Method for fabricating a semiconductor structure having a crystalline alkaline earth metal oxide interface with silicon

A method for fabricating a semiconductor structure comprises the steps of providing a silicon substrate (10) having a surface (12); forming on the surface of the silicon substrate an interface (14) comprising a single atomic layer of silicon, oxygen, and a metal; and forming one or more layers of a single crystal oxide (26) on the interface. The interface comprises an atomic layer of silicon, oxygen, and a metal in the form XSiO.sub.2, where X is a metal.

FIELD OF THE INVENTION
 The present invention relates in general to a method for fabricating a
 semiconductor structure including a crystalline alkaline earth metal oxide
 interface between a silicon substrate and other oxides, and more
 particularly to a method for fabricating an interface including an atomic
 layer of an alkaline earth metal, silicon, and oxygen.
 BACKGROUND OF THE INVENTION
 An ordered and stable silicon (Si) surface is most desirable for subsequent
 epitaxial growth of single crystal thin films on silicon for numerous
 device applications, e.g., ferroelectrics or high dielectric constant
 oxides for non-volatile high density memory and logic devices. It is
 pivotal to establish an ordered transition layer on the Si surface,
 especially for subsequent growth of single crystal oxides, e.g.,
 perovskites.
 Some reported growth of these oxides, such as BaO and BaTiO.sub.3 on
 Si(100) was based on a BaSi.sub.2 (cubic) template by depositing one
 fourth monolayer of Ba on Si(100) using reactive epitaxy at temperatures
 greater than 850.degree. C. See for example: R. McKee et al., Appl. Phys.
 Lett. 59(7), pp 782-784 (Aug. 12, 1991); R. McKee et al., Appl. Phys.
 Lett. 63(20), pp. 2818-2820 (Nov. 15, 1993); R. McKee et al., Mat. Res.
 Soc. Symp. Proc., Vol. 21, pp. 131-135 (1991); U.S. Pat. No. 5,225,031,
 issued Jul. 6, 1993, entitled "Process for Depositing an Oxide Epitaxially
 onto a Silicon Substrate and Structures Prepared with the Process"; and
 U.S. Pat. No. 5,482,003, issued Jan. 9, 1996, entitled "Process for
 Depositing Epitaxial Alkaline Earth Oxide onto a Substrate and Structures
 Prepared with the Process". However, atomic level simulation of this
 proposed structure indicates that it likely is not stable at elevated
 temperatures.
 Growth of SrTiO.sub.3 on silicon (100) using an SrO buffer layer has been
 accomplished. T. Tambo et al., Jpn. J. Appl. Phys., Vol. 37 (1998), pp.
 4454-4459. However, the SrO buffer layer was thick (100 .ANG.), thereby
 limiting application for transistor films, and crystallinity was not
 maintained throughout the growth.
 Furthermore, SrTiO.sub.3 has been grown on silicon using thick metal oxide
 buffer layers (60-120 .ANG.) of Sr or Ti. B. K. Moon et al., Jpn. J. Appl.
 Phys., Vol. 33 (1994), pp. 1472-1477. These thick buffer layers would
 limit the application for transistors.
 SUMMARY OF INVENTION
 Therefore, a method for fabricating a thin, stable crystalline interface
 with silicon is set forth below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 To form the novel interface between a silicon (Si) substrate and one or
 more layers of a single crystal oxide, various approaches may be used.
 Several examples will be provided for both starting with a Si substrate
 having a clean surface, and a Si substrate having silicon dioxide
 (SiO.sub.2) on the surface. SiO.sub.2 is amorphous rather than single
 crystalline and it is desirable for purposes of growing additional single
 crystal material on the substrate that a single crystal oxide be provided
 as the interface.
 Turning now to the drawings in which like elements are designated with like
 numbers throughout, FIGS. 1 and 2 illustrate a semiconductor structure
 including a Si substrate 10 having a clean surface 12. A clean (2.times.1)
 surface 12 may be obtained with any conventional cleaning procedure, for
 example, with thermal desorption of SiO.sub.2 at a temperature greater
 than or equal to 850.degree. C., or by removal of the hydrogen from a
 hydrogen terminated Si(1.times.1) surface at a temperature greater than or
 equal to 300.degree. C. in an ultra high vacuum. Hydrogen termination is a
 well known process in which hydrogen is loosely bonded to dangling bonds
 of the silicon atoms at surface 12 to complete the crystalline structure.
