SELECTIVELY DOPING ISOLATION TRENCHES UTILIZED IN CMOS DEVICES

Abstract: To provide more effective dielectric-filled isolation trenches in CMOS integrated circuit devices, one surface portion of the trenches is masked (42) and a dopant is introduced into the unmasked surface portion to enhance or increase the doping level thereof. The mask is then removed and a dopant of the opposite type conductivity is introduced into all the trench surfaces. The doping levels are selected so that the resulting doping concentrations of the trench surface portions are enhanced in comparison with the original doping levels thereof to prevent conductivity type inversion of the surface portions during device operation.

Description: SELECTIVELY DOPING ISOLATION TRENCHES UTILIZED IN CMOS DEVICES Background of the Invention This invention relates to integrated-circui t devices made in com plernentary-metal-ox ide-se iconductor (CMOS) form and , more particularly , to a method for fabricating isolation trenches in such devices . It is known to utilize dielectric-filled trenches in a CMOS device to achieve isolation between adjacent n-channel and p-channel transistor regions. In that way, latchup-free operation of a CMOS device can be ensured. In a CMOS device utilizing trenches for isolation, inversion layers may form along the sidewalls of the trenches. Such layers can deleteriously affect device performance by causing transistors adjacent to the trenches to exhibit undesirably high leakage to the device substrate. In some cases, the sidewall inversion layers can actually lead to device failure by causing source/drain regions along the sidewalls to be shorted together. Selectively doping the sidewalls has the potential to eliminate or substantially reduce inversion effects in a trench-isolated CMOS device. But such an approach, which requires that each sidewall of a trench be doped independently, is made exceedingly difficult by the extremely narrow and deep structure of typical trenches in a very-large-scale-integrated (VLSI) CMOS device. Available lithographic techniques, for example, are not feasible for selectively masking the sidewalls of such trenches to establish a basis for selective doping. Such selective doping is accomplished by this invention. Summary of the Invention In accordance with this invention, n- and p- type regions of trenches are independently doped by nonlithographically forming a dopant mask only on the P-type regions of the trench. Preferrably, the mask on the p-type regions of the trench is formed by selective anodization. In one embodiment, the p-type regions are etched and rendered porous in a wet anodization step. The porous silicon is then oxidized to form a mask layer made of silicon dioxide. In another embodiment, the p-type regions of each trench are selectively converted to silicon dioxide in a one-step dry anodization procedure carried out in a plas a. After forming a mask on the p-type regions of the trenches by either of the techniques outlined above, an n-type dopant is introduced into the n-type trench regions. Subsequently, the mask is removed and a p-type dopant is introduced into all the trench regions. In the n-type trench regions, this p-type dopant partially compensates the priorly introduced n-type dopant. But the net n-type dopant concentration thereby established in the n-type regions of the trenches and the p-type dopant concentration established in the p-type regions are at the desired levels to avoid inversion effects. Brief Description of the Drawing FIG. 1 is a cross-sectional depiction of a portion of a standard CMOS device that includes dielectric-filled isolation trenches; FIG. 2 shows a portion of an unfilled trench in a partially fabricated CMOS device that is to be processed in accordance with the principles of the present invention; FIG. 3 shows the FIG. 2 device immersed in a wet bath during selective anodization; and FIGS. 4 through 6 depict the device at subsequent stages of the herein-described fabrication sequence. Detailed Description FIG. 1 shows a portion of a conventional CMOS device. The device includes a p region 10 having a p- type epitaxial layer 12 formed thereon. By way of example, the thickness t ^ of the layer 12 is approximately 5.0 micrometers (μm) . In a manner well known in the art, n-type tubs are formed in the layer 12. One such tub 14 is shown in FIG. 1. Source and drain regions of typical p-channel