USE OF CHLORINE TO FABRICATE TRENCH DIELECTRIC IN INTEGRATED CIRCUITS

Abstract

Chlorine is incorporated into pad oxide (110) formed on a silicon substrate (120) before the etch of substrate isolation trenches (134). The chlorine enhances the rounding of the top corners (140C) of the trenches when a silicon oxide liner (150.1) is thermally grown on the trench surfaces. A second silicon oxide liner (150.2) incorporating chlorine is deposited by CVD over the first liner (150.1), and then a third liner (150.3) is thermally grown. The chlorine concentration in the second liner (150.2) and the thickness of the three liners (150.1, 150.2, 150.3) are controlled to improve the corner rounding without consuming too much of the active areas (140).

Description

BACKGROUND OF THE INVENTION

The present invention relates to integrated circuits, and more particularly to dielectric formed in trenches in a silicon substrate. Some embodiments are suitable for substrate isolation for integrated circuits.

FIG. 1 illustrates an intermediate structure in a flash memory fabrication process using shallow trench isolation (STI). Silicon dioxide 110 (“pad oxide”) is thermally grown on silicon substrate 120. Silicon nitride 130 is formed on oxide 110 and patterned photolithographically to define substrate isolation trenches 134 to be formed between active areas 140. Oxide 110 and substrate 140 are etched through the openings in nitride 130 to form the isolation trenches. Silicon dioxide 150 is deposited to fill the trenches and cover the wafer. Oxide 150 is polished by chemical mechanical polishing (CMP) until the top surface of nitride 130 is exposed. A planar top wafer surface is provided.

Oxide 150 can be etched down (FIG. 2) to achieve a more planar topography in subsequent steps. Nitride 130 and pad oxide 110 are etched away (FIG. 3). Silicon dioxide 410 (“tunnel oxide”) is thermally grown to a desired thickness (e.g. 9 nm). A doped polysilicon layer 510 (FIG. 5) is deposited on oxide layers 410, 150 to provide the floating gates and is partially patterned. Dielectric 520 (e.g. a sandwich of silicon dioxide, silicon nitride, silicon dioxide, i.e. ONO) is formed over the structure. Doped polysilicon 530 is deposited on ONO 520. Layers 530, 520, 510 are patterned together to create wordlines form the layer 530 and to finish the patterning of the floating gate. Source/drain regions 610 (top view in FIG. 6) are formed on each side of each floating gate. The cross sectional plane of FIG. 5 is marked V-V in FIG. 6. Floating gates 510 are shown with crosses in FIG. 6. Additional layers (not shown) are deposited and patterned to provide conductive bitlines contacting some of the source/drain regions. See e.g. U.S. Pat. No. 6,265,292 issued Jul. 24, 2001 to Parat et al. and incorporated herein by reference.

The electric field is undesirably increased at sharp corners 140C (FIGS. 4, 5) during the circuit operation. In addition, the growth of tunnel oxide 410 (FIG. 4) is retarded at these corners, so oxide 410 is thinner at the corners than in the middle of the active areas. The oxide thinning further increases the electric field at the corners, creating overerase and/or other problems (depending on the memory operation). See U.S. patent application published as no. US 2004/0014269 on Jan. 22, 2004, incorporated herein by reference. It is desirable to round the trench corners 410C to provide a uniform thickness oxide 410 and reduce the electric field at the corners, as illustrated in FIG. 7 (showing the wafer with rounded corners 410C at the stage of FIG. 1) and in FIG. 8 (the wafer with rounded corners at the stage of FIG. 3).

To round the corners 140C, oxide 150 can be formed by first growing a thin silicon dioxide liner on the trench surface by thermal oxidation. The oxidation rounds the corners 140C. Then the rest of oxide 150 can be deposited (by a high density plasma process, i.e. HDP, or some other technique). The rounding should be controlled to minimize the active area consumption. If the corners are at the [111] crystallographic plane and the trench sidewalls are at [100], a chlorine source can be used in the liner formation to provide a desired rounding without an undue consumption of the active area. See PCT application published as WO 01/47010 on 28 Jun. 2001 and incorporated herein by reference.

Improved corner rounding techniques for flash memories and other integrated circuits are desirable.

SUMMARY

This section summarizes some features of the invention. Other features are described in the subsequent sections. The invention is defined by the appended claims which are incorporated into this section by reference.

