Method of making an integrated circuit structure having gate electrode and underlying oxide

Abstract: An improved electrically erasable programmable read only memory (EEPROM) integrated circuit structure and method for its fabrication is disclosed, having an enhanced interface between the floating gate electrode (30) and the underlying tunnel oxide (14). The structure is capable of a relatively higher floating gate breakdown voltage and is less subject to charge migration from the floating gate and resultant charge trapping in the tunnel oxide. The improvements comprise forming the floating gate electrode from amorphous silicon and doping the silicon floating gate by implantation with a dopant, such as arsenic or phosphorus, under conditions wherein the doping agent will not easily migrate into the underlying tunnel oxide layer.

Description: This invention relates to an electrically erasable programmable read only memory (EEPROM) device. More particularly, this invention relates to an improved EEPROM device having higher breakdown voltage and lower defect density, including reduced charge trapping in oxide layers and a method of making same. In the construction of some types of EEPROMs of the MOS type, for example, using Fowler-Nordheim tunneling, an additional gate, called a floating gate, is placed between the control gate and the substrate. This floating gate is used to store a charge which changes the threshold voltage of the device. This charge must be erased or removed for the transistor to become operable under the normal bias of the control gate. The two gates are separated by a layer of oxide called an interlevel oxide. Another layer of oxide, known as a tunnel oxide, is placed between the floating gate and the underlying substrate. This tunnel oxide layer is usually less than 10⁻⁸m (100 Angstroms) in thickness. While MOS devices initially were constructed using, respectively, metal, oxide, and silicon layers, the metal layer has been conventionally replaced with polycrystalline silicon (polysilicon) which is doped to enhance the conductivity of the polysilicon material. For most MOS devices, wherein the usual gate oxide layer is about 2.5-3x10⁻⁸m (250-300 Angstroms) in thickness, the use of polysilicon for the gate electrode is very satisfactory. However, when the oxide layer is very thin, as in the above described tunnel oxide layer beneath the floating gate of an EEPROM device, the uneven interface formed between the polysilicon and the thin tunnel oxide can result in unacceptably low breakdown voltages in such devices. This is may be caused by virtue of the crystalline nature of the polysilicon. This problem is best shown in the prior art illustrations of Figures 1A and 1B. In Figure 1A, a silicon substrate 10, having a smooth surface, has formed thereon a thin layer of oxide 14. A layer of polysilicon 18 is deposited over oxide layer 14. As shown in exaggerated form in Figure 1A, the lower surface 17 of polysilicon layer 18 abutting oxide layer 14 is not smooth. While the upper surface 19 of polysilicon layer 18 could be polished smooth, basically little can be done about the uneven undersurface 17. When the structure is exposed to subsequent processing steps, including heating of the structure, oxide growth on polysilicon surface 17 as well as diffusion at the polysilicon/oxide interface can result in the structure shown in Figure 1B wherein the uneven surface 17 of polysilicon layer 18 has been replaced by the uneven interface 16 between oxide layer 14 and polysilicon layer 18. The result is an uneven oxide thickness between silicon substrate 10 and polysilicon 18 which can allow intense electrical fields to exist in the oxide at the thinnest points and eventually cause unacceptable current leakage or breakdown across the tunnel oxide of such a device. The use of polysilicon as the electrode requires the addition of a dopant to the polysilicon to achieve the desired conductivity. Conventionally, phosphorus oxychloride (POC1₃) is used to dope the polysilicon to provide a N+ type layer. However, the doping material sometimes migrates from the polysilicon into the underlying tunnel oxide which can affect oxide integrity. This causes charge trapping which will interfere with the erasable feature of the EEPROM device, and also affects the breakdown voltage of the structure. This charge migration may be due to the high concentration of the phosphorus oxychloride dopant as well as the inability to accurately control the concentration of the dopant. It would therefore be desirable to provide an integrated circuit structure of the EEPROM type having a floating gate which will have a smoother interface between the tunnel oxide and the floating gate and which will have a lower migration of dopant into the adjoining tunnel oxide layer to reduce the occurrence of charge trapping and a method of fabricating such a structure. Reference may be made to EP-A-53013, GB-A-2130009, and EP-A-160965. With regard to EP-A-160965, there is disclosed therein a floating gate MOS device having a phosphorous implanted floating gate of silicon. EP-A-53013 describes a non-volatile memory device having an undoped amorphous silicon layer formed over a gate oxide layer. GB-A-2130009 describes the formation of a gate electrode using amorphous silicon rather than polysilicon. The present invention provides a method of constructing a MOS-type integrated circuit EEPROM comprising, forming an amorphous silicon material layer over a gate oxide layer, thereby forming a floating gate electrode, characterised by; doping said amorphous silicon material of said floating gate electrode by implanting arsenic therein. In this way migration of dopant into the underlying gate oxide is inhibited. These and other objects of the invention will be apparent from the following description and accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1A and 1B are fragmentary cross-sections of prior art