MOS transistors formed in the tubs 14 are represented in FIG. 1 by p regions 16 and 18. MOS transistors of the n-channel type are also formed in the device structure of FIG. 1. These transistors are defined in portions of the p-type layer 12 that have not been converted to n-type tubs or, alternatively, can be formed within p-tubs (not shown) of a conventional twin-tub CMOS device. In any case, source and drain regions of typical such n-channel transistors are represented in FIG. 1 by n + regions 20 and 22. Trenches are utilized in the FIG. 1 CMOS device to achieve electrical isolation between adjacent n-channel and p-channel transistor regions. (In that way, as previously discussed, latchup-free operation of the device can be ensured.) One such trench 24 that completely surrounds n-type tub 14 is depicted in FIG. 1. The dimensions a_, b and £ of the indicated trench are, for example, approximately 1.25, 5.5 and 0.25 jam, respectively. After being formed in conventional ways known in the art, the trench 24 shown in FIG. 1 is filled with dielectric material. Illustratively, this involves initially growing a 1000-Angstrom-unit (8)-thick layer 26 of silicon dioxide on the walls and bottom of the trench 24. The trench 24 is then filled and overstuffed with a relatively thick layer of undoped polycrystalline silicon that covers the entire top surface of the depicted device structure. Subsequently, after a standard planarization step, the poly layer 28 filling the trench 24 appears as shown in FIG. 1. In practice, the n-type tub 14 of the CMOS device represented in FIG. 1 is typically connected to a positive voltage such as +5 volts and the p region 10 (and thus the p-type layer 12) is connected to a point of reference potential such as ground. A potential difference thus appears across , each dielectric-filled trench of the device. As a result, it is possible for the doped region on one side of a trench to act as a gate with respect to the doped region on the other side thereof. In turn, this can cause an inversion layer to form along the sidewalls of each trench. Additionally, induced charge in the dielectric material in the trenches can itself cause inversion effects along the - sidewalls or enhance the priorly described inversion effects. The inversion layers thereby established along the trench sidewalls of the aforedescribed CMOS device are depicted in FIG. 1. An n-type inversion layer, designated by minus signs, is formed along the sidewall of the trench 24 within the p-type layer 12. Similarly, a p-type inversion layer, designated by plus signs, is formed along the sidewall of the trench 24 within the n-type layer 14. The p-type inversion layer represented in FIG. 1 serves as a conductive bridge between the source/drain regions 16 and 18 and the portion of the p-type layer 12 that directly underlies the n-type tub 14. As a result, relatively high leakage currents can thereby flow from the source/drain regions 16 and 18 to the p region 10. In turn, this deleteriously affects the operating characteristics of the constituent p-channel transistors included in the depicted device. Further, the n-type inversion layer represented in FIG. 1 serves in effect to increase the junction area between the n source/drain regions 20 and 22 and the p-type region 12. In turn, this can lead to relatively high and deleterious leakage currents from the regions 20 and 22 to the p + region 10. In accordance with this invention, the dopant density or concentration adjacent the sidewalls of isolation trenches included in CMOS devices is selectively increased. In particular, the n-type dopant density of the portions of the n-type tub 14 adjacent to the trenches 24 is increased to a value that considerably exceeds the orignally established density in the tub 14. Similarly, the p-type dopant density of the portions of the p-type layer 12 adjacent to the trenches 24 is increased to a value that exceeds the originally established density in the layer 12. In both cases, the resulting dopant density along the trench sidewalls is thereby established sufficiently high that, during normal operation of the device, no inversion layer of the respective opposite type will form along the sidewalls. Consequently, the af oredescribed deleterious effects stemming from inversion are avoided. FIG. 2 shows a portion of the FIG. 1 device at an earlier point in the fabrication sequence therefor. The FIG. 