Crystal puller for growing monocrystalline silicon ingots

Details Crystal puller for growing monocrystalline silicon ingots Crystal puller for growing monocrystalline silicon ingots

2002-09-25

2003-04-29

Kunemund, Robert (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Apparatus

For crystallization from liquid or supercritical state

C117S200000, C117S204000, C117S208000

Reexamination Certificate

active

06554898

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to crystal growing apparatus used in growing monocrystalline silicon ingots, and more particularly to a heater assembly for use in such a crystal growing apparatus.
Single crystal silicon, which is the starting material for most semiconductor electronic component fabrication, is commonly prepared by the so-called Czochralski (“Cz”) method. The growth of the crystal is most commonly carried out in a crystal pulling furnace. In this method, polycrystalline silicon (“polysilicon”) is charged to a crucible and melted by a heater surrounding the outer surface of the crucible side wall. A seed crystal is brought into contact with the molten silicon and a single crystal ingot is grown by slow extraction via a crystal puller. After formation of a neck is complete, the diameter of the crystal ingot is enlarged by decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached. The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process, the crystal diameter must be reduced gradually to form an end-cone. Typically, the end-cone is formed by increasing the pull rate and heat supplied to the crucible. When the diameter becomes small enough, the ingot is then separated from the melt.
It is now recognized that a number of defects in single crystal silicon form in the growth chamber as the ingot cools from the temperature of solidification. More specifically, as the ingot cools intrinsic point defects, such as crystal lattice vacancies or silicon self-interstitials, remain soluble in the silicon lattice until some threshold temperature is reached, below which the given concentration of intrinsic point defects becomes critically supersaturated. Upon cooling to below this threshold temperature, a reaction or agglomeration event occurs, resulting in the formation of agglomerated intrinsic point defects.
The type and initial concentration of these point defects in the silicon are determined as the ingot cools from the temperature of solidification (i.e., about 1410° C.) to a temperature greater than about 1300° C. (i.e., about 1325° C., 1350° C. or more); that is, the type and initial concentration of these defects are controlled by the ratio v/G
0
, where v is the growth velocity and G
0
is the average axial temperature gradient over this temperature range. Accordingly, process conditions, such as growth rate (which affect v), as well as hot zone configurations (which affect G
0
), can be controlled to determine whether the intrinsic point defects within the single crystal silicon will be predominantly vacancies (where v/G
0
is generally greater than the critical value) or self-interstitials (where v/G
0
is generally less than the critical value).
Defects associated with the agglomeration of crystal lattice vacancies, or vacancy intrinsic point defects, include such observable crystal defects as D-defects, Flow Pattern Defects (FPDs), Gate Oxide Integrity (GOI) Defects, Crystal Originated Particle (COP) Defects, and crystal originated Light Point Defects (LPDs), as well as certain classes of bulk defects observed by infrared light scattering techniques (such as Scanning Infrared Microscopy and Laser Scanning Tomography).
Also present in regions of excess vacancies, or regions where some concentration of free vacancies are present but where agglomeration has not occurred, are defects which act as the nuclei for the formation of oxidation induced stacking faults (OISF). It is speculated that this particular defect, generally formed proximate the boundary between interstitial and vacancy dominated material, is a high temperature nucleated oxygen precipitate catalyzed by the presence of excess vacancies; that is, it is speculated that this defect results from an interaction between oxygen and “free” vacancies in a region near the V/I boundary.
Defects relating to self-interstitials are less well studied. They are generally regarded as being low densities of interstitial-type dislocation loops or networks. Such defects are not responsible for gate oxide integrity failures, an important wafer performance criterion, but they are widely recognized to be the cause of other types of device failures usually associated with current leakage problems.
Agglomerated defect formation generally occurs in two steps; first, defect “nucleation” occurs, which is the result of the intrinsic point defects being supersaturated at a given temperature. Once this “nucleation threshold” temperature is reached, intrinsic point defects agglomerate. The intrinsic point defects will continue to diffuse through the silicon lattice as long as the temperature of the portion of the ingot in which they are present remains above a second threshold temperature (i.e., a “diffusivity threshold”), below which intrinsic point defects are no longer mobile within commercially practical periods of time. While the ingot remains above this temperature, vacancy or interstitial intrinsic point defects diffuse through the crystal lattice to sites where agglomerated vacancy defects or interstitial defects, respectively, are already present, causing a given agglomerated defect to grow in size. Growth occurs because these agglomerated defect sites essentially act as “sinks,” attracting and collecting intrinsic point defects because of the more favorable energy state of the agglomeration.
Accordingly, the formation and size of agglomerated defects are dependent upon the growth conditions, including v/G
0
(which impacts the initial concentration of such point defects), as well as the cooling rate or residence time of the main body of the ingot over the range of temperatures bound by the “nucleation threshold” at the upper end and the “diffusivity threshold” (which impacts the size and density of such defects) at the lower end. Thus, control of the cooling rate or residence time enables the formation of agglomerated intrinsic point defects to be suppressed over much larger ranges of values for v/G
0
; that is, controlled cooling allows for a much larger “window” of acceptable v/G
0
values to be employed while still enabling the growth of substantially defect-free silicon.
As an example, one crystal puller used for controlling the cooling of monocrystalline ingots above the nucleation threshold of intrinsic point defects is disclosed in co-assigned U.S. patent application Ser. Nos. 09/344,003 and 09/338,826, which are incorporated in their entirety herein by reference. The crystal puller includes an electrical resistance heater mounted in the pull chamber of the crystal puller housing generally toward the bottom of the pull chamber of the housing. The electrical resistance heater has heating segments that may be constructed of equal length (e.g., a non-profiled heater) or of stepped, or staggered lengths (e.g., a profiled heater). As portions of the ingot grown in the puller are pulled upward into radial registration with the heater, heat is radiated by the heater to these portions of the ingot to reduce the cooling rate of the ingot.
Co-assigned U.S. patent application No. 09/661,745, the full disclosure of which is incorporated herein by reference, discloses a quenching process for growing a monocrystalline silicon ingot according to the Czochralski method in which the nucleation and/or growth of interstitial type defects is suppressed by controlling the cooling rate of the ingot through nucleation. More particularly, initial growth conditions are selected to provide an ingot containing silicon self-interstitials as the predominant intrinsic point defect from the center to the edge of the ingot, or a central core in which vacancies are the predominant intrinsic point defect surrounded by an axially symmetric region in which silicon self-interstitials are the predominant intrinsic point defect. As the ingot cools while being pulled upward within the crystal puller

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