Some embodiments of the present invention incorporate chlorine into pad oxide 110. It is well known that chlorine increases oxygen diffusion through silicon dioxide. Chlorine has also been used in silicon dioxide layers, at a concentration of at most three atomic percent, to immobilize metal atoms. In some embodiments of the present invention, chlorine incorporation into pad oxide 110 increases the oxygen diffusion through oxide 110 during the liner formation. This oxygen diffusion increases the oxidation rate at corners 410C relative to the trench sidewalls, to create a desired rounded corner profile.

In some embodiments, the chlorine concentration in oxide 110 is more than three atomic percent. An exemplary range is 5˜15 atomic percent. Greater concentrations can also be used. Some embodiments use 5˜10 atomic percent of chlorine.

In some embodiments, the oxidation time is shortened to prevent undue consumption of the active area. Hence the liner is very thin (3˜10 nm in some embodiments). After the liner formation, a second oxide liner is deposited by chemical vapor deposition (CVD), and then a third liner is thermally grown. The CVD liner protects the active areas from excessive oxidation when the third liner is being formed, but the corner rounding can be enhanced during the third liner fabrication. In some embodiments, chlorine is incorporated into the CVD liner to speed up oxidation of the trench sidewall and bottom surfaces during the third liner fabrication and provide a desired corner profile. The CVD liner can be used with or without chlorine incorporation into pad oxide 110.

The invention is not limited to the features and advantages described above. The invention includes non-memory integrated circuits. The corner rounding techniques can be used with trenches other than STI trenches, and the invention is not limited to substrate isolation. Other features are described below. The invention is defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-5 show cross sections of prior art integrated circuits in the process of fabrication.

FIG. 6 is a plan view of a prior art integrated circuit in the process of fabrication.

FIGS. 7, 8 show cross sections of prior art integrated circuits in the process of fabrication.

FIG. 9-14 show cross sections of integrated circuits in the process of fabrication according to some embodiments of the present invention.

DESCRIPTION OF SOME EMBODIMENTS

The embodiments described in this section illustrate but do not limit the invention. The invention is not limited to particular fabrication techniques or numerical values and ranges. The invention is defined by the appended claims.

FIG. 9 illustrates initial STI fabrication stages in one embodiment of the present invention. Silicon dioxide layer 110 comprising chlorine atoms is formed on silicon wafer 120. In some embodiments, the chlorine concentration is more than three atomic percent, or at least 5 atomic percent. A range from 5 to 15 atomic percent is believed to be suitable, and other concentrations are possible.

In one embodiment, oxide 110 is formed by thermal oxidation at 800˜1000° C. The oxygen flow is 10±5 l/m in (liters per minute). The chlorine is provided by hydrogen chloride (HCl) flown at 1 l/min. Other possible chlorine sources include the chlorine gas (Cl), TCA (trichloroethane, C₂H₃Cl₃), TCE (trichloroethylene, C₂HCl₂), dichloroethylene (C₂H₂Cl₂). Other chlorine sources, known or to be invented, may also be suitable.

Oxide 110 can also be formed by CVD. For example, dichlorosilane can be used: SiH₂Cl₂+2N₂O→SiO₂+2N₂+2HCl. Oxygen is also flown in the reaction chamber, reacting with the hydrogen chloride (HCl) to form chlorine. Chlorine is incorporated into the SiO₂ layer 110. Other fabrication techniques, known or to be invented, can also be used. An exemplary thickness of oxide 110 is 5˜15 nm, but this is not limiting.

Silicon nitride 130 is formed on oxide 110 by known techniques (e.g. CVD). An exemplary thickness of nitride 130 is 100˜200 nm, but this is not limiting.

A photoresist mask (not shown) is formed on nitride 130 and patterned to have openings over the positions of STI trenches 134. Nitride 130 is etched through the openings to form a hard mask for the STI trenches. Oxide 110 and silicon 120 are etched through the openings in nitride 130 to form the trenches between active areas 140. In some embodiments, the trench depth is 0.3 μm. In some embodiments, the trenches have sharp corners 140C, but this is not necessary. Smoother corners can be obtained by suitably controlling the etch process, as described for example in U.S. patent application publication US 2004/0014269 A1 published Jan. 22, 2004 and incorporated herein by reference. The etch can be anisotropic (e.g. RIE). The silicon substrate etch can be controlled to provide vertical or sloped sidewalls for the trenches. In some embodiments, the trench sidewalls are at 60˜9° to the horizontal. See the aforementioned U.S. publication 2004/0014269 A1 and U.S. Pat. No. 6,265,292.

