Method for improved semiconductor device reliability

[0002] (1) Field of the Invention

[0003] This invention relates to the field of semiconductor electronic devices and more specifically to semiconductor integrated circuit devices in which an improved method for depositing protective layers of silicon oxynitride is used to provide increased manufacturability by improving yield and reliability for such devices.

[0004] (2) Description of the Prior Art

[0005] The manufacture of semiconductor integrated circuit devices is achieved by the use of a variety of thin films of materials which are deposited and shaped to perform each of the specialized functions required for the effective performance of such devices. A semiconductor substrate is used for the fabrication of electrical elements and components which are then interconnected electrically to form the desired circuit and function. Due to the need for constantly increased performance, the dimensions of the components and their separation have become smaller to minimize the separation and hence the time delay for propagation of electrical signals in the circuit. This has resulted not only in smaller device dimensions but also in reduced dimensions of all other aspects of device fabrication. Thus, the thicknesses of the thin films of conductive materials and the insulating and protective layers employed in the devices have decreased in proportion. The decreasing thicknesses of interconnection patterns of aluminum, as an example of a widely used conductive material, results in an increased susceptibility to corrosion by such substances as moisture, which can lead to an increase in electrical resistance or even open circuits. Likewise, the decreased thicknesses of protective layers of material deposited over components and elements such as aluminum may give rise to a diminished ability to afford a protective barrier against moisture.

[0006] Agents such as moisture or certain gaseous ambients can also give rise to changes in the surface electrical properties of the semiconductor substrate and its various elements. In addition to permitting greater degrees of permeation by chemical species, the reduced thickness of insulating and passivating films and layers also leads to a greater degree of transmission of electrically charged species such as energetic or “hot” electrons from one device region to another where such charge may be trapped, interfering with the electrical operation of the device. To minimize this phenomenon, thin insulating layers with superior dielectric properties are increasingly required.

[0007] For the reasons mentioned, thin films of barrier materials have been devised to act as blocking or hindering agents to deleterious species when deposited over the various elements of semiconductor integrated circuit devices. Originally, layers of silicon nitride, Si₃N₄, were found to be superior to silicon oxide for reducing the transmission of deleterious species such as sodium and potassium ions, and the higher value of the relative dielectric constant was desirable for electrical purposes. More recently, an even more effective material for this purpose is the compound formed among the elements silicon, nitrogen, and oxygen which is generically referred to as silicon oxynitride and is written as SiO_xN_ywhere x and y refer to the variable amounts of oxygen and nitrogen respectively depending on the details of the method of deposition. This type of material in the form of a thin film can function to minimize the sensitivity of the semiconductor surface electrical potential to various ambients during the processes of device fabrication and later in operation. It also serves as a barrier to diffusion of moisture and such unwanted ionic species as sodium and potassium ions. As a very thin film, it is more effective in minimizing the passage and trapping of hot electrons.

[0008] Silicon oxynitride layers have been deposited from various combinations of gases such as silane SiH₄, silane derivatives Rsi, or organosiloxanes ROSi to supply the silicon atoms and oxygen- and nitrogen-containing gases to supply these atomic species in the desired proportions in layers deposited by the process of chemical vapor deposition. A particularly useful gas is nitrous oxide N₂O which contains both. Other possibilities are combinations of gases such as ammonia NH₃, oxygen O₂, etc. The use of an electrical plasma sustained in the reacting gases to enhance the deposition rate or improve the properties of the deposited film or both has been discussed in the literature. For example, the use of the combination of tetraethoxysilane (TEOS)+N₂+NH₃plasma-enhanced CVD processes to produce films of silicon oxynitride is discussed by Harada in U.S. Pat. No. 5,362,686. The use of nitrous oxide is not discussed and the method of plasma generation and sustenance is not described in any detail. Other descriptions of the use of silicon oxynitride deposited layers for various purposes in electrical device fabrication by (U.S. Pat. No. 4,381,595) and (U.S. Pat. No. 5,071,790) mention electrical interconnections on a semiconductor substrate, but neither the use of nitrous oxide nor the details of the plasma process are discussed.

