tube 11. Each 
individual graphite support member is about 25 mm in axial thickness and 
about 4,5 mm in radial thickness. The spacing between member ends can vary 
but is generally preferably at least about 250 mm. Each individual such 
member has a curvature such that it is adapted to make face-to-face 
engagement with adjoining surfaces of tube 11. 
Each set 3 and 4 of graphite support members is held in place by respective 
pairs of copper blocks sets 5 and 6. Each block set 5 and 6 is itself 
composed of halves which are adapted to mount over the radially outer 
surfaces of sets 3 and 4, respectively. Each block set has an axial 
thickness about equal to the axial thickness of sets 3 and 4. Each block 
set half is about equal to the others in size. In each block set half 
adjacent the transversely opposed end portions thereof a chennel 7 is 
provided which is adapted to receive therethrough screw or nut and bolt 
assemblies 8 which function as positioning and clamping means for the 
subassembly of block sets 5 and 6 with support sets 3 and 4. The sizing of 
the individual block set halves is such that they are preferably in 
spaced, adjacent relationship to one another in the assembled oven 10, the 
spacing therebetween ranging from about 0.5 to 2 cm. The graphite supports 
with respect of each tube end 14 and 15 are dimensioned in such a way that 
the ratio of outer diameter 13 of the tube 11 to contacting length of the 
graphite cheeks along the tube surface is about 2:1; thus, each graphite 
support extends circumferentially over tube 11 a distance of about 15 mm. 
Electric terminals 15 and 16 (not detailed) are provided for each block set 
5 and 6. The block sets 5 and 6 are each provided with tubes 17 for 
conducting cooling water therethrough during operation of oven 10. 
A thermal insulator layer 9 of approximately 30 mm thickness is positioned 
circumferentially about the heated portions of tube 11 between the 
opposite ends 14 and 15 thereof. Preferably layer 9 is axially spaced at 
each opposed end thereof from respective sets 3 and 4, and sets 5 and 6. A 
very good temperature constancy is achieved even in the case of high 
diffusion temperatures so that a zone of very even temperature over a 
fairly large portion of the tube 11 interior is gained during operation of 
oven 10. 
A voltage of about 10V is applied in order to obtain a diffusion 
temperature of about 1300.degree. C, at a current strength of about 100 
amperes. The pre-heating period is typically about 60 minutes. 
An oven of this invention may be conventionally used as a diffusion 
furnace. In the tube interior during oven operation a zone is 
characteristically produced which is constant in temperature. In such zone 
semiconductor crystal disks can be diffused and/or annealed. If desired, 
an oven can be provided with additional connections for flushing with gas. 
Other and further objects, aims, purposes, alternative embodiments and the 
like will be apparent to those skilled in the art from a reading of the 
present specification and appended drawings, without departing from the 
spirit and scope of this invention.