2 device structure includes a protective mask on the entire top surface thereof. By way of example, the mask comprises a 350-&-thick layer 30 of silicon dioxide and a 2000-8-thick layer 32 of silicon nitride. Thus, the only portions of the structure that are not protected by the noted two-layer mask are the sidewalls and the bottom of the trench 24. The sidewalls and bottom of the trench 24 shown in FIG. 2 are next selectively masked. Preferrably, this is done in a two-step procedure that includes an initial wet anodization step in which the p-type trench sidewalls and bottom, but not the n-type trench sidewalls, are etched and rendered porous. FIG. 3 shows an illustrative apparatus for carrying out the wet anodization step. In FIG. 3, the entire device structure (36) of FIG. 2 is shown immersed in an anodization bath 34. Illustratively, the bath 34 comprises an electrolyte containing 5 percent hydrofluoric acid in a 1:1 solution of acetic acid and water. The device structure 36 is connected to the positive terminal of a dc power supply 38 to whose negative terminal a platinum electrode 40 is connr-zted. By passing approximately 0.75 millia peres per square centimeter through the electrolyte for about 10 minutes, the p-type trench sidewalls of the device structure 36 are selectively etched and rendered porous. As indicated in FIG. 4, only the p-type region 42 of the trench sidewall and bottom is etched and rendered porous. Illustratively, the thickness d_ of the region 42 is approximately 0.22 μm. Oxidation of the porous silicon region 42 is then carried out by exposing the device structure to oxygen in a furnace at about 900 degrees Celsius for about 5 minutes or by carrying out a rapid-thermal- annealing (RTA) step at approximately 1050 degrees Celsius for about 60 seconds. The resulting silicon dioxide region, designated by reference numeral 44 in FIG. 5, occupies about the same volume as the p-type silicon that was anodized and made porous. During the aforedescribed oxidation step in which the silicon dioxide region 44 (FIG. 5) is formed, a thin layer (for example about 200 8) of silicon dioxide is unavoidably formed on the n-type trench walls. This thin layer is then removed in a known etching process. During the removal step, a thin surface layer of the region 44 is also removed. But the remaining thickness of the region 44 (typically at least about 1500 A) is more than adequate to serve as a protective mask or barrier during a subsequent dopant enhancement step. Next, an n-type dopant such as arsenic is introduced into the unmasked or n-type sidewalls trenches to enhance the dopant concentration therein. Illustratively, an average dopant concentration of at least approximately 7 x 10 dopant ions per cubic centimeter is established along the n-type sidewalls in the region indicated by minus signs along the left-hand side of the tub 14 shown in FIG. 5. (The dopant concentration in other parts of the n-type tub 14 is about 10 16 dopant ions per cubic centimeter.) The thickness e> of the enhanced doped region is approximately 0.25 μm . Various known techniques are available for carrying out the aforedescribed dopant enhancement step. As schematically represented in FIG. 5 by dashed arrows 46, an ion implantation procedure can be utilized for doping the unmasked n-type trench sidewalls. For steeply inclined trench walls, wobbling of the implantation source relative to the device structure may be necessary to achieve uniform dopant enhancement. Other known techniques can be employed to carry out the aforedescribed dopant enhancement step. For example, an n-doped polysilicon layer (not shown) formed in the trench 24 (FIG. 5) can be utilized as a source for increasing the doping of the sidewalls of the n-type tub 14. Alternatively, conventional solid or gaseous sources can be used. Also, the technique described in U. S. Patent No. 4,472,212 for forming shallow highly doped surface layers can be employed. Next, the dopant mask layer 44 shown in FIG. 5 is removed. Illustratively, this is done in a conventional wet etching step utilizing dilute hydrofluoric acid. After this step in the fabrication sequence, the contour of the trench 24 is approximately as shown in FIG. 6. Then, by any of various known techniques, a p-type dopant such as boron is introduced into the trench bottom and sidewalls, as indicated by plus signs in FIG. 6, to enhance the dopant concentration