Semiconductor device manufacturing: process – Coating of substrate containing semiconductor region or of... – By reaction with substrate
Reexamination Certificate
2000-05-16
2001-08-07
Le, Vu A. (Department: 2824)
Semiconductor device manufacturing: process
Coating of substrate containing semiconductor region or of...
By reaction with substrate
C438S778000, C438S773000, C438S774000, C438S588000, C438S591000
Reexamination Certificate
active
06271152
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods for fabricating semiconductor integrated circuits, and in particular, to methods for isolating devices in semiconductor integrated circuits.
BACKGROUND OF THE INVENTION
Individual devices are conventionally isolated on a semiconductor substrate using a technique termed LOCal Oxidation of Silicon (LOCOS). LOCOS comprises forming a thin pad oxide layer on an underlying substrate. A blanket silicon nitride layer is then formed on the thin pad oxide layer. Silicon nitride is very slow to oxidize. Thus, if a sufficiently thick layer of silicon nitride is used as an oxidation mask, the underlying substrate is protected from oxidation. The thin pad oxide layer serves to prevent introduction of crystallographic dislocations in the underlying substrate, caused by lattice strain between silicon and silicon nitride. Active regions are then defined in the blanket silicon nitride layer by masking and etching to remove the silicon nitride in field regions. Active regions remain covered by silicon nitride, preventing subsequent oxidation of the underlying substrate. The field regions are then oxidized using an oxidizing ambient to form field oxide. Subsequent to LOCOS isolation, devices are formed in the active regions.
Conventionally, oxidation in a LOCOS process comprises utilizing an oxidizing ambient at atmospheric pressure. However, field oxide formed using atmospheric LOCOS techniques is plagued by lateral encroachment of oxide under the silicon nitride mask layer, forming a “bird's beak.” Lateral encroachment is undesirable because it severely decreases semiconductor chip density. In order to fulfill consumer desires for faster microprocessors and other semiconductor chips, chip density must be maximized. Thus, the “bird's beak” problem prevents semiconductor devices from achieving their optimal electrical performance.
Many ways of decreasing such encroachment have been studied. One particularly effective method of alleviating the “bird's beak” phenomenon is by oxidizing the underlying substrate using pressures higher than conventionally-used atmospheric pressure. Oxide formed using high pressures is also less prone to crystal defects throughout the oxide layer. Crystal defects are undesirable because they allow diffusion through the oxide layer, potentially resulting in leakage currents that are detrimental to optimal device performance. For example, refresh times for dynamic random access memory arrays are optimized by decreasing leakage current from memory cells. Optimized refresh times are critical for faster electrical performance and lower power consumption. Power consumption is minimized because the memory cell array does not need to be refreshed as often when the leakage current is minimized.
While oxide formed using high pressure is relatively free of crystal defects, the oxide thinning effect prevents the effective use of oxide formed using high pressure, particularly in narrow pitch structures. The oxide thinning effect results in increased leakage current when thinning is severe. For illustration, given 0.6 &mgr;m pitch field oxide regions produced by a 0.25 &mgr;m process, the thickness of the field oxide is approximately 2,500 angstroms in the center of the field oxide regions, but only approximately 1,750 angstroms at the edges of the field oxide regions. This 70% thickness undesirably increases the leakage current through such field oxide regions. Narrow pitch structures are common in semiconductor integrated circuits, particularly memory and logic circuitry. Thus, oxide formed using high pressure has not been effectively used in such applications.
Another problem experienced with the use of high pressures for forming field oxide is that surface contamination often causes “hot spots” in the field oxide layer at the interface with the substrate. Contaminants, such as sodium, potassium, and fluorine are often undesirably introduced during the semiconductor fabrication process, particularly from deposition chambers and fueeaces. When such contaminants are present on a substrate during high pressure oxidation, they act as nucleation sites. Excess oxide growth occurs at such nucleation sites. High pressure oxidation significantly magnifies this problem. Excess growth has been known to cause field oxide regions on a wafer that are approximately 30 to 40% thicker than the rest of the field oxide. On large wafers, such excessive growth regions have been reported covering areas as large as a dime, and even a quarter. This is extremely undesirable because it causes problems with back end of line (BEOL) technology, particularly presenting step coverage problems for interconnect and metal line fabrication, severely decreasing yields, which increases fabrication cost.
There is a need for a method for forming field oxide having a low crystal defect density. There is a further need for a method for forming field oxide that is not adversely affected by the oxide thinning effect. A method to alleviate the “hot spot” effect seen when oxidizing substrates at high pressures is needed in order to take advantage of the advantageous effects of high pressure oxidation. As device density is becoming increasingly important and cost pressures increase, it is critical to provide isolation techniques having minimal leakage current and providing high yield.
SUMMARY OF THE INVENTION
The present invention teaches a method for forming field oxide using high pressure. As in a conventional LOCal Oxidation of Silicon (LOCOS) process, a substrate is blanketed with a blanket silicon nitride layer, supported by a pad oxide layer. Active regions are then defined in the substrate underlying the silicon nitride layer using conventional mask and etching techniques. Oxidation of field regions between the active regions is accomplished in a two-step process. A first oxide layer is formed in the field region. Then, a second oxide layer is formed, joining with the first oxide layer and consuming further silicon from the substrate. The second oxide layer is formed at a pressure of at least approxinmaely 5 atmospheres.
In one embodiment, the first oxide layer is formed at atmospheric pressure using a conventional oxidation technique, such as rapid thermal oxidation (RTO), wet oxidation, or dry oxidation. Temperatures of approximately 600 to 1,100 degrees Celsius are used for the oxidation step. The first oxide layer is formed to a thickness of approximately 1000 angstroms or less, and typically greater than 200 angstroms. The actual thickness will depend on the ratio of thickness to other layers that is desired.
In another embodiment, the first oxide layer is formed at or near atmospheric pressure, approximately 1 to 5 atmospheres. Wet or dry oxidation is used for the oxidizing ambient. Temperatures of approximately 600 to 1,100 degrees Celsius are used for the oxidation step. The first oxide layer is formed to a thickness of approximately 1000 angstroms or less, and typically greater than 200 angstroms. The pressure is then ramped upwards to form the second oxide layer.
Field oxide formed using the method of the invention has the best characteristics of both atmospheric pressure oxide and one-step high pressure oxide. The resulting field oxide has superior electrical characteristics to those of one-step high pressure oxidation. Since there is no thinning, improved device and substrate threshold voltage characteristics are obtained. The number of crystal defects in the total thickness of the field oxide is also minimized. Therefore, devices, such as memory cells, can be formed between field oxide produced using the method of the invention having a lower leakage current.
Structurally, resulting field oxide has similar characteristics to those of atmospheric pressure oxide. By forming the first oxide layer prior to the high pressure oxide, the “hot spot” effect is minimized, improving the uniformity of the resulting field oxide layer. The oxide thinning effect in such field oxide is also minimized to approximately 90% thickness, as compared
Chapek David L.
Thakur Randhir P. S.
Le Vu A.
Micro)n Technology, Inc.
Owens Beth E.
Schwegman Lundberg Woessner & Kluth P.A.
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