Methods of forming electrical contacts

[0002] This invention relates generally to methods of forming electrical contacts to semiconductive material.

[0003] In semiconductor circuitry fabrication, electrical connections or contacts are commonly made between conductive lines and conductive diffusion areas formed within a semiconductive substrate. In the context of this document, 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. Such electrical contacts are typically accomplished by initially etching a contact opening through insulative material over a conductive diffusion region comprising highly doped semiconductive material to which electrical connection is desired. Conductive material, such as conductively doped semiconductive material, is thereafter formed within the contact opening in electrical connection with the diffusion region within the semiconductive substrate. The conductive contact filling material can then either be planarized or, if deposited to a sufficient thickness, patterned into a conductive line or other desired electronic component.

[0004] One problem associated with such connections is exemplified in FIG. 1. There illustrated is a semiconductor wafer fragment 10 comprised of a p− doped monocrystalline silicon substrate having a pair of conductive gate lines 14 and 16 formed thereover. Source/drain diffusion regions 18, 20, and 22, constituting n+ dopant, are formed within substrate 12 to form field effect transistors. In this example, electrical connection with an overlying conductive line is desired to be made with respect too diffusion region 20.

[0005] An insulating layer 24 is formed over substrate 12 and gates 14 and 16, and is subsequently planarized. A contact opening 26 is then patterned and formed, typically by dry etching, through insulating layer 24 over diffusion region 20. Such has the effect of roughening or otherwise damaging the outer surface of silicon substrate 12 within diffusion region 20 upon outward exposure thereof. This also undesirably has a tendency to change an outer portion 28 of the semiconductive material of substrate 12 exposed by the contact etching. This change typically manifests itself in a modified crystalline structure of the silicon material of substrate 12. Subsequently, a buried contact ion implant region 30 is formed into and through region 28 within diffusion region 20. Implant 30 is provided to achieve enhanced electrical contact between diffusion region 20 and a subsequently deposited conductive material. The substrate is typically subjected to an anneal in an attempt to repair or overcome the silicon damage caused by both the dry etch forming contact opening 26 and that caused by the implant to form region 30.

[0006] The wafer is subjected to an HF clean for a short period of time to clear any native oxide formed over region 20. A conductive layer 32 is then formed within contact opening 26 and over insulating layer 24, for example by depositing or otherwise forming a conductively doped semiconductive material such as polysilicon.

[0007] Unfortunately, the combined effect of both the changed outer portion 28 from the dry etch and the subsequent implant region 30, even with a subsequent anneal, forms a less than desired electrical connection between the diffusion region and conductive material 32, particularly where the conductive material within contact opening 26 is essentially void of any silicide material. It would be desirable to overcome some of these drawbacks associated with forming electrical contacts to semiconductive material.

[0008] In one aspect of the invention, a method of forming an electrical contact to semiconductive material includes forming an insulative layer over a contact area of semiconductive material. A contact opening is etched through the insulative layer to the semiconductive material contact area. Such etching changes an outer portion of the semiconductive material exposed by the etching. The change is typically in the form of modifying crystalline structure of only an outer portion from that existing prior to the etch. The changed outer portion of the semiconductive material is etched substantially selective relative to semiconductive material therebeneath which is unchanged. The preferred etching chemistry is a tetramethyl ammonium hydroxide solution. A conductive material within the contact opening is formed in electrical connection with the semiconductive material.

[0009] In another implementation, the changed outer portion is etched with a basic solution regardless of selectivity in the etch relative to semiconductive material therebeneath which is unchanged.

[0010] The preferred conductive material is conductively doped semiconductive material which is formed in the contact opening to be in contact with semiconductive material which is unchanged. Further, the conductive material within the contact opening is preferably void of any silicide material.

[0011] Preferred embodiments of the invention are described below with reference to the following accompanying drawings.

[0012] FIG. 1 is a cross-sectional view of a prior art semiconductor wafer fragment, and is discussed in the “Background” section above.

[0013] FIG. 2 is a cross-sectional view of a semiconductor wafer fragment at one process in accordance with the invention.

