Non-contact processing of crystal materials

A system and method for forming spherical semiconductor crystals is disclosed. The system includes a receiver tube 18 for receiving semiconductor granules 104. The granules are then directed to a chamber 14 defined within an enclosure 20. The chamber maintains a heated, inert atmosphere with which to melt the semiconductor granules into a molten mass. A nozzle, 40, creates droplets from the molten mass, which then drop through a long drop tube 16. As the droplets move through the drop tube, they form spherical shaped semiconductor crystals 112. The drop tube is heated and the spherical shaped semiconductor crystals may be single crystals. An inductively coupled plasma torch positioned between the nozzle and the drop tube melts the droplets, but leaving a seed in-situ. The seed can thereby facilitate crystallization.

BACKGROUND OF THE INVENTION 
The invention relates generally to semiconductor devices, and more 
particularly, to an apparatus and method for forming a device such as a 
spherical-shaped semiconductor crystal. 
Conventional integrated circuits, or "chips," are formed from a flat 
surface semiconductor wafer. The semiconductor wafer is first manufactured 
in a semiconductor material manufacturing facility and is then provided to 
a fabrication facility. At the latter facility, several layers are 
processed onto the semiconductor wafer surface. Once completed, the wafer 
is then cut into one or more chips and assembled into packages. Although 
the processed chip includes several layers fabricated thereon, the chip 
still remains relatively flat. 
Manufacturing the wafers requires creating rod-form polycrystalline 
semiconductor material; precisely cutting ingots from the semiconductor 
rods; cleaning and drying the cut ingots; manufacturing a large single 
crystal from the ingots by melting them in a quartz crucible; grinding, 
etching, and cleaning the surface of the crystal; cutting, lapping and 
polishing wafers from the crystal; and heat processing the wafers. 
Moreover, the wafers produced by the above process typically have many 
defects. These defects can be attributed to the difficulty in making a 
single, highly pure crystal due to the cutting, grinding and cleaning 
processes as well as impurities associated with containers used in forming 
the crystals. These defects become more and more prevalent as the 
integrated circuits formed on these wafers contain smaller and smaller 
dimensions. 
In co-pending U.S. patent application Ser. No. 08/858,004 filed on May 16, 
1997, a method and apparatus for manufacturing spherical-shaped 
semiconductor integrated circuit devices is disclosed. Although certain 
manufacturing methods for making spherical shaped crystals are disclosed 
in the above-referenced application, an improved method of making the 
spherical shaped crystals, which includes fewer defects and is more 
manufacturable, is desired. 
SUMMARY OF THE INVENTION 
The present invention, accordingly, provides an apparatus and method for 
processing crystals. To this end, one embodiment provides a receiver tube 
for receiving semiconductor granules. The granules are then directed to a 
chamber defined within an enclosure. The chamber maintains a heated, inert 
atmosphere with which to melt the semiconductor granules into a molten 
mass. A nozzle, located at one end of the chamber, creates droplets from 
the molten mass, which then drop through a long drop tube. As the droplets 
move through the drop tube, they form the spherical shaped semiconductor 
crystals. 
In another embodiment, the drop tube is heated with an inductively coupled 
plasma torch located between the nozzle and the drop tube. The plasma 
torch melts the droplets, thereby decreasing the number of crystalline 
directions in the droplet. 
In yet another embodiment, spherical shaped crystals are formed from 
polycrystal granules. The polycrystal granules are melted into a seed and 
a molten mass. The molten mass then solidifies around the in-situ seed. As 
a result, the molten mass creates a crystal with crystalline directions 
identical to those of the seed. 
In still another embodiment, before the polycrystal granules are formed 
into the spherical shaped crystals, they are coated with a nucleating 
agent. Once coated, the granules are completely melted and then 
re-solidified. By so doing, the resulting polycrystal granules have fewer 
crystalline directions, which promotes the ability to form a single 
crystal seed. 