structures. Figure 2 is a fragmentary cross-sectional view of the structure of the invention. Figure 3 is a flow sheet illustrating the process of the invention. Figure 4 is a graph illustrating average breakdown voltages across the tunnel oxide for EEPROM devices constructed with a polysilicon floating gate doped with a phosphorus oxychloride dopant. Figure 5 is a graph illustrating average breakdown voltages across the tunnel oxide for EEPROM devices constructed with a polysilicon floating gate doped by arsenic implantation. Figure 6 is a graph illustrating average breakdown voltages across the tunnel oxide for EEPROM devices constructed with an amorphous silicon floating gate doped by arsenic implantation. Figure 7 is a graph illustrating average breakdown voltages across the interlevel oxide for EEPROM devices constructed with a polysilicon floating gate doped with a phosphorus oxychloride dopant. Figure 8 is a graph illustrating average breakdown voltages across the interlevel oxide for EEPROM devices constructed with a polysilicon floating gate doped by arsenic implantation. Figure 9 is a graph illustrating average breakdown voltages across the interlevel oxide for EEPROM devices constructed with an amorphous silicon floating gate doped by arsenic implantation. Figure 10 is a graph plotting the percentage of failures of prior art devices and devices constructed in accordance with the invention when cycled two thousand times. DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the invention, as illustrated in Figures 2 and 3, floating gate 30 is constructed using amorphous silicon, to thereby provide a smooth interface between the amorphous silicon floating gate and the underlying tunnel oxide and increase the breakdown voltage of the tunnel oxide layer. The term "breakdown voltage", as used herein in relation to a voltage applied across the tunnel oxide layer of an EEPROM device, is intended to define the voltage at which the current through the tunnel oxide layer exceeds 1 microampere. While the applicants are not constrained by one particular theory, it would appear that the non-crystalline nature of amorphous silicon and the absence of grain boundaries between adjoining crystals (as occurs in a polycrystalline structure) permits the formation of a surface which, without polishing or using any other planarizing techniques, provides a much smoother interface between the floating gate and the underlying oxide than the polycrystalline silicon material conventionally used. Formation of intense electrical fields at regions in the tunnel oxide where the oxide would (but for this invention) have been thinner due to irregularities in the tunnel oxide/floating gate interface are therefore avoided resulting in a higher breakdown voltage across the tunnel oxide. In a preferred embodiment, the amorphous silicon is formed on the oxide surface 14 by depositing silicon at a temperature below the temperature at which polycrystalline silicon forms, i.e., below about 600°C. Preferably, the silicon should be deposited at a temperature of about 580°C or less, e.g., a range of about 550 to 580°C, to insure that amorphous silicon, not polysilicon, is formed. The thickness of the amorphous silicon layer formed for a floating gate electrode for an EEPROM device should be from about 0.20 to 0.35 micrometer (2000 to 3500 Angstroms), preferably about 0.25 micrometer (2500 Angstroms). As shown in Figure 2, subsequent to the formation of amorphous silicon floating gate layer 30, an interlevel oxide layer 40 is formed over layer 30 followed by control gate layer 50, which may comprise a metal layer. Formation of floating gate layer 30 from amorphous silicon then provides an additional benefit in the formation of a smooth interface between floating gate layer 30 and interlevel oxide layer 40. To further enhance the performance of the device, particularly with respect to charge trapping which may prevent erasure, amorphous silicon layer 30 is doped by ion implantation using an arsenic dopant rather than a phosphorus dopant, as will be discussed below. Subsequent heating of the amorphous layer to redistribute the dopants more uniformly or any other heating of the structure during subsequent fabrication steps, will not result in migration of the dopant into the underlying oxide layer 14 or, for that matter, into interlevel oxide layer 40, because of the implantation conditions. According to the invention, migration of the doping agent is further inhibited by implantation with arsenic ions, presumably due to the size of the arsenic ion compared to the phosphorus ion. The arsenic implantation of amorphous silicon layer 30 is carried out using a concentration of from about 1.75 x 10¹⁵ to about 2.5 x 10¹⁶ atoms per square centimeter. If a phosphorus dopant is implanted, the dosage should be from about 5 x 10¹⁵ to about 2 x 10¹⁶ atoms per square centimeter, preferably about 1 x 10¹⁶ atoms per square centimeter. The implantation voltage will depend upon the thickness of the amorphous silicon layer. Typically, for a 0.25 micrometer thick amorphous silicon floating gate, the implantation voltage should be about 50 KEV for arsenic or 30 KeV for phosphorus. After implantation, the implanted silicon is heated to a temperature of from about 950° to about 1100°C, typically, about 1000°C for about 45 minutes to 2 hours, typically about 1 hour, with the longer time period used for the lower temperature to redistribute the dopant more uniformly as well as to alleviate any damage done during the implantation step. The exact time and temperature used will also be dependent upon the level of dopant used. To further illustrate the invention, a number of EEPROM devices were constructed, respectively, using a polysilicon floating gate conventionally doped with POCl₃ at a concentration of about 1 x 10¹⁸ atoms per square centimeter (representing the prior art), a polysilicon floating gate doped with arsenic, implanted at 50 