US3653977A - Method of preventing ion channeling in crystalline materials - Google Patents

Abstract

The invention discloses a method of preventing ion channelling in crystalline materials by preirradiating the material with sufficiently energetic electrons, X-rays or gamma rays to produce a sufficient density of crystal imperfections known as point defects. These defects are readily annealed away at temperatures insufficient to diffuse dopant atoms or produce a chemical or electrical effect in the material.

Description

I United States Patent [15] 3,653,977 Gale 51 Apr. 4, 1972 [54] METHOD OF PREVENTING ION [56] References Cited CHANNELING IN CRYSTALLINE UNITED STATES PATENTS MATERIALS 2,911,533 11/1959 Damask 250/49.5 [72] Inventor: Alfred J. Gale, Lexington, Mass. 2,968,723 1/1961 Steigerwaid ..250/49.5 3,341,754 9/1967 Kellett et al. ..3l7/234 [73] Ass'gnee: a Phys'cs Corporation Buflmgton 3,383,567 5/1968 King et a] ..3 1 7/234 ass.

[ Filed: p 0 9 Primary EXamir er-Ralph NllSOll Assistant Examiner-A. L, Birch [21] Appl. No; 720,023 Attorney-Francis J. Thornton [57] ABSTRACT [52] US. Cl ..l48/1.5, 317/234, 250/495,

The invention discloses a method of preventing ion chan- 29/25.3, 252/623 51 I t Cl on nellmg in crystalline materials by preu'radlatmg the material with sufficiently energetic electrons, X-rays or gamma rays to [58] Flflld of Search ..250/49.5; 148/15; 317/234, produce a sufficient density of crystal imperfections known as point defects. These defects are readily annealed away at temperatures insufficient to diffuse dopant atoms or produce a chemical or electrical effect in the material.

7 Claims, 2 Drawing Figures I l l l 1 l0 5 ox+4 2 9 3O x I x+a Z 9 40 a E x+2 ion lmplunled 2 Gaussian Curve |n 8 Poini Defect 2 Environment 0 0X44 O l l X l l l 1 O 4 5 6 2 3 DEPTH MICRONS Patented April 4, 1972 CONCENTRATION IONS/CM 2 Channeling Toil Dle IO Theoretical Curve Major Crystallographic r ct n FIG. I

DEPTH -M|CRONS I I I I I KDXUfi FIG. 2 2E x04 2 m I Z x 9 so 1 IO o 40 a E [O L Ion Implanted z Gousslun Curve m 8 Point Defect 2 m Envuronment 8 l0 x l l l l I I0 0 2 3 5 6 DEPTH MICRONS INVENTOR ATTORNEY METHOD OF PREVENTING ION CHANNELING IN CRYSTALLINE MATERIALS DESCRIPTION OF THE DRAWINGS FIG. 1 shows, in solid line, a typical curve of implanted ion distribution in a crystal body and, in broken line, the ion distribution for amorphous material.

FIG. 2 shows in solid line the ion distribution in a crystalline body after it has been treated in accordance with the inventron.

DISCLOSURE With the advent of further advances in solid state physics and the need for more exotic, active, electronic devices workers in the art have turned to ion implantation as the mechanism for producing these devices.

Ion implantation is defined as a process in which a beam of energetic ions is directed against a body of material to selectively effect electrical and/or chemical changes in the body by causing the ions, of the beam, to pass into the body of treated material.

If the body of treated material is amorphous or non crystalline the highly energetic ions thus introduced come to rest in the target as a result of either electronic stopping or nuclear stopping or both. Electronic stopping is defined as inelastic collisions with the atomic electrons of the target material which absorbs the energy of the bombarding ions by exciting and ionizing the target atoms. Nuclear stopping is defined as elastic collisions between the bombarding ion and the screened nuclear field of the target atom which absorbs the ions energy by displacement of the target atom.

J. Lindhard and M. Scharff reported, in 1961, in the Physical Review 124, 128, a theory and provided criteria whereby the distribution of ions in amorphous material may be determined. For amorphous material this theoretical distribution is determined to be Gaussian as illustrated in FIG. 1 as curve with a mean range determined by the energy and a straggle (width at half maximum) determined primarily by the relative contributions of the electronic and nuclear stopping.

This theory of Lindhard et al. has been found to be appropriate to amorphous target material but failing when applied to crystalline material. This failure of the theory is especially noted when the beam of incident ions is directed along an open crystallographic direction of the crystal.

If a crystal is examined, it will be seen that there are certain directions in the crystal along which an undeflected ion can travel without encountering lattice atoms. These directions lie between planes of atoms and along tubes walled by atoms belonging to intersecting planes and are known as channels.

Theory first predicted that occasional large angle scattering would prevent any impinging ion from travelling far along such channels. However, experimental results show that once an ion is aligned with a channel, either by deliberate orientation or by scattering, the ion is focused in the channel by the nuclear charge of the lattice atoms and by the repulsive force of two electron clouds trying to occupy the same volume.

This focusing action permits the channelled ions to penetrate significantly deeper into the target body before they are finally stopped by electronic stopping. However, because the electronic stopping power in the channel is approximately three times smaller than that predicted by Lindhard a deeply penetrating component or tail appears on the ion distribution curve. This tail is shown by curve in FIG. 1.

Such channelling is undesirable since in order to get quantitatively reproducible results, it is necessary to obtain consistency in the initial material and to align the crystallographic directions of the targets to within a few tenths of a degree of the desired orientation each time. Such identity in the initial materials of the bombarded samples is as a practical matter impossible to obtain. Moreover there is experimental evidence that shows surface films and the effect of other implanted ions can also add to the irreproducibility of the distribution of channelled ions even if perfect alignment is obtained in each case.