In different embodiments, different structures may be utilized, some of 
which may not come into physical contact with any of the polycrystal 
granules, droplets, or other material.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, the reference numeral 10 designates, in general, one 
embodiment of a processor for forming spherical shaped semiconductor 
crystals and/or solar cell crystals. Formation of the crystals may be 
facilitated in different manners by varying parameters described herein, 
including repetitive processing through portions of the processor 10. 
The processor 10 can be separated into three sections: an input section 12, 
a main furnace section 14, and a dropper section 16. The input section 12 
includes a receiver tube 18 for receiving processing materials, such as 
granules, gases and the like. The receiver tube 18 is about 2 centimeters 
in diameter and registers with the main furnace section 14 
An enclosure 20 surrounds the main furnace section 14 and supports a 
general environment for processing. The enclosure is filled with an 
insulative material 22 to contain the relatively high temperatures 
produced in the main furnace section. The enclosure 20 and insulative 
material 22 provide an inert atmosphere, which prevents burnout of the 
insulation material 22 and other components stored therein. Disposed 
within the insulative material is a crucible 24. The crucible serves to 
hold molten semiconductor material, yet not react with the material. 
A lid 26 of the crucible 24 connects to the receiver tube 18. In the 
present embodiment, the lid is threadably engaged to the receiver tube to 
facilitate removal and separation of the various components. The lid 26 
further maintains the inert atmosphere inside the enclosure. Alternative 
embodiments may have other types of lids that either temporarily or 
permanently secure the receiver tube 18 to the crucible 24. The receiver 
tube 18 can either batch feed or continuously feed raw semiconductor 
material from the crucible 24. For each type of feeding, a different lid 
26 may be required. 
Immediately surrounding the outside of the crucible 24 is a furnace 30. In 
the present embodiment, the furnace is a fluid-heat type furnace, although 
other sources of heat may be used. The furnace 30 includes a fluid nozzle 
32 through which the fluid may pass. The fluid nozzle 32 further maintains 
the inert atmosphere inside the enclosure 20. Although not shown, another 
device may be used to heat the fluid before it passes through the fluid 
nozzle 32. Also, a heat measurement device 34, such as a thermocouple, is 
attached to the furnace 30 for monitoring the temperature of the furnace 
30 and of the crucible 24. 
The enclosure 20, along with the crucible 24, rests on a support platform 
36. The platform has several apertures to facilitate the various devices 
and processes herein disclosed. The platform 36 is also able to withstand 
some of the severe heat that radiates from the furnace 30 while 
maintaining the inert atmosphere inside the enclosure 20. 
Attached to a bottom portion of the crucible 24, as seen in FIG. 1, is a 
dropper 40. Although not shown, the dropper 40 may include a nozzle that 
injects precise sized droplets of molten semiconductor material from the 
crucible 24 and into the dropper section 16. In one embodiment, the nozzle 
may be according to U.S. Pat. No. 5,560,543, entitled Heat-resistant 
Broad-bandwidth Liquid Droplet Generators. Alternatively, or in 
combination with the nozzle, inert gas may also be applied to facilitate 
the precise amounts of molten semiconductor material being injected into 
the dropper section 16. 
The dropper section 16 includes a long drop tube 42. For example, the drop 
tube 42 may be stainless steel with an electro-polished inside finish, 
about five to ten centimeters in diameter and about ten meters in length. 
The drop tube 42 includes apertures so that a cooling gas 44 may flow 
therein. The cooling gas may also include impurities for doping the 
semiconductor material to a desired level. In some embodiments, a heater 
46 is placed adjacent to the drop tube 42. The heater 46 helps to reduce 
the number of different crystalline growth directions by slowing the 
cooling process. 
Referring to FIG. 2, a method 100 may be used in conjunction with the 
processor 10. At step 102, material 104 is placed into the receiver tube 
18. For the sake of example, the material includes silicon, it being 
understood that different types of semiconductor material may also be 
used. The material 104 may also include an inert carrier gas, such as 
argon, and one or more dopant materials. 
