Methods of forming SOI insulator layers, methods of forming transistor devices, and semiconductor devices and assemblies

[0002] This patent is a continuation application of U.S. patent application Ser. No. 09/302,167 which was filed on Apr. 29, 1999, now U.S. Pat. No. 6,174,755; which is a continuation of U.S. patent application Ser. No. 08/916,773 filed Apr. 20, 1997, now U.S. Pat. No. 5,940,691, issued on Aug. 20, 1999.

[0003] The invention pertains to methods of forming silicon-on-insulator layers, methods of forming transistor devices, and to semiconductor devices and assemblies.

[0004] Numerous semiconductor devices and assemblies may be formed utilizing silicon-on-insulator (SOI) constructions. Such assemblies can include, for example, fully depleted SOI devices or partially-deleted SOI devices. Among SOI devices are n-channel transistors and p-channel transistors. Such transistors can, depending on the desired characteristics, be either fully depleted SOI devices or partially depleted SOI devices. Also, such transistors can be incorporated into specific types of devices, such as, for example, memory array transistor devices and peripheral transistor devices.

[0005] There are generally two ways of providing a starting substrate for SOI fabrication. In a first method, oxygen is implanted at a desired depth into a silicon wafer. The wafer is then subjected to an anneal to form a buried silicon dioxide layer having an outward monocrystalline silicon layer thereover. The anneal can also repair damage caused by the implant, although the repair is typically not perfect.

[0006] In a second method, a silicon wafer is initially provided with an outer silicon dioxide layer. Such outer silicon dioxide layer can be formed, for example, by exposing the wafer to an oxidizing ambient. After formation of the outer silicon dioxide layer, a separate silicon wafer is positioned against the silicon dioxide layer to form a composite comprising the silicon dioxide layer sandwiched between a pair of silicon wafers. The composite is heated in a furnace to cause fusing of the silicon wafers with the silicon dioxide. Thereafter, the second silicon wafer is mechanically polished down to a desired thickness such that its remnants constitute an SOI construction.

[0007] In many applications, it is desired to have an SOI construction in which the silicon layer has a substantially uniform thickness throughout its construction. A method for improving the uniformity of thickness of a silicon layer in an SOI construction is described with reference to FIG. 1, which illustrates an apparatus 10 configured for electrostatically etching a silicon layer of a SOI construction. Apparatus 10 comprises a vessel 12 within which is an etching composition 14 preferably comprising potassium hydroxide. A heater 16 is provided within etching composition 14 to control a temperature of the composition during an etching process. An SOI construction 18 is supported within vessel 12 by a TEFLON (™) holder 20 which comprises a back support 22 and a front support 24. SOI construction 18 is compressed between back support 22 and front support 24. A first O-ring 26 is between SOI construction 18 and front support 24 and seals a back of SOI construction 18 from exposure to etching composition 14. A second O-ring 34 seals front support 24 against back support 22. An electrode 28 extends across a back of SOI construction 18 and supports the back of SOI construction 18 while also providing an electrical connection to SOI construction 18. Electrode 28 is electrically connected through a voltage supply 30 to a platinum electrode 32 extending within etching composition 14.

[0008] An expanded view of zone 2 of FIG. 1 is shown in FIG. 2. As shown, SOI construction 18 comprises a substrate portion 40, an insulator layer 42, and a silicon layer 44. Silicon layer 44 is a “frontside” of construction 18 and substrate 40 is a “backside” of construction 18. Frontside 44 is exposed to etching composition 14.

[0009] In operation, silicon layer 44 is generally lightly doped, with, for example, a p-type conductivity-enhancing dopant. A voltage is provided with voltage supply 30 to force a depletion region 46 to be formed within silicon layer 44. Etching composition 14 then etches silicon layer 44 to about depletion region 46 and stops. A thickness of depletion region 46 can be controlled by controlling a voltage provided by voltage supply 30. Although it is not clear if the etching composition stops etching at the depletion layer, or at some location near the depletion layer, it is clear that the amount of silicon etched from layer 44 can be controlled by controlling a thickness of depletion layer 46.

[0010] In operation, a 20% (wt) potassium hydroxide solution is typically used as an etchant and 50-75 volts are applied by voltage supply 30 for a typical etching duration of about six minutes. The temperature of the potassium hydroxide solution is typically controlled to be about 70° C. with temperature controller 16.

