Method for Fabricating MNOS Structures Utilizing Hydrogen Ion Implantation

The invention relates generally to methods of improving the performance of MNOS structures and more particularly pertains to a method for reducing the density of electronic trapping states and fixed insulator charge in an MNOS structure with a thin oxide layer.

There is a growing class of applications for solidstate electronic devices, such as Charge-Coupled-Devices (CCD) and Field-Effect Transistors (FET), that require operability at cryogenic temperatures and tolerance to ionizing radiation. Such applications include the use of CCDs in conjunction with large area infrared photodetector arrays for imaging and signal processing. Another application is the use of solid state electronic devices in outer space.

In order to achieve radiation hardening and operability at cryogenic temperatues an MNOS (metal-silicon nitride-silicon dioxide-semiconductor) dual dielectric insulator structure is often used in place of pure silicon dioxide as the gate insulator in electronic MOS (metal-oxide-semiconductor) devices such as FETs and CCDs (the "gate" insulator refers to the insulator in the active regions of the devices such as the FET gate).

A nitride/oxide insulator structure utilized as an active gate insulator in an MOS device is described in the paper by M. C. Peckerar et al. entitled "Hydrogen Annealed Nitride/Oxide Dielectric Structures for Radiation Hardness", IEEE Transactions on Nuclear Science, Vol. NS-27, No. 6, Dec. 1980. In that article superior radiation tolerance of nitride compared with oxide is discussed along with the need for a thin oxide layer between the nitride and silicon regions to prevent memory effects. The authors found that high temperature post-dielectric processing degraded the performance of the gate due to the introduction of electron trapping states and fixed insulator charge in the oxide layer. However, they discovered that a high temperature hydrogen anneal reduced the interface density states to a value of about 1.times.10.sup.10 states/cm.sup.2 eV. This high temperature hydrogen anneal includes the steps of exposing the surface of the MNOS structure to hydrogen gas (H.sub.2) and heating the structure to a predetermined temperature.

It is well established that the diffusion rate of hydrogen is substantially higher in silicon dioxide compared to silicon nitride at typical annealing temperatures. Therefore, in MNOS structures, it has been shown that the hydrogen enters the oxide layer through vent holes in the nitride layer during the high temperature anneal. These vent holes, or other openings, must be formed in the nitride layer prior to the high temperature hydrogen anneal. The hydrogen then diffuses laterally into the gate region, and chemically reacts at the oxide-silicon interface. Thus, a very high annealing temperature (typically 650.degree.-1000.degree. C.) is required to obtain lateral diffusion of hydrogen through the oxide layer in MNOS structures when the layer is very thin (.ltoreq.100 .ANG.) as is required for radiation-tolerant devices.

This high annealing temperature, required by existing techniques, has several disadvantages. First, the heating of the structure increases the fixed insulator charge thereby degrading device performance. Second, the aluminum gate contact must be deposited after the hydrogen anneal since the aluminum can only tolerate temperatures up to 500.degree. C. However, aluminum deposition increases surface charge density. Finally, the density of electronic states has not been reduced below the level originally present before high temperature post dielectric processing.

Accordingly, it is an object of the invention to decrease the temperature, required by current techniques, of the hydrogen anneal.

It is a further object of the invention to remove the requirement to open vent holes in the nitride layer as is required by conventional high-temperature hydrogen annealing.

It is still a further object of the invention to decrease the density of electronic states and surface charge below a level achievable by current techniques.

It is still a further object of the invention to permit the formation of the aluminum interconnect and/or gate metallization prior to the hydrogen anneal.

The above and other objects are achieved in the present invention which comprises a novel method for reducing the density of electronic states and the fixed insulator charge in the thin oxide layer of an MNOS structure of the type having a gate and field and gate regions. The method includes the steps of implanting hydrogen ions into the oxide layer of the MNOS structure such that the gate blocks the hydrogen implant from the gate region of the oxide layer, and then annealing to cause the hydrogen ions to diffuse laterally into the gate region.

The method of the invention reduces the required temperature of the hydrogen anneal to below 500.degree. C. and provides for a lower density of electronic trapping states and fixed insulator charge than previously achieved in thin oxide layers of MNOS structures.

Briefly, the method of the invention utilizes ion implantation to position hydrogen ions in the thin oxide layer of an MNOS structure without requiring vent holes. A subsequent annealing step causes the hydrogen to diffuse laterally into the gate region of the oxide layer and react chemically at the Si--SiO.sub.2 interface to reduce the interface state density and density of fixed oxide charge. The annealing temperature required is lower than in existing methods most likely because ion implantation produces a high density of chemically reactive H.sup.+ in the oxide layer, whereas less reactive H.sub.2 is the dominant species in the conventional hydrogen gas anneal.

FIG. 1 shows an MNOS structure for processing by the method of the present invention.