At step 106, the material 104 passes through the lid 26 and into the main 
furnace section 14. The furnace 30 can produce temperatures of about 
1600.degree. C., which far exceed the melting point of silicon 
(1410.degree. C.). This high temperature causes the material 104 to become 
a molten mass 108. At step 110, the nozzle 40 allows droplets 112 of the 
molten mass to leave the crucible 24 and enter the dropper section 16. At 
step 114, the droplets 112 fall down the drop tube 42. The drop tube will 
allow the droplets to cool and form a polycrystalline structure. The 
cooling gases 44 may be, for example, helium, hydrogen, argon, or nitrogen 
to facilitate the cooling of the droplets. The cooling gases 44 may also 
be used to control the rate of descent of the droplets 112. 
In some embodiments, the processor 10 controls the rate at which the 
droplets 112 cool. This may occur by many different methods. For example, 
the cooling gases 44 may be heated. Also, the drop tube 42 may be heated 
by the heaters 46. As a result, the droplets 112 will cool very slowly, 
thereby forming crystals. In additional embodiments, such as those 
discussed with reference to FIG. 3 below, the heaters 46 may actually melt 
the droplets. At step 114, the cooled droplets are then transferred to a 
next operation for processing. 
Referring to FIGS. 3, 3a, and 3b, in another embodiment, an inductively 
coupled plasma ("ICP") torch 150 is positioned above the drop tube 42. The 
ICP torch 150 includes a quartz tube 152 made of a high temperature 
material, for example, ceramic. Surrounding the quartz tube 152 is a 
cooling system 154. The cooling system may include water or any other type 
of coolant to help prevent the quartz tube 152 from melting. 
Also surrounding the quartz tube 152 is a conductive coil 156. In the 
present embodiment, the conductive coil 156 is a hollow copper coil 
attached to a radio frequency ("RF") energy generator 158. Because the 
coil 156 is hollow, air or other fluid can flow therethrough to help lower 
the temperature of the coil. 
An entry tube 160 is attached to the quartz tube 152 for receiving the 
droplets 112 (FIG. 1) and directing them towards a central chamber 162 of 
the quartz tube. Although not shown, a shield may be provided to prevent 
radiation losses from plasma in the tube 160. 
In operation, an argon gas flows at atmospheric pressure inside the quartz 
tube 152. The RF generator 158 operates at a desired frequency to heat the 
gas, thereby creating a plasma flame 164. The plasma flame 164 is at a 
temperature between 8000.degree. K. to 10,000.degree. K., which depends on 
the gas flow rate and torch dimensions. A portion of the flame 164a 
extends into the entry tube 160 and serves as a preheater. 
When a droplet 112 enters the entry tube 160, it is quickly preheated by 
the plasma flame 164a. The droplet 112 then enters the central chamber 162 
where the plasma flame 164 melts it. Because the temperature of the flame 
164 is so high, impurities in the droplet 112 will vaporize. As the 
droplets exit the central chamber 162, they solidify to form crystals 
112a. The crystals 112a advance through the drop tube 42 where they cool, 
as described above with reference to FIGS. 1 and 2. 
In many instances, the crystals 112a are single crystal granules with a 
consistent, uniform crystalline direction. However, in other instances, 
the crystals 112a may be polycrystal granules with multiple crystalline 
directions. It is understood, however, that the polycrystals produced by 
the drop tube 42 will have fewer different crystalline directions than the 
droplets 112, i.e., there are fewer crystalline structures in a single 
granule. 
Referring to FIGS. 4a-4d, the quartz tube 152 can take on various shapes 
and sizes that will affect both the plasma flame 164 as well as the 
portion 164a. Therefore, it is anticipated that different quartz tubes, 
such as quartz tubes 152a-d, can be utilized to accommodate different 
requirements of the heater 46. 