[0011] The above-discussed methods of forming SOI constructions are utilized to form constructions in which the silicon layer has a relatively uniform thickness. However, in accordance with the invention which follows it is recognized that there may be some applications in which it is desirable to form SOI constructions having a silicon layer of varying thickness. Accordingly, methods are described for creating SOI constructions in which the silicon layer has a varying thickness. Also described are assemblies and devices designed to take advantage of an SOI construction within which the silicon layer has a varied thickness.

[0012] The invention encompasses methods of forming SOI constructions having varying thicknesses within the silicon layer. The invention also encompasses methods of forming transistor devices from such SOI constructions. Additionally, the invention encompasses semiconductor devices and assemblies utilizing SOI constructions which have varying thicknesses of the silicon layer.

[0013] In one aspect, the invention encompasses a method of forming a semiconductor-on-insulator layer wherein a substrate is provided, an insulator layer is provided over the substrate and a semiconductive layer is provided over the insulator layer. The semiconductive layer has a first portion and a second portion. A depletion region is formed within the semiconductive layer proximate the insulator layer. A thickness of the depletion region is controlled to form a different thickness in the first portion than in the second portion. The semiconductive layer is etched to about the depletion region.

[0014] In another aspect, the invention encompasses a method of forming a semiconductor-on-insulator layer wherein a semiconductive substrate is provided. The semiconductive substrate has a substantially uniform doping with a first impurity. A first portion of the semiconductive substrate is doped with a second impurity. A second portion of the semiconductive substrate is doped with a third impurity. A third portion of the semiconductive substrate is left undoped with either of the second impurity or the third impurity. An insulator layer is formed over the semiconductive substrate. A semiconductive layer is formed over the insulator layer. The semiconductive layer is substantially uniformly doped with a p-type impurity. A depletion region is formed within the semiconductive layer. The depletion region is formed over the first portion, the second portion and the third portion. The depletion region is thicker over the first portion of the substrate relative to over the second and third portions. The depletion region is thinner over the second portion of the substrate relative to over the first and third portions. The semiconductive layer is exposed to an electrolytic etching composition to etch the semiconductive layer to about the depletion region.

[0015] In another aspect, the invention encompasses a method of forming a thin film transistor wherein a substrate is provided. A source template portion, a channel template portion and a drain template portion of the substrate are defined. The source and drain template portions are doped differently than the channel template portion. An insulator layer is formed over the substrate. A semiconductive layer is formed over the insulator layer. A depletion region is formed within the semiconductive layer. The depletion region is over the source template portion, the channel template portion and the drain template portion. The depletion region is proximate the insulator layer, and is thinner over the channel template portion than over the source and drain template portions. The semiconductive layer is exposed to an electrolytic etching composition to etch the semiconductive layer to about the depletion region. The etched semiconductive layer is thinner over the channel template portion than over the source and drain template portions. The etched semiconductive layer over the channel template portion comprises a transistor channel, the etched semiconductive layer over the source template portion comprises a transistor source, and the etched semiconductive layer over the drain template portion comprises a transistor drain. A transistor gate is formed proximate the transistor channel. The transistor source, transistor drain, transistor channel and transistor gate are incorporated into a thin film transistor.

[0016] Preferred embodiments of the invention are described below with reference to the following accompanying drawings.

[0017] FIG. 1 is a diagrammatic cross-sectional view of a prior art apparatus for electrostatically etching a silicon layer of an SOI construction.

[0018] FIG. 2 is an expanded view of the zone labeled 2 in FIG. 1.

[0019] FIG. 3 is a fragmentary cross-sectional view of a silicon-on-insulator construction shown at a preliminary step of a first embodiment process of the present invention, and shown in an expanded view similar to that of FIG. 2.

[0020] FIG. 4 is a view of the FIG. 3 silicon-on-insulator construction shown at a processing step subsequent to that of FIG. 3.

[0021] FIG. 5 is a fragmentary cross-sectional view of a silicon-on-insulator construction shown at a preliminary step of a second embodiment process of the present invention, and shown in an expanded view similar to that of FIG. 2.

[0022] FIG. 6 is a fragmentary cross-sectional view of a silicon-on-insulator construction shown at a preliminary step of a third embodiment process of the present invention, and shown in an expanded view similar to that of FIG. 2.

[0023] FIG. 7 is a fragmentary cross-sectional view of a silicon-on-insulator construction at a preliminary step of a fourth embodiment process of the present invention.

[0024] FIG. 8 is a fragmentary cross-sectional view of the FIG. 7 silicon-on-insulator construction shown at processing step subsequent to that of FIG. 7.

[0025] FIG. 9 is a fragmentary cross-sectional view of a semiconductor assembly formed according to a fifth embodiment method of the present invention.