[0014] FIG. 3 is a view of the FIG. 2 wafer fragment at a processing step subsequent to that shown by FIG. 2.

[0015] FIG. 4 is a view of the FIG. 2 wafer fragment at a processing step subsequent to that shown by FIG. 3.

[0016] FIG. 5 is a view of the FIG. 2 wafer fragment at a processing step subsequent to that shown by FIG. 4

[0017] FIG. 6 is a diagrammatic cross-sectional view of an alternate semiconductor wafer fragment at an alternate processing step in accordance with the invention.

[0018] FIG. 7 is a view of the FIG. 6 wafer fragment at a processing step subsequent to that shown by FIG. 6.

[0019] FIG. 8 is a view of the FIG. 6 wafer fragment at a processing step subsequent to that shown by FIG. 7.

[0020] FIG. 9 is a diagrammatic cross-sectional view of another alternate embodiment semiconductor wafer fragment at another alternate processing step in accordance with the invention.

[0021] FIG. 10 is a view of the FIG. 9 wafer fragment at a processing step subsequent to that shown by FIG. 9.

[0022] FIG. 11 is still another cross-sectional view of an alternate embodiment wafer fragment at an alternate processing step in accordance with the invention.

[0023] 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).

[0024] FIG. 2 illustrates a semiconductor wafer fragment 40 similar to the prior art depiction of FIG. 1. Like numerals have been utilized where appropriate, with differences being indicated with different numerals. Diffusion region 20 constitutes a contact area 42 of semiconductive material to which electrical contact is desired. Gates 14 and 16 are provided over substrate 12, with source/drain regions 18, 20 and 22 being formed within substrate 12, thus forming desired field effect transistors. Insulating layer 24 is formed over contact area 42 and the gates and diffusion regions of such field effect transistors.

[0025] Referring to FIG. 3, contact opening 26 is etched through insulating layer 24 to semiconductive material contact area 42, typically utilizing a suitable dry etch chemistry which is selective to etch silicon dioxide relative to underlying silicon. An example etch chemistry includes C₂F₅H, CHF₃, and CH₂F₂in a plasma etcher. Such etching changes an outer portion 28 of the semiconductive material exposed by the contact etching, typically by undesirably modifying the crystal structure of the silicon lattice within substrate 12, which in this embodiment is an outer portion of source/drain diffusion region 20. Regardless and alternately considered in one implementation, outer portion 28 is provided to have a different crystalline structure than semiconductive material immediately therebeneath whether resulting from the etch, and whether occurring before or after such etch.

[0026] Referring to FIG. 4, and in accordance with but one aspect of the invention, changed outer portion 28 is etched substantially selective relative to semiconductive material therebeneath of diffusion region 20 which is unchanged by the contact etching which formed opening 26. Such outer portion etching preferably comprises wet etching using, for example, a solution of tetramethyl ammonium hydroxide comprising from about 0.5% to 10% tetramethyl ammonium hydroxide by weight relative to deionized water. Pressure during etching is preferably ambient, with an example etching temperature being 30° C. In the context of this document, a substantially selective etch is defined by an etch which etches one material relative to another material at a removal rate of at least 2:1. Selectivity in this etch using the above example tetramethyl ammonium hydroxide solution wherein substrate 12 comprises monocrystalline silicon and diffusion region 20 constitutes phosphorus doping to a level of at least 1×10²⁰ions/cm³is expected to be greater than or equal to 5:1.

[0027] In accordance with an alternate implementation, outer portion 28 is etched using some basic solution which etches outer portion 28 regardless of selectivity relative to underlying semiconductive material therebeneath. Regardless, preferably the etching of the changed outer portion is conducted to etch all exposed changed outer portion material. FIG. 4 illustrates outer portion 28 having been substantially removed from source/drain diffusion region 20 of semiconductive substrate 12.