KeV at a concentration of from 1.75 x 10¹⁵ to 2.5 x 10¹⁶ atoms per square centimeter, and an amorphous silicon floating gate similarly doped with arsenic. The devices were tested to determine the average breakdown voltage of the three classes of devices. Breakdown voltage was determined by measuring the current flowing across the tunnel oxide between the silicon substrate and the floating gate. The voltage at which a current of 1 microamp was measured was defined as the breakdown voltage of the oxide layer. The results are shown in the graphs of Figures 4 to 6 which each plot breakdown voltages against the number of devices reaching the 1 microamp current level at that particular voltage. The Sigma or standard deviation shown in the graphs indicate that the results are consistent among the devices tested. Figure 4 shows the results for the prior art structures constructed using a polysilicon floating gate doped with a conventional phosphorus oxychloride dopant. Figure 5 shows that some improvement in average breakdown voltage occurs when the polysilicon is doped by implantation with arsenic in accordance with the invention. Figure 6 shows even further improvement in the average breakdown voltage when the amorphous silicon floating gate of the invention is used instead of the prior art polysilicon material in the floating gate. Another group of devices, constructed in the same respective manner, were tested to determine the effect of the improvements of the invention on breakdown voltage between the floating gate and the control gate through the interlevel oxide therebetween. Figures 7 through 9 illustrate the average breakdown voltage plotted against the number of devices exhibiting 1 microamp of current flow across the oxide layer at the particular voltage level. Figure 7 shows the results for devices representing the prior art construction of a polysilicon floating gate conventionally doped with a phosphorus oxychloride dopant as described above with respect to Figure 4, while Figure 8 depicts the test results for devices constructed with polysilicon floating gates doped by implantation with arsenic as previously described. Figure 9 shows the test results for devices constructed with amorphous silicon floating gates doped by implantation with arsenic. The results indicate that the devices, constructed in accordance with the invention, maintain similar or better quality in the breakdown voltage levels across the interlevel oxide layer between the floating gate and the control gate compared to the test results shown in Figure 7 for the prior art constructions. Figure 10 illustrates further testing of the devices just described by cycling the devices through on and off states two thousand times. Percentages of failures are shown for each of the three types of devices previously described with respect to Figures 4-6. It will be noted that for each of the three runs, the percentage of failures for the devices constructed in accordance with the invention is lower than the prior art devices. Thus, we provide an improved integrated circuit device having improved charge trapping characteristics and wherein the average breakdown voltage across the gate oxide, particularly the tunnel oxide layer of an EEPROM device, is increased by constructing the conducting layer above the gate oxide using amorphous silicon to provide a more planarized interface with adjoining oxide layers. The silicon gate electrode is doped by implantation with arsenic instead of conventional chemical doping with phosphorus oxychloride to further increase the average breakdown voltage and to inhibit charge migration into the adjoining oxide layers which, in the case of an EEPROM device, can cause problems in the form of charge trapping in the tunnel oxide layer.

Claims: A method of constructing a MOS-type integrated circuit EEPROM comprising: forming an amorphous silicon material layer over a gate oxide layer (14), thereby forming a floating gate electrode (30); characterised by; doping said amorphous silicon material of said floating gate electrode by implanting arsenic therein. The method of claim 1 characterised in that said step of forming an amorphous silicon layer over a gate oxide layer further comprises depositing a layer of silicon on said gate oxide layer at a temperature below the temperature at which polycrystalline silicon forms. The method of claim 2 characterised in that said step of forming said amorphous silicon layer over said gate oxide layer comprises depositing said silicon over said gate oxide layer at a temperature of less than 600°C. The method of claims 3 characterised in that said step of forming said amorphous silicon layer over said gate oxide layer comprises depositing said silicon over said gate oxide layer at a temperature of from 550 to 580°C. The method of any of claims 1 to 4 characterised in that said step of forming said amorphous silicon layer over said gate oxide layer comprises depositing said layer of amorphous silicon over said gate oxide layer to a thickness of from 0.20 to 0.35 micrometer. The method of claim 5 characterised in that said step of forming said amorphous silicon layer over said gate oxide layer comprises depositing said layer of amorphous silicon over said gate oxide layer to a thickness of 0.25 micrometer. The method of any of claims 1 to 6 characterised in that said step of doping said amorphous silicon material by implanting arsenic therein further comprises implanting arsenic at a concentration of from 1.75 x 10¹⁵ to 2.5 x 10¹⁶ atoms per square centimetre at an energy level of about 50 KeV. The method of any of claims 1 to 7 characterised by annealing the implanted amorphous silicon layer at a temperature of from 950°C to 1100°C for from 45 minutes to 2 hours.

Inventors: COX WILLIAM P, LIANG MONG-SONG, COX, WILLIAM P., LIANG, MONG-SONG
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1986
Main classification: H01L21
IPC codes: H01L29, H01L21