Referring to FIG. 5, to facilitate the formation of single crystalline 
material with the above described devices and methods, an in-situ seeding 
process 200 may be used. Forming single crystalline materials by 
non-contact processing is difficult because the solidification of the 
material may not be unidirectional, and hence polycrystal. Uniform 
crystalline direction is facilitated using a seed or a crystal nucleating 
agent. The in-situ seeding process 200 may utilize a non-contact 
processing technique in which a seed is introduced in-situ into the 
material by controlling the melting of the feed material. 
FIGS. 6a-6f provide illustrations and examples of intermediate material 
configurations for each step of the in-situ seeding process 200. 
Furthermore, exemplary processing details are provided with FIGS. 6a-6f. 
It is understood, however, that the illustrations and examples of FIGS. 
6a-6f are only provided for the sake of clarity and are not intended to 
limit the invention in any way. 
At step 202 of FIG. 5, a first step in the formation of single crystal 
silicon spheres is to obtain a polycrystalline granule. 
Referring also to FIG. 6a, an illustration of a polycrystalline granule is 
designated with a reference numeral 204a. The polycrystalline granule 204a 
has several crystalline structures 205 and may be produced by one of the 
above-described processes or another process all together. Referring to 
FIG. 1 for one example, the processor 10 may be used to produce the 
polycrystalline granule 204a, which in FIG. 1, is designated as the 
droplet 112. 
At step 206 of FIG. 5, a nucleating agent is provided to coat the 
polycrystalline granule to produce a coated granule. The nucleating agent 
may be supplied from an external source and sprayed onto the 
polycrystalline granule. 
Referring also to FIG. 6b, an illustration of a coated granule is 
designated with a reference numeral 204b. A nucleating agent 208 in the 
form of a seeding powder is applied around the polycrystalline granule 
204a to produce the coated granule 204b. For example, the seeding powder 
may be boron nitride or quartz. Other nucleating agents can be used, 
although materials that provide a good nucleating site and have minimal 
solubility in liquid silicon are desirable. Referring to FIG. 3 for 
example, the nucleating agent 208 can be applied in the entry tube 160. 
At step 210 of FIG. 5, the coated granule passes through a heating zone 
until it is completely melted. 
Referring also to FIG. 6c, an illustration of a completely melted granule 
is designated with a reference numeral 204c. For example, the melted 
granule 204c may pass through an ICP torch at a controlled rate, such as 
is described with respect to FIG. 3. The melted granule 204c is extremely 
pure from the non-contact processing. Non-contact processing techniques 
may also include levitating the coated granules 204b by an energy source 
such as electromagnetic induction, acoustic, electrostatic, aerodynamic, 
plasma or combination-type sources. In the case of electromagnetic and 
plasma levitation processes, the levitation energy may also be used for 
melting. Although not shown, the nucleating coat is still present around 
the outside of the melted granule 204c. 
At step 212 of FIG. 5, the molten granule solidifies to form a 
course-grained granule with a relatively few number of polycrystals. 
Referring also to FIG. 6d, an illustration of a solidified course-grained 
granule is designated with a reference numeral 204d. The granules 204d may 
solidify while dropping through a tube, such as the drop tube 42 described 
in FIG. 1. The temperature for solidification may be controlled by heaters 
surrounding the tube and an atmosphere of Ar, He or a mixture of both may 
be maintained in the tube. The course-grained granule 204d has a plurality 
of polycrystals 214, however, the number of polycrystals 214 is less than 
the number of polycrystals 205 of FIG. 6b. This is because the nucleating 
agent 208 seeds crystal growth in a limited number of directions as the 
granule 204d solidifies. 
At step 216 of FIG. 5, the solidified course-grained granule is remelted. 
This time, however, the material is partially melted and a seed is left 
in-situ, or in place. The seed will be in-situ because it will "float" 
inside the molten material. Also, because the course-grained granule from 
step 212 consists of a relatively few number of polycrystals, it is likely 
that the seed will be single crystal, with a single growth direction. As 
in step 212 above, it is understood that any type of heating and melting 
may be used to remelt the material. 