[0009] It is a principal object of the invention to describe a method for producing a barrier layer of silicon oxynitride deposited on a semiconductor integrated circuit device to act protectively with respect to penetration by moisture or other deleterious species to protect the semiconductor surface and the interconnections of the device.

[0010] It is another object of the invention to describe a process for producing optimized properties of silicon oxynitride layers by plasma-enhanced chemical vapor deposition from nitrous oxide and a silicon-containing gas.

[0011] These objects are achieved by a process of plasma-enhanced chemical vapor deposition of silicon oxynitride from a gas mixture of nitrous oxide and a silicon-containing gas employing a dual-power source of plasma generation and sustenance to produce optimum properties of the deposited layer, for passivation of the semiconductor surface, minimization of trapping energetic electrons, and protection of the elements of the device from moisture and other potentially deleterious ambients.

[0012] FIG. 1 is a schematic cross-sectional diagram of a portion of a semiconductor integrated circuit device of the prior art.

[0013] FIG. 2 is a schematic cross-sectional diagram of a portion of a semiconductor device of the current invention.

[0014] FIG. 3 is a graph of the properties of silicon oxynitride layers deposited by variations of the process of the current invention.

[0015] FIG. 4 is a graph of the improved properties of silicon oxynitride layers deposited in accordance with the current invention.

[0016] Referring now more particularly to FIG. 1, there is shown a schematic cross-section of a portion of a semiconductor integrated circuit device fabricated according to the prior art. There is shown a semiconductor substrate 10 containing electrically active elements coated with an insulating oxide layer 12. Interconnecting the semiconductor circuit elements is a layer of conductor material 14 formed into an appropriate pattern. A barrier or “liner” layer 16 is deposited over the previous layers, and a final layer of chemically vapor-deposited silicon oxide 18 is placed over the barrier layer. One function of the barrier layer is to protect the underlying device structures during the deposition of the final oxide layer, which is typically done by a CVD process employing ozone O₃and tetraethoxysilane TEOS.

[0017] Referring now more particularly to FIG. 2, there is shown a schematic cross-section of a portion of a semiconductor integrated circuit device of the current invention. A semiconductor substrate 20 containing circuit elements is coated with a silicon oxide layer 22 and an interconnection pattern 24. Over these is deposited a barrier or “liner” layer of silicon oxynitride 26 deposited from a reactive gas mixture consisting of nitrous oxide N₂O and a silane derivative RSi by a plasma-assisted CVD process using a dual power supply to energize and sustain the plasma. Finally, the device structure is covered with a silicon oxide layer 28 deposited by a CVD process from a gas mixture of O₃and TEOS.

[0018] The use of a dual power supply to energize and sustain the plasma during the deposition of silicon oxynitride is beneficial in that the plasma is driven by radio frequency power at a frequency of 13.56 mHz while a separate low-frequency bias power supply at between 300 and 380 kHz permits the bombardment of the depositing film by ions from the plasma, resulting in improved film properties. The use of N₂O and silane derivatives (see line 30 in FIG. 3) as reactive gases is an improvement over the use of TEOS and O₂(see line 32) as is shown by FIG. 3. The higher value for the refractive indices of deposited films from N₂O is a measure of improved electrical properties since refractive index and dielectric constant are directly proportional.

[0019] The flow rate of N₂O during CVD of the barrier layer should be kept in a ratio of 20 or more times that of the silane derivative employed to obtain reproducible and desirable film properties. This is shown in FIG. 4, in which the amount of residual carbon found in the deposited oxynitride film is given for various combinations of N₂O and silane derivatives at various flow rate ratios. It is seen that ratios of 20 or more, and a carbon concentration of less than about 1% on the interconnection pattern and the elements and the devices, are preferred, and that derivatives such as tetramethylsilane (TMS) 34 and tetraethylsilane (TES) 36 are preferred to oxysilane derivatives such as TEOS 38. The process parameters for optimum properties of the silicon oxynitride layer are given in Table I:

[0020] While the invention has been particularly shown and described with reference to the preferred embodiments thereof it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.