in the p-type regions of the device along the sidewalls and bottom of each trench. Illustratively, an average dopant concentration of at least approximately 10 1 dopant ions per cubic centimeter is established in the p-type regions. This is sufficient to prevent conductivity type inversion during operation of the completed device. (The dopant concentrations in other parts of the p-type layer 12 and the p + region 10 are about 3 x 10 15 and 5 x 10 18 dopant ions per cubic centimeter, respectively.) The thickness _f of the enhanced p-type doped region is approximately 0.25 μm. As indicated in FIG. 6, the p-type dopant is not only introduced into portions of the p-type regions adjoining the trench 24 but is also introduced into the previously enhanced portions of the n-type region adjoining the trench. The p-type dopant thus- partially compensates the n-type dopant previously introduced into the trench sidewall adjoining the tub 14. But, the resulting net enhancement of n-type dopant concentration in the sidewall adjoining the tub 14 is still sufficient to prevent the enhanced n-type region of the depicted device from exhibiting inversion effects during normal operating conditions. Illustratively, the net n-type dopant concentration in the sidewall adjoining the tub 14 is at least approximately 6 x 10 16 dopant ions per cubic centimeter. A dry rather than a wet anodization procedure can be utilized to selectively mask the trench walls of a CMOS device of the type represented in FIG. 2. In the dry approach, the p-type sidewalls and bottoms of the trenches are selectively converted to silicon dioxide in a one-step anodization procedure carried out in a plasma. Such a dry anodization procedure involves placing the device whose p-type regions are to be converted to silicon dioxide on an anode electrode in a plasma reaction chamber. Either oxygen or a mixture of oxygen and chlorine is introduced into the chamber. A plasma is then generated in the chamber. The properties of the resulting oxide selectively formed on p-type silicon portions of the device are comparable to those of thermally grown oxide. Further details of one-step dry anodization procedures of the type described above are set forth in "Selective Anodic Oxidation of Silicon in Oxygen Plasma" by V. Q. Ho et al , IEEE Transactions on Electron Devices, Vol. ED-27, No. 8, August 1980, pages 1436-1443 and in "Anodic Oxidation of Si in Oxygen/Chlorine Plasma" by N. Hanej i et al, IEEE Transactions on Electron Devices, Vol. ED-32, No. 2, February 1985, pages 100-105. The structure depicted in FIG. 6 can thereafter be processed in known manner to form a complete CMOS device. Such further processing includes, for example, removing the layer 32 from the top surface of the device structure and then growing a thin high- quality layer of silicon dioxide on the sides and bottom of the isolation trench 24. Subsequently, the trenches of the device are filled with a material such as undoped polysilicon. Conventional MOS transistors can then be formed in the structure.

Claims: Claims A method of independently doping n- and p-type regions of isolation trenches formed in a CMOS device, said method CHARACTERIZED BY the steps of forming a dopant mask (42) only on the p-type region of each trench, doping only the n-type region of each trench with an n-type dopant, remo ing said dopant mask, and then simultaneously doping both n- and p- type regions of each trench with a p-type dopant, the resulting dopant concentration levels of both said trench regions being sufficiently high to prevent conductivity type inversion of said regions during device operation. 2. A method as in claim 1 wherein said forming step includes selective anodization. 3. A method as in claim 2 wherein the p-type region of each trench is etched and rendered porous in a wet anodization step. 4. A method as in claim 3 wherein the p-type region of each trench is then oxidized to form a mask comprising silicon dioxide. 5. A method as in claim 2 wherein the p-type region of each trench is selectively converted to a mask (44) comprising silicon dioxide in a one-step dry anodization procedure carried out in a plasma.

Inventors: LEBOWITZ JOSEPH, SEIDEL THOMAS EDWARD, LEBOWITZ, JOSEPH, SEIDEL, THOMAS, EDWARD
Applicants: AMERICAN TELEPHONE & TELEGRAPH, AMERICAN TELEPHONE & TELEGRAPH COMPANY
Application year: 1986
Main classification: H01L21
IPC codes: H01L21, H01L27