FIG. 2 depicts a cross-sectional view of an MNOS structure processed by the present method.

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1 thereof, an MNOS structure for processing by the method of the present invention is depicted. The process steps for making the structure of FIG. 1 are conventional. A silicon substrate 10, of either the P or N type, has a silicon dioxide (oxide) layer 12 formed thereon in a known manner by thermal oxidation. The oxide layer 12 is a thin layer about 90 Angstroms thick. Next a silicon nitride (nitride) layer 14 is deposited on the oxide layer in a known manner by chemical vapor deposition. A gate electrode 16 of polysilicon and a gate contact 18 are subsequently deposited by known techniques. Note that the section of the oxide layer 12 disposed below the gate electrode 16 is designated the gate region 20 while the section of the oxide layer 12 not disposed below the gate electrode 16 is designated the field region 22.

The structure depicted in FIG. 1 is exposed to a beam of hydrogen ions 30 so that the ions are implanted in the structure. The energy and dose of the beam is selected, utilizing techniques well known in the art, so that the peak of the implanted hydrogen profile is in the thin oxide layer 12 with a fluence in the range of 10.sup.15 -10.sup.16 H.sup.+ /cm.sup.2.

The gate electrode 16 should be thick enough to block the hydrogen implant from the gate region 20 of the thin oxide layer 12 thereby precluding the effects of implantation damage in the active region of the device. Thus, hydrogen ions are implanted solely in the field region of the oxide layer 12, whereas no hydrogen ions are implanted in the gate region 20.

Subsequent to this ion implantation step the MNOS structure is annealed at approximately 400.degree. for 1-2 hours. This annealing step causes the ions implanted in the field region 22 to diffuse laterally into the gate region 20 and to chemically react at the substrate 10-oxide layer 12 interface. This chemical reaction reduces the density of interface electronic trapping states.

The efficiency of the lateral diffusion of hydrogen into the gate region 20 during the anneal is increased by the nitride layer 14 disposed over the oxide layer 12. This increase of efficiency is due to the low diffusion rate of hydrogen in the nitride layer 14 which prevents the hydrogen from diffusing vertically out of the oxide layer 12 during the anneal. However, small openings, necessary for device fabrication or other purposes, may be formed in the nitride layer 14 without seriously affecting the lateral diffusion of the hydrogen into the gate region 20 of the oxide layer 12.

The MNOS structure processed by the present method has been shown experimentally to have very low electronic trapping state densities. Ultra-low densities as low as 7.times.10.sup.7 states/cm.sup.2 -eV at midgap have been measured. This density is at least an order of mangitude lower then the lowest values currently achievable in either MNOS or MOS structures by any existing annealing method.

The net fixed insulator charge is also reduced for MNOS structures processed by the current method. Total fixed charge of less than .+-.5.times.10.sup.10 e/cm.sup.2 has been measured.

Additionally, since the annealing temperature is below 500.degree. C., the aluminum electrodes may be deposited prior to the annealing step. This is an advantage due to the fact that the deposition of the aluminum electrodes after the anneal increases fixed insulator charge density.

FIG. 2 depicts a cross sectional view of an MNOS structure of a CCD processed by the present method. The characteristics and methods of fabrication of the CCD are reported in the article by N. S. Saks entitled "Fabrication of Surface-Channel Charge-Coupled Device with Ultralow Density of Interface States", Appl. Phys. Lett. 41(8), Oct. 15, 1982.

Referring now to FIG. 2, an oxide layer 12, 90 Angstroms thick in the gate region 20 and 1000 Angstroms thick in the field region 22 is formed on the surface of a P-type silicon substrate 10. A nitride layer 14, 750 Angstroms thick is then formed on the oxide layer 12. The polysilicon gate 16 is 5,000 Angstroms thick.

The CCD is then implanted with 5.times.10.sup.15 H.sup.+ ions/cm.sup.2 at 20 keV (equivalent to 1.5.times.10.sup.16 H.sup.+ ions/cm.sup.2 at 6.7 keV) to reduce the interface state density. At this energy, the range of the implanted hydrogen is not long enough to penetrate the 5,000 Angstrom polysilicon gate layer 16, which prevents direct implant damage from occurring in the sensitive MNOS gate insulator. The implant energy is chosen such that most of the hydrogen will penetrate the silicon nitride layer 14 in the field insulator region not covered by the gate 16 and stop in the 1000 Angstrom field oxide layer 12. During the 400.degree. C. anneal following implantation, the implanted hydrogen diffuses laterally into the thin gate oxide layer 12 where it reduces the interface state density. Following implantation and anneal the interface state density measured by an ac conductance technique on on-chip test capacitors is in the range 1-3.times.10.sup.8 /cm.sup.2 -eV.

It will be understood that various changes in the details, materials, steps and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.