US 3917491 A
A method for fabricating radiation resistant MOS devices in which the metal sodium is removed after the oxide is grown by utilizing negative ion implantation. Low energy negative ions penetrate the SiO2 oxide layer depositing their negative charge on the surface of the layer and transferring enough energy to the oxide itself to release the bound sodium in the form of positive ions. The sodium ions drift to the oxide surface due the negative charges thereon. A thin uppermost portion of the SiO2 oxide layer contains both the implanted negative ion charges and the sodium ions. This uppermost layer is etched away using hydrofluoric acid. Removal of the sodium metal from the oxide layer significantly reduces the sensitivity of the MOS device to radiation.
Description (OCR text may contain errors)
United States Patent 1 1 Oswald, Jr. et al.
[ NOV. 4, 1975 METHODS FOR FABRICATING RESISTANT MOS DEVICES Primary ExaminerG. Ozaki Attorney, Agent, or Firm-Nathan Edelberg; Robert P.
 Inventors: Robert B. Oswald, Jr., Bethesda;
Anthony J. Baba, Gaithersburg, Glbson Saul Elbaum b th f Md. O O  ABSTRACT  Asslgnee: The Umted States of Amenca as A method for fabricating radiation resistant MOS derepresented the Secretary of the vices in which the metal sodium is removed after the Army Washington oxide is grown by utilizing negative ion implantation.  Filed: Jan. 8, 1974 Low energy negative ions penetrate the SiO oxide layer depositing their negative charge on the surface  Appl 431822 of the layer and transferring enough energy to the oxide itself to release the bound sodium in the form of 52 US. Cl. 148/15; 148/187; 357/91; positive km The sodium ions dn'ft to the Oxide 427/93 face due the negative charges thereon. A thin upper-  Int. C1. H0lL.7/54 most Portion of the 2 Oxide layer contains both the 58 Field of Search 148/15, 187; 357/91 implanted negative ion charges and the Sodium ions- This uppermost layer is etched away using hydroflu-  Refer Cit d oric acid. Removal of the sodium metal from the oxide UNITED STATES PATENTS layer significantly reduces the sensitivity of the MOS device to radiation. 3,507,709 4/1970 Bower 148/l.5 3,573,454 4/1971 Andersen et al 148/l.5 x 4 Claims, 2 Drawing Figures mg zgbisme i ON QSM T\-\\CKNESS A q ETCHMG 4 SRO).
LAYER US. Patent Nov. 4, 1975 somuM \ON Accumuumou ETG-NG Si 01 LAYER \ON BOMBNZDMENT METHODS FOR FABRICATING RESISTANT MOS DEVICES RIGHTS OF THE GOVERNMENT The invention specified herein may be manufactured, used, or licensed by or for the United States Government for governmental purposes without the payment to the inventor of any royalty thereon.
BACKGROUND OF THE INVENTION The present invention relates to radiation hardening of metal-oxide semiconductor devices, and more particularly, to a method whereby ion bombardment is used to remove substances from a layer of a semiconductor device by ionic migration.
The adverse affects of high energy radiation upon electronic components, particularly of the solid state or semiconductor type, are well known. Gamma-rays, for example, will break sodium-oxygen bonds in oxide layers, producing positive sodium ions. These ions will drift under the influence of the internal electrical fields present during the operation of MOS devices, and will cause the threshold voltage of the devices to shift, rendering them inoperable.
It is accordingly an object of the present invention to provide an improved method for fabricating radiation resistant MOS devices.
A further object of the invention is to provide a simple method of removing sodium from the oxide layer of an MOS device utilizing ion implantation and hydrofluoric acid etching.
SUMMARY OF THE INVENTION The invention is a simple method for removing sodium from the SiO layer once the layer is grown. The thickness of the layer is first increased by approximately 1,000 Angstroms. The layer is then bombarded with negative ions of either oxygen, argon, or helium. The energies of these ions are controlled such that they come to rest in the oxide layer and transfer enough energy to the oxide to release the bound sodium as positive ions of sodium. These sodium ions migrate to the surface of the oxide layer to combine with the large negative charge deposited there by the negative ions. The uppermost layer of the oxide containing the migrated sodium is removed by a buffered hydrofluoric acid etchant.
DESCRIPTION OF THE DRAWINGS These and other objects of this invention will become apparent from the following description and from the accompanying drawings, in which:
FIG. 1 is a combined isometric and cross sectional view illustrating a typical MOS device with an increased oxide layer grown thereon.
FIG. 2 is a flow chart illustrating the essential steps of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention is embodied in a metal-oxidesemiconductor circuit structure having a silicon dioxide (SiO layer depleted of a substantial portion of the sodium atoms normally contained therein.
