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Isolation Scheme for MOS Technology: MINI-ROX

IP.com Disclosure Number: IPCOM000060901D
Original Publication Date: 1986-May-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 3 page(s) / 60K

Publishing Venue

IBM

Related People

Lu, NC: AUTHOR

Abstract

Semi-ROX (recessed isolation) isolation techniques have been used for many years to isolate adjacent transistors in MOS technology. The typical process is briefly illustrated in Figs. 1-3. After forming a composite layer of oxide and nitride, windows are opened by a lithographic technique and boron ions are implanted through the window into the substrate (an NMOS process is used here as an example, and the same concept can be applied for PMOS transistors) (Fig. 1). Then the thick thermal oxide is locally grown in the window region to isolate the adjacent devices (called local oxide), but no thick oxide is grown over the nitride layer. Assuming the local oxide has thickness T, "birds' beaks" of approximately the same magnitude occur on both sides of the oxide under the nitride layers.

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Isolation Scheme for MOS Technology: MINI-ROX

Semi-ROX (recessed isolation) isolation techniques have been used for many years to isolate adjacent transistors in MOS technology. The typical process is briefly illustrated in Figs. 1-3. After forming a composite layer of oxide and nitride, windows are opened by a lithographic technique and boron ions are implanted through the window into the substrate (an NMOS process is used here as an example, and the same concept can be applied for PMOS transistors) (Fig.
1). Then the thick thermal oxide is locally grown in the window region to isolate the adjacent devices (called local oxide), but no thick oxide is grown over the nitride layer. Assuming the local oxide has thickness T, "birds' beaks" of approximately the same magnitude occur on both sides of the oxide under the nitride layers. About 40% of the local oxide thickness is below the substrate surface since silicon is consumed during the oxidation (Fig. 2). After the removal of the nitride layers and several subsequent processing steps used for defining gate electrodes, phosphorus or arsenic ions are implanted into the substrate to form the MOS source and drain regions out of the local oxide regions (Fig. 3). This conventional ROX isolation has several drawbacks, while the MOS device dimensions are scaled down to the sub-micron regions. One drawback is that a long and high temperature oxidation cycle is required to grow the thick local oxide. Therefore, the implanted dopants for field-isolation are driven deeply. The low surface concentration gives a low threshold voltage for the field-isolation device, thus degrading the isolation efficiency. Besides, the existing dopant profiles before the field-oxide growth can be disturbed by the long temperature cycle. Another drawback is that the occurrence of the bird's beak increases the isolation dimension after processing to g+2T if the thick oxide opening width is g which can be the minimal feature size. Since the oxide in the tapered region of the bird's beak is thin, the source/ drain implants usually penetrate through the bird's-beak edge such that the source/drain N+ regions significantly overlap the field-isolation P+ region. This gives a large perimeter component of source/drain junction capacitance and lower breakdown voltage. Also, more oxidation- induced defects occur in the bird's-beak regions, which cause large leakage current. This article describes a new isolation scheme called MINI-ROX, which can solve these drawbacks. The process is described in the following steps with reference to Figs. 4-6. (Step 1) Similar local oxidation using a composite layer of oxide and nitride is adopted; however, the local oxide is thermally grown to a thickness T1 (Fig. 4) which is much thinner than the final isolation oxide thickness T. Since the source/drain junction depth is becoming very shallow in the sub-micron technology, both the oxide isolation and the field-implant profile beneath the si...