[0028] Referring to FIG. 5, conductive material 32 is formed within contact opening 26 in electrical connection with the semiconductive material of diffusion region 20. Such material preferably comprises polysilicon or some other semiconductive material conductively doped to a concentration of about 1×10²⁰ions/cm³. Contact opening 26 is thereby preferably formed to be void of any silicide, with material 32 being in contact with semiconductive material of diffusion region 20 which is unchanged from the contact etching.

[0029] An alternate preferred embodiment is described with reference to FIGS. 6-8. Like numerals from the FIGS. 2-5 embodiment are utilized where appropriate, with differences being indicated with different numerals or with the suffix “a”. Wafer fragment 40ain FIG. 6 depicts wafer fragment 40 of the FIGS. 2-5 embodiment at a step immediately subsequent to that depicted by FIG. 3. Specifically, after contact opening 26 has been formed, a conductivity enhancing impurity is ion implanted into changed outer portion 28. An example implant is phosphorus at a dose of 9×10¹²atoms/cm²and energy of 80 keV. Region 30 is shown as having been implanted into and through region 28. Alternately, the implant could be conducted to be shallower, for example to substantially only place the implanted impurity into the changed outer portion. Further alternately, the implanting can be conducted to place the dopant concentration of the implant substantially central relative to the thickness or depth of region 28. Such latter implanting will focus most of the implant to within region 28, but perhaps not necessarily only place the impurity into the changed outer portion.

[0030] Regardless, such implanting further adversely modifies or changes the exposed semiconductive material such that selectivity in the subsequent etch utilizing at least the tetramethyl ammonium hydroxide solution can be substantially increased, such as to a selective etch ratio of 100:1. FIG. 7 illustrates such a subsequent etch removing material of regions 28 and 30 selectively relative to semiconductive material therebeneath of region 20 which is substantially unchanged by either of the contact opening 26 etch or the exemplary subsequent ion implant. An exemplary depth to the base of the source/drain diffusion regions is 2000 Angstroms, with an example depth to the base of region 30 being 1000 Angstroms. An example etch with a 2% by weight solution of tetramethyl ammonium hydroxide for 90 seconds at a temperature of 30° C. and ambient pressure produced a 1000 Angstroms selective removal of the damaged silicon to the base of region 30.

[0031] Referring to FIG. 8, a conductive layer 32 is subsequently formed within and over contact opening 32 and ideally in contact with diffusion region 20.

[0032] Another alternate embodiment is described with reference to FIGS. 9 and 10. Like numerals from the FIGS. 2-8 embodiments are utilized where appropriate, with differences being indicated by the suffix “b” or with different numerals. FIG. 9 illustrates a wafer fragment 40bat a processing step immediately subsequent to processing of wafer 40 as depicted in FIG. 4. In FIG. 9, a conductivity enhancing impurity has been implanted into semiconductive material of diffusion region 20 after having etched changed outer portion 28 to form region 30b.Such is ideally a shallow buried contact implant intended to enhance conductivity of the contact, like the implant 30 of the second described embodiment. Although the implant produced will have some effect in adversely modifying conductive properties of the silicon at the contact, such does not produce the double-damage effect in the contact of the prior art due to both remaining silicon damage from the dry etch to form the contact and a subsequent buried contact implant.

[0033] Referring to FIG. 10, a conductive layer 32 is subsequently formed within and over contact opening 32 and ideally in contact with diffusion region 20.

[0034] Yet another alternate embodiment is depicted in FIG. 11. Like numerals from the FIGS. 2-10 embodiments are utilized where appropriate, with differences being indicated with the suffix “c” or with different numerals. FIG. 11 depicts a wafer fragment 40cat a processing step immediately subsequent to that depicted with wafer 40ain FIG. 7. Accordingly, wafer fragment 40chas previously been processed by an ion implant immediately prior to etching the changed outer portion, ideally to enhance selectivity in the resulting etch of the damaged silicon relative to undamaged silicon therebeneath. Further in FIG. 11, a subsequent buried contact conductivity enhancing ion implant is conducted to form region 50 within diffusion region 20.

[0035] In compliance with the statute, the invention has been. described 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.