Referring now to FIG. 1, a semiconductor and oxide structure of an MOS device 30 has an increased oxide layer 20 undergoing ionic bombardment. The semiconductor substrate 10 comprises an n-type material of monocrystalline silicon. Other semiconductor materials such as GaAs, GaP, et cetera, may be employed. In accordance with the use of an n-type semiconductor substrate 10 regions 22 are doped in accordance to methods commonly known in the industry to produce p-regions. This, of course, is done after our method is used to remove the sodium from the oxide layer. After depleting the oxide layer 20 of the sodium, windows are etched into the oxide layer 20 for diffusion of the re gions 22. The final step in the processing of the the MOS device is deposition of the metal source, drain, and gate. 1
In our method, once the semiconductor substrate is prepared, the oxide layer 20 comprising SiO is permitted to be increased in the growing process by approximately 1,000 Angstroms. After growth of the additional 1,000 Angstroms, the SiO layer is bombarded with negative ions of either oxygen, argon, or helium from an ion implantation machine. The energy of the bombarding ions is maintained ata low level in order to prevent the penetration into the oxide layer beyond the 1,000 uppermost Angstroms. As these implanted ions come to rest in the oxide layer their negative charge is deposited on the surface of the oxide and the energy transfer from the bombardment by the ion causes bound sodium in the oxide layer to be released in the form of positive sodium ions. These sodium ions then drift to the oxide-surface due to the large negative charge concentrated there by migration of the implanted ion charge after implantation. After bombardment, the shallow 1,000 Angstrom upper layer of the oxide contains both the implanted atoms and the sodium atoms. These are removed with a standard buff ered hydrofluoric acid etch solution. Hence, a significant portion of the sodium within the oxide film is reduced. Note: The etching rate of the solution is controlled by means commonly known in the art such that only the uppermost additional 1,000 Angstroms of the oxide layer are removed. The structure is then subjected to impurity diffusion in the conventional manner.
This method is further explained as illustrated in the following example for the conditions specified and outlined.
EXAMPLE Utilizing a MOS transistor, at the point of the deposition of the SiO oxide layer the oxide layer thickness is increased by 1,000 Angstroms. Initially in the fabrication of a transistor the oxide thickness is 2,000 Angstroms. For the purpose of radiation hardening, the oxide thickness is increased from 2,000 Angstroms to 3,000 Angstroms by increasing growth time. Once the 3,000 Angstrom thickness is acquired, the active side of the substratum of the transistor is subjected to negative ion beam bombardment with the oxygen ion, 0', produced by a Cameca model IMS 300 ion microanalyzer. It is found that by maintaining the oxygen ion energy at 10 Kev, these ions penetrate only a few hundred Angstroms into the oxide. To remove substantially all the sodium ions from the oxide layer an ion bombardment density of 10 ions per square centimeter is sufficient. After bombardment, the transistors are immersed in a dilute solution of acid (HF) to remove the top 1,000 Angstroms of oxide. The dilute solution of hydrofluoric acid consists of a standard 48 percent solution diluted with high purity water. This dilute solu- 3 tion of hydrofluoric acid reduces the etch rate and provides better control of said etch rate. This insures that the final desired thickness of the oxide layer is accurately obtained.
Referring to FIG. 2, the essential steps of the invention comprise: Step 1 increasing the silicon dioxide layer thickness by approximately 1,000 Angstroms; step 2 bombarding the oxide layer with ions of either oxygen, argon, or helium, of such an energy that these ions only penetrate a few hundred Angstroms into the uppermost portion of the oxide; step 3 allowing the sodium ions to accumulate on the surface of the oxide layer; and, step 4 etching away the uppermost 1,000 Angstroms of the oxide layer.
From the above description and drawings it will be apparent that various modifications in the method described in detail may be made within the scope of this invention. Therefore, the invention is not intended to be limited to the specific method described except as may be required by the following claims.
What is claimed is:
l. A method for enabling a metal-oxide semiconductor device to withstand the effects of a high radiation environment comprising the steps of:
a. increasing the thickness of the oxide layer;
b. directing a beam of negative ions into the oxide layer;
c. implanting said beam of negative ions only in the uppermost 1,000 Angstroms of said oxide layer and then d. allowing the sodium atoms in the oxide layer to accumulate on the surface of said layer as sodium atoms and ions; and
e. removing said sodium atoms and ions.
2. The invention according to claim 1 wherein said sodium atoms and ions are removed by etching away the uppermost portion of said oxide layer.
3. The invention according to claim 2 wherein said etching is performed by a diluted buffered hydrofluoric acid solution.
4. The invention according to claim 1 wherein said oxide layer is increased by at least 1,000 additional Angstroms.