 The interface 14 of a crystaline material may be formed by supplying (as
 shown by the arrows in FIG. 1) controlled amounts of a metal, Si, and
 O.sub.2, either simultaneously or sequentially to the surface 12 at a
 temperature less than or equal to 900.degree. C. in a growth chamber with
 O.sub.2 partial pressure less than or equal to 1.times.10.sup.-9 mBar. The
 metal applied to the surface 12 to form the interface 14 may be any metal,
 but in the preferred embodiment comprises an alkaline-earth-metal, such as
 barium (Ba) or strontium (Sr).
 As the application of the Ba, Si, and O.sub.2 form BaSiO.sub.2 as the
 interface 14, the growth is monitored using Reflection High Energy
 Electron Diffraction (RHEED) techniques which are well documented in the
 art and which can be used in situ, i.e., while performing the exposing
 step within the growth chamber. The RHEED techniques are used to detect or
 sense surface crystalline structures and in the present process change
 rapidly to strong and sharp streaks by the forming of an atomic layer of
 the BaSiO.sub.2. It will of course be understood that once a specific
 manufacturing process is provided and followed, it may not be necessary to
 perform the RHEED techniques on every substrate.
 The novel atomic structure of the interface 14 will be described in
 subsequent paragraphs.
 It should be understood by those skilled in the art that the temperatures
 and pressures given for these processes are recommended for the particular
 embodiment described, but the invention is not limited to a particular
 temperature or pressure range.
 Referring to FIGS. 3-6, another approach comprises forming a Si substrate
 10 having a surface 12, and a layer 16 of SiO.sub.2 thereupon. The layer
 16 of SiO.sub.2 naturally exists (native oxide) once the Si substrate 10
 is exposed to air (oxygen) or it may be formed purposely in a controlled
 fashion well known in the art, e.g., thermally by applying (arrows) oxygen
 onto the surface 12. The novel interface 14 may be formed at least in one
 of the two suggested embodiments as follows: By applying an
 alkaline-earth-metal to the surface 18 of SiO.sub.2 layer 16 at
 700-900.degree. C., under an ultra high vacuum. More specifically, the Si
 substrate 10 and the amorphous SiO.sub.2 layer 16 are heated to a
 temperature below the sublimation temperature of the SiO.sub.2 layer 16
 (generally below 900.degree. C.). This can be accomplished in a molecular
 beam epitaxy chamber or Si substrate 10 can be at least partially heated
 in a preparation chamber after which it can be transferred to the growth
 chamber and the heating completed. Once the Si substrate 10 is properly
 heated and the pressure in the growth chamber has been reduced
 appropriately, the surface 12 of the Si substrate 10 having SiO.sub.2
 layer 16 thereon is exposed to a beam of metal, preferrably an
 alkaline-earth-metal, as illustrated in FIG. 5. In a preferred embodiment,
 the beam is Ba or Sr which is generated by resistively heating effusion
 cells or from e-beam evaporation sources. In a specific example, Si
 substrate 10 and SiO.sub.2 layer 16 are exposed to a beam of Ba. The Ba
 joins the SiO.sub.2 and converts the SiO.sub.2 layer 16 into the interface
 14 comprising BaSiO.sub.2 in a crystalline form. Alternatively, an
 alkaline-earth-metal may be provided to the surface 18 at lower
 temperatures, annealing the result at 700-900.degree. C., in an ultra high
 vacuum.
 Once the interface 14 is formed, one or more layers of a single crystal
 oxide may be formed on the surface of the interface 14. However, an
 optional layer of an alkaline-earth-metal oxide, such as BaO or SrO, may
 be placed between the interface 14 and the single crystal oxide. This
 alkaline-earth-metal oxide provides a low dielectric constant
 (advantageous for certain uses such as memory cells) and also prevents
 oxygen from migrating from the single crystal oxide to the Si substrate
 10.