[0026] FIG. 10 is a fragmentary cross-sectional view of a semiconductor assembly formed according to a sixth embodiment method of the present invention.

[0027] This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8).

[0028] The invention generally encompasses methods of controlling a thickness of a semiconductor layer in a semiconductor-on-insulator assembly, encompasses methods of incorporation of such SOI assemblies into circuitry devices, and encompasses circuitry devices incorporating such SOI assemblies.

[0029] A first embodiment method of the present invention is discussed 11 with reference to FIGS. 3 and 4. Referring to FIG. 3, an SOI assembly 50 is illustrated. SOI assembly 50 is illustrated in an expanded view similar to that of FIG. 2, with SOI construction 50 being treated with electrolytic etching apparatus 10 of FIG. 1. SOI construction 50 comprises a substrate 52, an insulator layer 54 and a semiconductive layer 56 over insulator layer 54. Semiconductive layer 56 preferably comprises a semiconductive material, such as silicon doped with a p-type conductivity enhancing dopant. Insulator layer 54 can comprise a number of materials known to persons of ordinary skill in the art, including, for example, silicon dioxide and silicon nitride.

[0030] An electrode 28 is electrically connected with substrate 52 and an etching composition 14 is adjacent an outer surface of semiconductive layer 56. Electrode 28 is connected to a voltage supply 30 (shown in FIG. 1) which is in turn connected to an electrode 32 (shown in FIG. 1). Electrode 32 can be a platinum electrode, or can comprise other electrode materials. Voltage supply 30 (shown in FIG. 1) is utilized to form a depletion region 58 (shown in dashed line) within semiconductive layer 56.

[0031] SOI construction 50 comprises three portions 60, 62 and 64, having differing thicknesses of depletion region 58. The differing thickness of depletion region 58 in portions 60, 62 and 64 is caused by forming differing electric fields in portions 60, 62 and 64. The shown first embodiment method of controlling the thickness of depletion region 58 encompasses providing doped regions 66 and 68 within substrate 52 to define portions 60 and 64, while portion 62 is defined by the uniform dopant concentration within substrate 52.

[0032] In the shown embodiment, substrate 52 comprises a semiconductive material, such as silicon, which is uniformly conductively doped to from is about 10¹⁵to about 10¹⁸atoms/cm³with either a p-type or an n-type conductivity enhancing dopant. Doped regions 66 and 68 are provided with conductivity enhancing dopant which is either a different type from that utilized to uniformly dope substrate 52, or which is at a different concentration from the uniform substrate doping of substrate 52. For instance, if substrate 52 is blanket doped with a p-type conductivity enhancing dopant to a concentration of from 10¹⁵to about 10¹⁸atoms/cm³, region 66 could comprise a p-type conductivity enhancing dopant provided to a concentration of from about 10¹⁸to about 10²⁰atoms/cm³, and more preferably from about 10¹⁸to about 10¹⁹atoms/cm³. Region 66 will comprise a higher concentration of p-type conductivity enhancing dopant than the portion of substrate 52 within portion 62. The higher concentration of p-type conductivity enhancing dopant within region 66 relative to the substrate 52 within portion 62 causes depletion region 58 to be thicker within portion 60 than within portion 62.

[0033] Dopant region 68 can comprise an n-type conductivity enhancing dopant provided to a concentration of from 10¹⁷to about 10²⁰atoms/cm³. The n-type dopant of region 68 causes depletion region 58 to be thinner within portion 64 than within portions 60 and 62.

[0034] Doped regions 66 and 68 can alternatively be referred to as conductivity-modifying diffusion regions 66 and 68. Conductivity-modifying diffusion regions 66 and 68 alter a conductivity induced by voltage supply 30 (shown in FIG. 1) within semiconductive layer 56 and thereby modify the thickness of depletion region 58 within portions 60 and 64.

[0035] An alternate way of viewing the FIG. 3 construction is that a predominate portion of depletion region 58 comprises a base thickness “X” defined by the thickness within portion 62. Depletion region 58 comprises stepped segments at portions 60 and 64 which have thicknesses “Y” and “Z” that are different from base thickness “X”.

[0036] Referring to FIG. 4, SOI construction 50 is shown after etching composition 14 (shown in FIG. 3) etches to about depletion region 58 (shown in FIG. 3) to form the illustrated undulating outer surface 70 on semiconductive layer 56, and after SOI construction 50 is removed from apparatus 10 (shown in FIG. 1).

