The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalties thereon.

BACKGROUND OF THE INVENTION

This invention relates in general to a method of treating silicon and in particular, to such a method that results in the fabrication of an MNOS field effect transistor with improved memory retention characteristics.

Heretofore, in the manufacture of the MNOS transistor device structure, the critical SiO₂ layer has been formed by employing vapor or liquid chemical techniques. The SiO₂ layer is only about 25 to 50 angstroms in thickness but its composition is significant in determining transistor turn-on threshold voltage and Si₃ N₄ /SiO₂ retention characteristics. With the vapor and liquid techniques, it has been difficult to precisely control the oxidation reaction in which SiO₂ is produced in the surface of the silicon wafer. This is because one cannot precisely control the quantity of oxygen atoms that are available for oxidation of the silicon. In addition, in the liquid and vapor chemical techniques, it is difficult to control the uniformity of the oxidation reaction across the silicon wafer and also to control the amount of oxygen atoms introduced into the silicon from one oxidation run to another.

Moreover, in the liquid and vapor chemical techniques, foreign atoms are introduced into the silicon during the oxidation. These foreign atoms or contaminants remain in the silicon oxide and contribute to the degradation of subsequent device structures.

SUMMARY OF THE INVENTION

The general object of this invention is to provide a method of forming a tunneling insulator interface layer of SiO₂ in the Si₃ N₄ /Si interface region of a silicon semiconductor substrate in which a layer of Si₃ N₄ has been deposited on the top surface of the substrate. A further object of the invention is to provide such a method that offers extreme precision in the control of the oxide profile shape. A still further object of the invention is to provide such a method in which the resulting structure has the form Si₃ N₄ /SiO₂ /Si and in which the SiO₂ layer is free from foreign atoms. A particular object of the invention is to provide such a metod in which the resulting Si₃ N₄ /SiO₂ /Si structure can be made into an MNOS field effect transistor device.

It has now been found that the foregoing objects can be attained by using an ion implanation technique for the formation of the SiO₂ tunneling insulator interface layer. More particularly, according to the invention, a suitable semiconductor substrate such as p-type silicon is first placed in a reaction chamber and the system either evacuated or purged with an inert gas. The top surface of the semiconductor substrate is then etched with HCl gas to remove oxide impurities and contaminants. A layer of Si₃ N₄ of about 500 angstroms in thickness is then deposited on the impurity free top surface of the silicon. A mask provided with windows representing device structures is then formed over the silicon nitride layer. The mask may be a metallic material or an organic material as for example, photoresist. Oxygen is then implanted through the window or unmasked portion of the silicon nitride layer into the silicon nitride/silicon interface region at room temperature for about 30 minutes and at an accelerating potential of about 30 to 60 kv. After removing the mask, the oxygen implanted silicon substrate is annealed in a flowing atmosphere of nitrogen at about 1000° C. for about 30 minutes. The final structure has the form Si₃ N₄ /SiO₂ /Si and can be made into a field effect transistor device by conventional diffusion, ion implant, and metallization processes.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A wafer of p-type silicon of <100> orientation is placed in a RF induction heated reaction chamber and the system then purged with helium to remove oxygen and water vapor. The top surface of the wafer is then etched with HCl gas at an elevated temperature of about 1000° C. to remove oxide impurities and other contaminants from the silicon surface. An amorphous layer of silicon nitride of about 500 angstroms in thickness is then deposited on the impurity free top surface of the silicon. A photoresist mask provided with windows representing device structures is then formed over the silicon nitride layer. The mask is about one micron thick. Oxygen is then implanted through the window portion of the silicon nitride layer into the silicon nitride/silicon interface region. The implanatation is carried out at room temperature for about 30 minutes at an accelerating potention of 30 to 60 kv. This results in a concentration of 2 to 5 (10¹⁶) oxygen atoms per square centimeter at the silicon nitride/silicon interface. A layer of silicon dioxide of about 20 to 50 angstroms is formed. The photoresist mask is then removed from the silicon nitride layer with vapor plasma. The treated silicon substrate is then annealed in a flowing atmosphere of nitrogen at about 1000° C. for about 30 minutes. The resulting structure has the form Si₃ N₄ /SiO₂ /Si. This structure is then made into a field effect transistor device by conventional diffusion, ion implant, and metallization processes.

In the method, the particular silicon substrate chosen and its orientation are not critical. That is, the silicon substrate might be p-type or n-type and its orientation might be <100> , < 111>, etc.

The silicon nitride is deposited on the impurity free top surface of the silicon substrate as an amorphous layer of about 300 to 700 angstroms in thickness in about 3 to 4 minutes. The silicon nitride can be conveniently obtained by the reaction of silane with ammonia at a temperature of about 750° C.

Examples of specific masking materials that can be used in the invention include aluminum, platinum or titanium.

Ion implantation can be conveniently carried out using a 300kv ion implanter. However, the particular ion implanter used is not critical. What is critical however is that during the implanatation, the surface accumulation of oxygen atoms or dose required to form SiO₂ at the Si₃ N₄ /Si interface region be on the order of 2 to 5 (10¹⁶) oxygen atoms/cm².

The method of the invention results in a tunneling insulator interface layer of silicon dioxide that is more uniform in thickness and reproducible from one oxidation run to another. The silicon dioxide layer is also free of contaminants. These characteristics of the silicon dioxide layer enable the resulting silicon nitride/silicon dioxide/silicon structure to be made into devices having improved memory retention and endurance.

We wish it to be understood that we do not desire to be limited to the exact details shown and described, for obvious modifications will occur to a person skilled in the art.