 Referring to FIGS. 7 and 8, the formation of alkaline-earth-metal oxide
 layer 22 may be accomplished by either the simultaneous or alternating
 supply to the surface 20 of the interface 14 of an alkaline-earth-metal
 and oxygen at less than or equal to 700.degree. C. and under O.sub.2
 partial pressure less than or equal to 1.times.10.sup.-5 mBar. This
 alkaline-earth-metal oxide layer 22 may, for example, comprise a thickness
 of 50-500 .ANG..
 Referring to FIGS. 9-12, a single crystal oxide layer 26, such as an
 alkaline-earth-metal perovskite, may be formed on either the surface 20 of
 the interface 14 or the surface 24 of the alkaline-earth-metal oxide layer
 22 by either the simultaneous or alternating supply of an
 alkaline-earth-metal oxide, oxygen, and a transition metal, such as
 titanium, at less than or equal to 700.degree. C. under an oxygen partial
 pressure less than or equal to 1.times.10.sup.-5 mBar. This single crystal
 oxide layer 26 may, for example, comprise a thickness of 50-1000 .ANG. and
 will be substantially lattice matched with the underlying interface 14 or
 alkaline-earth-metal oxide layer 22. It should be understood that the
 single crystal oxide layer 26 may comprises one or more layers in other
 embodiments.
 Referring to FIG. 13, a side view (looking in the &lt;l10&gt; direction) of the
 atomic configuration of the Si substrate 10, interface 14, and
 alkaline-earth-metal metal oxygen layer 26 is shown. The configuration
 shown comprises, in relative sizes, for illustrative purposes, from larger
 to smaller, barium atoms 30, silicon atoms 32, oxygen atoms 34, and
 titanium atoms 36. The Si substrate 10 comprises only silicon atoms 32.
 The interface 14 comprises metal atoms (which in the preferred embodiment
 are illustrated as barium atoms 30), silicon atoms 32, and oxygen atoms
 34. The alkaline-earth-metal metal oxygen layer 26 comprises barium atoms
 30, oxygen atoms 34, and titanium atoms 36.
 Referring to FIG. 14, a top view of the interface along view line AA of
 FIG. 13, shows the arrangement of the barium, silicon, and oxygen atoms
 30, 32, 34.
 Referring to FIG. 15, a top view along line AA of FIG. 13, shows the
 interface 14 and the top atomic layer 11 of the Si substrate 10.
 For this discussion, a monolayer equals 6.8.times.10.sup.14 atoms/cm.sup.2
 and an atomic layer is one atom thick. It is seen that the interface 14
 shown in the FIGS. comprises a single atomic layer, but could be more than
 one atomic layer, while the Si substrate 10 and the alkaline-earth-metal
 metal oxide layer may be many atomic layers. Note that in FIG. 13, only
 four atomic layers of the Si substrate 10 and only three atomic layers of
 the alkaline-earth-metal metal oxide layer 26 are shown. The interface 14
 comprises a half monolayer of the alkaline-earth-metal, a half monolayer
 of silicon, and a monolayer of oxygen. Each barium atom 30 is
 substantially equally spaced from four of the silicon atoms 32 in the Si
 substrate 10. The silicon atoms 32 in the interface 14 are substantially
 on a line and equally spaced between the alkaline-earth-metal atoms in the
 &lt;110&gt; direction. Each silicon atom 32 in the top layer of atoms in the Si
 substrate 10 is bonded to an oxygen atom 34 in the interface 14 and each
 silicon atom 32 in the interface 14 is bonded to two oxygen atoms 34 in
 the interface 14. The interface 14 comprises rows of barium, silicon, and
 oxygen atoms 30, 32, 34 in a 2.times.1 configuration on a (001) surface of
 the Si substrate 10, 1.times.in the &lt;l10&gt; direction and 2.times.in the
 &lt;110&gt; direction. The interface 14 has a 2.times.1 reconstruction.
 A method for fabricating a thin, crystalline interface 14 with silicon 10
 has been described herein. The interface 14 may comprise a single atomic
 layer. Better transistor applications are achieved by the interface 14
 being thin, in that the electrical coupling of the overlying oxide layers
 to the Si substrate 10 is not compromised, and in that the interface 14 is
 more stable since the atoms will more likely maintain their crystalinity
 in processing.