Referring also to FIG. 6e, an illustration of a partially-melted granule is 
designated with a reference numeral 204e. The granule 204e has a 
significant portion of molten material 218 and a small seed 220 floating 
therein. The melting process may be the same process used above with 
respect to FIG. 212, except instead of levitating the granule, it is 
allowed to drop through a drop tube, such as the drop tube 42 of FIG. 1. 
The temperature for solidification may be controlled by heaters, such as 
the heaters 46, and an atmosphere of Argon and/or Helium may be maintained 
in the tube 42. 
At step 222 of FIG. 5, the partially-melted granule is cooled at a control 
rate. Because the granule includes a seed in-situ, crystalline patterns 
will be determined from the seed. If the seed is single crystal, as 
desired, the granule will cool into a larger, single crystal device. 
Referring also to FIG. 6f, an illustration of a single crystal 
semiconductor device is designated with a reference numeral 204f. The 
device 204f has a grain direction 224 that is consistent with the grain 
direction of the seed 220 (shown in phantom) from FIG. 6e. It is 
understood that the device 204f may not be entirely spherical in shape. 
For the sake of explanation, the device 204f is illustrated as having a 
"tear drop" shape. If the tear drop shape is unacceptable, various 
polishing techniques can be used to smooth the device 204f into a more 
perfect sphere. 
As a result, the single crystal semiconductor device 204f is very pure, due 
to the non-contact processing and use of the in-situ seeding. Also, if for 
some reason, the seed produced in step 216 is polycrystal, the process 200 
can be repeated, beginning at step 216. 
Referring to FIG. 7, another method 300 may be used to facilitate the 
formation of single crystalline material with the above described devices 
and methods. The method 300 uses systems and methods described above as 
well as in co-pending U.S. patent application Ser. No. 08/858,004 filed on 
May 16, 1997, which is herein incorporated by reference. 
At step 302, small single crystal particles are formed, such as by 
epitaxial growth using a method of presently incorporated U.S. patent 
application Ser. No. 08/858,004. At step 304, the single crystal particles 
are then fed into a fluidized bed reactor, which can be heated to a 
temperature that meets or exceeds the melting of Si. One example of a 
fluidized bed reactor is disclosed in the presently incorporated U.S. 
patent application Ser. No. 08/858,004. At step 306, a gas mixture, such 
as monosilane and Argon, is fed into the reactor. At step 308, when the 
monosilane gas reaches a critical temperature, it decomposes to silicon 
vapor and hydrogen. At step 310, the silicon vapor deposits on the single 
crystal particle and grows in size as a granule. If the temperature is 
high enough, epitaxial growth can occur and the resulting granule will be 
a single crystal. Otherwise, the resulting granule will be a polycrystal 
with a single crystal core. 
At step 312, the granule is partially melted so that the outer 
polycrystalline layer is melted. This is similar to step 216 of FIG. 5. 
The unmelted single crystal core now acts as a seed for the rest of the 
molten silicon. Upon solidification, the partially molten granule converts 
to a single crystal. This is similar to step 222 of FIG. 5. 
Several advantages result from the above-described embodiments. For one, 
the material seldom, if ever, comes in physical contact with any other 
device or any part of the processor 10. By melting the material without 
physical contact, less contaminants are introduced. Also, as the material 
drops or is levitated, it becomes spherical due to surface tension. 
Furthermore, single crystals can be formed through controlled melting and 
cooling of the material. Further still, many crystals can be processed in 
rapid succession, with the crucible 24 continually being refilled. 
It is understood that several variations may be made in the foregoing. For 
example, different heating steps may be used in different parts of the 
processor. Further still, a catcher (not shown) may be included to receive 
the material and facilitate the heating or cooling process. The catcher 
may also be used to return the material to a furnace section for 
additional processing. Other modifications, changes and substitutions are 
also intended in the foregoing disclosure and in some instances some 
features of the invention will be employed without a corresponding use of 
other features. Accordingly, it is appropriate that the appended claims be 
construed broadly and in a manner consistent with the scope of the 
invention.