[0037] A second embodiment method for controlling a thickness of a depletion region within an SOI assembly is described with reference to FIG. 5. In referring to FIG. 5, similar labeling will be utilized as was used in describing the first embodiment of FIG. 3, with differences being indicated by the suffix “a,” or by different numerals.

[0038] Referring to FIG. 5, an SOI construction fragment 50ais illustrated. Fragment 50ais shown in an expanded view similar to the views of FIGS. 2 and 3, and is within an apparatus 10 (shown in FIG. 1). SOI construction 50acomprises a substrate 52a, an insulator layer 54aand a semiconductive material layer 56a. Substrate 52apreferably comprises a semiconductive material conductively doped with either p-type conductivity enhancing dopant or with n-type conductivity enhancing dopant. Semiconductive material 56apreferably comprises a material, such as silicon, conductively doped with p-type conductivity enhancing dopant.

[0039] SOI construction 50acomprises three portions 60a, 62aand 64a, each defined by a differing thickness of depletion region 58a.

[0040] Insulator layer 54ais modified to form regions of varying insulator thickness and thereby to form the differing thicknesses of portions 60a, 62aand 64a. Insulator layer 54acomprises a first material 80 at portion 62aand a second material 82, which is different from first material 80, at portion 60a. In the shown embodiment, second material 82 has a higher dielectric constant than first material 80. Such could be accomplished by, for example, utilizing silicon nitride as second material 82 and silicon oxide as first material 80. The high dielectric constant of material 82 causes depletion region 58ato be thicker at portion 60athan at portion 62a.

[0041] Insulator layer 54ais thickened at portion 64amore than at portion 62a, and is displaced inwardly into substrate 52aat portion 64arelative to portion 62a. Such thickening and inward displacement of layer 54acan be accomplished by, for example, forming a trench 84 within substrate 52aat portion 64aand subsequently filling trench 84 with insulator layer 54a. The thickening and inward displacement of insulator layer 54awithin portion 64a, relative to portion 62a, causes depletion region 58ato be thinner at portion 64arelative to portion 62a.

[0042] After formation of depletion region 58a, electrolytic etching component 14 (shown in FIG. 1) can be utilized to etch to about depletion region 58ato form an SOI construction having an undulating outer surface shaped similar to depletion region 58a.

[0043] Referring to FIG. 6, a third embodiment of the present invention is illustrated. In referring to FIG. 6, similar labels to those utilized above with respect to FIGS. 3-5 will be used, with differences indicated with the suffix “b” or with different numerals. FIG. 6 shows an SOI construction 50bwithin an apparatus 10 (shown in FIG. 1), in a view similar to that of FIGS. 2, 3 and 5. Construction 50bcomprises a substrate 52b, an insulator layer 54b, and a semiconductive material layer 56b. Substrate 52bpreferably comprises a uniformly doped semiconductive substrate, insulator layer 54bpreferably comprises an 41 insulative material such as silicon nitride or silicon dioxide, and semiconductive material 56bpreferably comprises a p-type doped silicon material.

[0044] A conductive material 90 is provided within insulative material 54b. Conductive material 90 changes an electric field within portion 60band thereby causes depletion region 58bto have a different thickness within portion 60bthan within portion 62b. Conductive material 90 could comprise, for example, a metallic material, such as titanium or tungsten. Although conductive material 90 is shown within insulator layer 54b, it will be recognized by persons of ordinary skill in the art that conductive material 90 could also be provided within substrate 52bto achieve a similar effect on depletion region 58bas that illustrated.

[0045] SOI assembly 50balso comprises a trench 84b, similar to trench 84 in FIG. 5. Trench 84bcauses a thickness of depletion region 58bto be altered within region 64brelative to the thickness of depletion region 58bwithin, for example, region 62b.

[0046] After formation of depletion region 58b, the electrolytic etching component 14 (shown in FIG. 1) can be utilized to etch to about depletion region 58bto form an SOI construction having an undulating outer surface shaped similar to depletion region 58b.

[0047] The SOI assemblies produced by the methods discussed above with reference to FIGS. 3-6 may be utilized to form semiconductor devices. For instance, a thin film transistor may be formed according to a process illustrated in FIGS. 7 and 8. In referring to FIGS. 7 and 8, similar labels to those utilized above with reference to FIGS. 3-6 will be utilized, with differences indicated by the suffix “c” or with different numerals.