The wafer is oxidized to form a silicon dioxide liner 150.1 (FIG. 10) on the trenches' silicon surfaces. In some embodiments, the liner is formed by dry oxidation at 850˜1050° C. Other oxidation processes can also be used. Oxygen diffuses through the exposed sidewalls of pad oxide 110 to round the top trench corners 140C. The bottom corners also become a little rounded.

In some embodiments, the oxidation is shortened to avoid undue consumption of active areas 140. This is particularly desirable if the active areas are narrow. In some embodiments, the active area width is 0.065˜0.18 μm, and smaller widths are also possible. The liner 150.1 is only 3˜10 nm thick in some embodiments.

In some embodiments, additional thermal oxide is desirable to provide better isolation and more rounding of the top and bottom corners of the trenches. Before growing the additional thermal oxide however, a silicon dioxide liner 150.2 (FIG. 11) is formed by CVD on the wafer to protect the active areas. In some embodiments, layer 150.2 incorporates chlorine for control of the top corner profile during the subsequent thermal oxidation. An exemplary thickness of layer 150.2 is 3˜20 nm, and the chlorine concentration is 10¹³˜10¹⁴ atoms/cm², or 1˜10 atomic percent. Other thickness and concentration parameters and also possible, and can be selected experimentally to obtain a desired profile for top corners 140C and a desired thickness uniformity for subsequently grown tunnel oxide 410 (FIG. 14).

The wafer is oxidized to grow the additional thermal silicon dioxide layer 150.3 (FIG. 12) on the trench surface. In some embodiments, a low oxidation rate is achieved by diluting oxygen in nitrogen or argon to a volume concentration of 10%. The oxidation temperature is 900˜1050° C. The thickness of layer 150.3 is 1˜5 nm. Other processes are also possible.

In each case, the appropriate values for the thickness of layers 110, 150.1, 150.2, 150.3 and the chlorine concentration in layers 110, 150.2 may depend on the trench and active area dimensions, the fabrication equipment, and desired circuit characteristics. In each case, the appropriate thicknesses and chlorine concentrations can be determined experimentally to obtain the desired corner rounding and thickness uniformity for oxide 410 (FIG. 14).

The remaining fabrication processes can be conventional. In one embodiment, silicon dioxide 150.4 (FIG. 13) is deposited by HDP to fill the trenches and cover the wafer. Oxide layers 150.4, 150.2 are polished by CMP to expose the nitride 130. Oxide 150.4, 150.2 is etched down (as in FIG. 2). Nitride 130 and pad oxide 110 are removed. Tunnel oxide 410 (FIG. 14) is thermally grown to a desired thickness. Layers 510, 520, 530 are deposited and patterned as described above, and the doping steps are performed, to form the structure of FIG. 6.

The invention is not limited to the embodiments described above. The invention is not limited to nonvolatile memories, MOS circuits, or substrate isolation. Other embodiments and variations are within the scope of the invention, as defined by the appended claims.

Claims

1. An integrated circuit comprising: a silicon substrate having a trench therein; a first dielectric in the trench, the first dielectric comprising: a first silicon oxide portion on a surface of the trench; and a second silicon oxide portion separated from the surface of the trench by the first silicon oxide portion, the second silicon oxide portion comprising chlorine at a higher concentration than the first silicon oxide portion.

2. The integrated circuit of claim 1 wherein the first silicon oxide portion does not comprise chlorine.

3. The integrated circuit of claim 1 wherein the chlorine concentration in the second silicon oxide portion is at least 1 atomic percent.

4. The integrated circuit of claim 3 wherein the chlorine concentration in the second silicon oxide portion is at most 10 atomic percent.

5. The integrated circuit of claim 1 wherein the chlorine concentration in the second silicon oxide portion is at least 1013 atoms/cm2.

6. The integrated circuit of claim 5 wherein the chlorine concentration in the second silicon oxide portion is at most 1014 atoms/cm2.

7. The integrated circuit of claim 1 wherein the first dielectric isolates adjacent active areas of the substrate from each other.

8. The integrated circuit of claim 1 wherein further comprising a transistor active area in the substrate adjacent to the trench.