Because of these drawbacks it is therefore desirable to eliminate as much as possible the probability of any ions being introduced along such channels.

The primary purpose of this invention therefore is to pro vide a method which will prevent such channelling and provides perfectly symmetrical ion distribution even in crystalline materials.

Broadly this purpose is accomplished by irradiating the crystalline target body with ions, electrons, X-rays or gamma rays which will produce in the body a sufficient density of point defects that will prevent or seriously inhibit channelling during subsequent ion implantations.

Although ions will create such point defects they are not preferred because they can, in semiconductor devices, produce unwanted chemical or electrical effects or both. X- rays or gamma rays are also not preferred in that they are highly penetrating and wasteful if they are of an energy sufficient to produce a significant number of Compton electrons in excess of the energy required to produce a lattice displacement in the material being treated and hence a point defect.

The energy required for any material may be determined by the following equation:

E, is the binding energy of an atom in the crystal lattice E is the minimum electron energy (threshold) required to create a point defect m is the mass of an atom of the crystal m is the rest mass of an electron v is the velocity of an electron of energy E,

c is the velocity of light.

When working with crystalline silicon this equation shows that electron beams exceeding KeV energy should be used to produce the density of point defects required to prevent channelling during a subsequent ion implantation.

Implantation of such moderately energetic electrons can be easily and readily achieved through the use of an electron accelerator apparatus sold by High Voltage Engineering Corporation.

The invention is practised by subjecting the crystalline body to be subsequently implanted with ions to a beam of such highly energetic electrons. The specified value of electrons used is of course dependent on the device material and thickness and for typical semiconductor devices, whose thickness is approximately 10 mils, 1 Mev electrons would be used. Thinner devices would use the lower energy electrons and thicker devices would require higher energies.

These electrons transfer their energy during their passage through the crystalline body to the lattice atoms of the body causing a slight displacement of the atom from its lattice site to an interstitial position to result in a lattice vacancy and an interstitial atom separated by distance dependent on the magnitude of the original energy transfer between the electron and the struck atom. Such disruption of the lattice causes at least one point defect that acts to block the crystal channel and prevent, by electronic stopping, the channelling or passage of subsequently implanted ions. There is however a high improbability that any one electron, introduced into the body, will create more than one point defect. Thus the number of created defects is substantially directly proportional to the number of electrons introduced into the body being treated.

Little is known about the initial behavior of the created interstitial atoms but it is known that the created vacancies wander through the crystal and tend to form quasi stable complexes with impurities in the crystal rather than recombining with the interstitial atoms. These created complexes also act as point defects. In any event these channel blocking interstitial atoms or point defects cause the previously perfect crystal to appear to the subsequently implanted ions is found to follow the symmetrical theoretically predicted Gaussian curve 30 of FIG. 2, and the normally expected tail 40 is not existent.

These artificially created point defects however are not permanent and can be annealed out. Fortunately they can be annealed out at temperatures much lower than those used in diffusion processing. The vacancies will themselves anneal out at temperatures as low as 190 C. while the more dominant point defects require higher temperatures. For example in N- type silicon, the vacancy-oxygen and vacancy-phosphorous complexes have been observed to anneal out below 400 C. (the temperature above which affects the lifetime of the silicon material). In general, however, substantially all damage is effectively removed at temperatures of about 300 C.

For Indium Antimonide crystals all the damage created by 1.0 Mev electron bombardment is completely annealed out at approximately 25 C. while for Gallium Arsenide temperatures of less than 600 C. are required.

Thus for the subsequent ion implantation it is necessary that the crystal body be held and maintained at a temperature below that at which annealing occurs. In many instances it may be desirable that the body be held at the temperature of liquid nitrogen (-1 90 C.).

In any event experimental results indicates for all semiconductor crystalline materials that such artificially created point defects anneal out of the crystal at or below the temperature that may be required to activate implanted substitutional ions in the material by causing them to become substitutional dopants. However when interstitially active ions are implanted in crystaline bodies, the annealing temperature need only be high enough to remove the point defects.

Thus such electron bombardment leaves no significant lasting effects in the crystal while permitting the significant advantage of preventing channelling to be obtained.

Having now described the invention it is desired that it be limited only by the following claims.

What is claimed is:

1. A method of treating a crystalline body to modify the physical characteristics of the body consisting of maintaining said body at a selected temperature, irradiating the body with an ionizing radiation whose energy is sufficient to create point defects in said body, bombarding said irradiated body with ions chosen to produce the selected effects in said body and heating said bombarded and irradiated body to remove said point defects.

2. The method of claim 1 wherein said ionizing radiation comprises electrons.

3. The method of claim 1 wherein said temperature is below that temperature at which said defects anneal out of said body.

4. The method of claim 3 wherein said ions are substitutionally active.

5. The method of claim 3 wherein said ions are interstially active.

6. The method of claim 1 wherein said energy is determined E is the binding energy of an atom in the crystal lattice E is the minimum electron energy (threshold) required to semiconductor.

lOI030 0434

Claims (6)

1. 2. The method of claim 1 wherein said ionizing radiation comprises electrons.
2. 3. The method of claim 1 wherein said temperature is below that temperature at which said defects anneal out of said body.
3. 4. The method of claim 3 wherein said ions are substitutionally active.
4. 5. The method of claim 3 wherein said ions are interstially active.
5. 6. The method of claim 1 wherein said energy is determined by the equation
6. 7. The method of claim 1 wherein said crystalline body is a semiconductor.

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