[0048] Referring to FIG. 7, an SOI assembly 50cis illustrated. Assembly 50ccomprises a substrate 52c, an insulator layer 54c, and a semiconductive material layer 56c. A source template region 100, a channel template region 102, and a drain template region 104 are defined within assembly 50c. A depletion region 58cis formed within semiconductive layer 56c. Depletion region 58ccan be formed by one or more of the methods discussed above with reference to FIGS. 3-6. Depletion region 58cis formed over source template region 100, channel template region 102 and drain template region 104. Depletion region 58cis proximate insulator layer 54c, and has a different thickness within channel template region 102 than within source and drain template regions 100 and 104.

[0049] While the depletion region 58cis formed, semiconductive layer 56cis exposed to an electrolytic etching composition (such as composition 14 shown in FIG. 1) and etched to about depletion region 58c.

[0050] Referring to FIG. 8, SOI assembly 50cis shown after such etching. As shown, the etched semiconductive layer 56ccomprises a different thickness within channel template region 102 than within source and drain template regions 100 and 104. In the shown embodiment, semiconductive material 56cis thinner within channel template region 102 than within source and drain template regions 100 and 104. However, as will be recognized by persons of ordinary skill in the art, alternative embodiments could be utilized in which the thickness of insulator layer 56cis greater within channel template region 102 than within source and drain template regions 100 and 104.

[0051] The etched semiconductive layer 56cwithin source and drain template regions 100 and 104 is then conductively doped to form source and drain regions 106 and 108, respectively. Also, a transistor gate 110 is formed over the etched semiconductive material 56cwithin channel template region 102 to utilize the material 56cwithin region 102 as a transistor channel 112. Channel 112, transistor gate 110, source region 106 and drain region 108 together comprise a transistor device 114.

[0052] A thickness of channel 112 is controlled by controlling a thickness of region 58cwithin channel template region-102. Such control of the thickness of channel 112 can be utilized to create a desired threshold voltage for transistor device 114.

[0053] Referring to FIG. 9, a method of the present invention can be utilized to selectively adjust thicknesses of peripheral n-channel transistor devices, peripheral p-channel transistor devices, and memory array devices, formed on SOI assemblies. In referring to FIG. 9 similar labels to those utilized above in describing FIGS. 1-8 will be used, with differences indicated by the suffix “d” or different numerals.

[0054] FIG. 9 illustrates an SOI assembly 50dcomprising a substrate 52d, an insulator layer 54dand a semiconductive layer 56d. Within SOI assembly 50dare defined a peripheral p-channel device region 120, a peripheral n-channel device region 122, and a memory array device region 124. Ultimately, p-channel transistors (not shown) will be formed within region 120, n-channel transistors (not shown) will be formed within region 122, and memory array transistors (not shown) will be formed within region 124. The methods discussed above regarding FIGS. 3-8 can be utilized to adjust relative thicknesses of semiconductive material 56d in the regions 120, 122 and 124 to enable the ultimately formed p-channel devices, n-channel devices and memory array devices to be formed over regions of semiconductive material 56dwith locally |varying thicknesses of channel regions and/or source/drain regions. In fully depleted SOI a threshold voltage can be influenced by a silicon layer thickness, as well as by variations of doping within the silicon layer. Thus, local variation of thickness of the semiconductive layer 56dwithin regions 120, 122 and 124 can be utilized to adjust a threshold voltage of the peripheral devices formed in regions 120 and 122 to be different from a threshold voltage of the memory array devices formed in region 124.

[0055] Referring to FIG. 10, a method of the present invention can be utilized to form SOI assemblies in which a semiconductive layer has a different thickness in a fully depleted SOI region than in a partially depleted SOI region. Generally, the semiconductive material would be thicker in a partially depleted SOI region than in a fully depleted SOI region, although, as will be recognized by persons of ordinary skill in the art, methods of the present invention can be utilized to form opposite structures as well.

[0056] In referring to FIG. 10, similar labels to those utilized in discussing FIGS. 3-9 above will be utilized, with differences indicated by the suffix “e” or by different numerals.

[0057] FIG. 10 illustrates an SOI assembly 50ecomprising a substrate 52e, an insulator layer 54e, and a semiconductive layer 56e. Within SOI assembly 50eare defined partially depleted SOI template regions 130 and fully depleted SOI template regions 132. Semiconductive material 56eis thicker within the partially depleted SOI regions 130 than within fully depleted SOI regions 132. Such variation in the thickness of semiconductive material 56ecan be accomplished by the methods discussed above regarding FIGS. 3-6.

[0058] To aid in interpretation of the claims that follow, the term “semiconductive substrate” is defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above.

[0059] Although the embodiments discussed above are described with reference to formation of SOI structures utilizing the apparatus of FIG. 1, it is to be understood that other apparatuses could be utilized in methods of the present invention.

[0060] In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.