System and method for gas-phase sulfur passivation of a semiconductor surface

Abstract

Improved methods and systems for passivating a surface of a high-mobility semiconductor and structures and devices formed using the methods are disclosed. The method includes providing a high-mobility semiconductor surface to a chamber of a reactor and exposing the high-mobility semiconductor surface to a gas-phase sulfur precursor to passivate the high-mobility semiconductor surface.

Claims

1. A method of passivating a surface of a semiconductor, the method comprising the steps of: providing the surface of the semiconductor to a reaction chamber of a reactor; exposing the surface of the semiconductor to a gas-phase sulfur precursor selected from the group consisting of H.sub.2S, NH.sub.4HS, and organosulfur compounds in the reaction chamber; and passivating the surface of the semiconductor in the reaction chamber using the gas-phase sulfur precursor to form a passivated semiconductor surface, wherein a pressure within the reaction chamber during the step of passivating is between 0.5 Torr and 750 Torr.

2. The method of passivating a surface of a semiconductor according to claim 1, wherein a source for the sulfur precursor is selected from the group consisting of organosulfur compounds.

3. The method of passivating a surface of a semiconductor according to claim 1, further comprising the step of depositing dielectric material onto the passivated semiconductor surface.

4. The method of passivating a surface of a semiconductor according to claim 3, wherein the step of depositing dielectric material and the step of exposing the surface of the semiconductor to a gas-phase sulfur-precursor are performed in the same reactor.

5. The method of passivating a surface of a semiconductor according to claim 3, wherein the step of depositing dielectric material and the step of exposing the surface of the semiconductor to a gas-phase precursor are performed in separate reactors.

6. The method of passivating a surface of a semiconductor according to claim 3, wherein the step of depositing dielectric material comprises depositing aluminum oxide.

7. The method of passivating a surface of a semiconductor according to claim 1, wherein the semiconductor is a high-mobility semiconductor selected from the group consisting of germanium and III-V semiconductor materials.

8. The method of passivating a surface of a semiconductor according to claim 1, further comprising the step of cleaning the surface of a semiconductor prior, using an in-situ gas-phase process, prior to the step of exposing the surface of the semiconductor to a gas-phase sulfur precursor.

9. The method of passivating a surface of a semiconductor according to claim 1, wherein the step of exposing the surface comprises exposing the surface of the semiconductor wafer to a plasma process.

10. The method of passivating a surface of a semiconductor according to claim 1, wherein the step of providing the surface of the semiconductor to a reaction chamber of a reactor comprises providing the surface within an atomic layer deposition reactor.

11. The method of passivating a surface of a semiconductor according to claim 1, further comprising the steps of providing a carrier gas and mixing the carrier gas with the gas-phase sulfur precursor.

12. A method of passivating a surface of a semiconductor, the method comprising the steps of: providing the surface of the semiconductor to a reaction chamber of a reactor; exposing the surface of the semiconductor to a gas-phase sulfur precursor selected from the group consisting of NH.sub.4HS, H.sub.2S, and organosulfur compounds in the reaction chamber; and passivating the surface of the semiconductor in the reaction chamber using the gas-phase sulfur precursor to form a passivated semiconductor surface.

13. A method of passivating a surface of a semiconductor, the method comprising the steps of: providing the surface of the semiconductor to a reaction chamber of a reactor; cleaning the surface of the semiconductor using an in-situ hydrogen gas-phase process, after the step of cleaning, exposing the surface of the semiconductor to a gas-phase sulfur precursor selected from the group consisting of NH.sub.4HS, H.sub.2S, and organosulfur compounds in the reaction chamber; and passivating the surface of the semiconductor in the reaction chamber using the gas-phase sulfur precursor to form a passivated semiconductor surface.

14. The method of passivating a surface of a semiconductor according to claim 12, further comprising the step of cleaning the surface of the semiconductor using an in-situ gas-phase process using a cleaning source selected from the group consisting of hydrogen gas and hydrogen plasma prior to the step of exposing the surface of the semiconductor to a gas-phase sulfur precursor.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

(1) A more complete understanding of the embodiments of the present disclosure may be derived by referring to the detailed description and claims when considered in connection with the following illustrative figures.

(2) FIG. 1 illustrates an exemplary system in accordance with various exemplary embodiments of the disclosure;

(3) FIG. 2 illustrates a comparison of an amount of sulfur on a surface of high-mobility semiconductor surfaces treated with gas-phase and aqueous-phase passivation processes;

(4) FIGS. 3(a)-3(c) illustrate XPS graphs of a semiconductor surface passivated with a gas-phase sulfur precursor;

(5) FIGS. 4(a)-4(d) illustrate capacitance-voltage characteristics of structures including passivated and non-passivated high-mobility semiconductor surfaces;

(6) FIGS. 5(a)-5(c) illustrate additional capacitance-voltage characteristics of structures including passivated and non-passivated high-mobility semiconductor surfaces; and

(7) FIGS. 6(a)-6(c) illustrate additional capacitance-voltage characteristics of germanium structures including passivated and non-passivated surfaces.

(8) It will be appreciated that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve the understanding of illustrated embodiments of the present disclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

(9) The description of exemplary embodiments of systems, methods, and devices provided below is merely exemplary and is intended for purposes of illustration only; the following description is not intended to limit the scope of the disclosure. Moreover, recitation of multiple embodiments having stated features is not intended to exclude other embodiments having additional features or other embodiments incorporating different combinations of the stated features.

(10) As set forth in more detail below, the systems and methods described herein can be used to passivate a surface of a semiconductor (e.g., a high mobility semiconductor, such as germanium (Ge) or III-V semiconductor). As used herein, surface means any portion of an exposed semiconductor surface. For example, the surface may be the entire exterior of a semiconductor wafer and/or layer or a portion thereof or a top surface of a semiconductor wafer and/or layer thereon or a portion of either.

(11) Turning now to FIG. 1, a system 100 for passivating a semiconductor surface is illustrated. System 100 includes a reactor 102, including a reaction chamber 103, a substrate holder 104, and a gas distribution system 106; a sulfur precursor source 108; a carrier or purge gas source 110; and valves 112, 114 interposed between the sources 108, 110 and reactor 102.

(12) Reactor 102 may be a standalone reactor or part of a cluster tool. Further, reactor 102 may be dedicated to a surface passivation process as described herein, or reactor 102 may be used for other processese.g., for layer deposition and/or etch processing. For example, reactor 102 may include a reactor typically used for chemical vapor deposition (CVD) and/or atomic layer deposition (ALD) processing, and may include direct plasma, and/or remote plasma apparatus. Using a plasma during the passivation process may enhance the reactivity of the sulfur precursor. Further, reactor 102 may operate under a vacuum or near atmospheric pressure. By way of one example, reactor 102 includes a reactor suitable for subsequent ALD deposition of a dielectric material onto substrate 116. An exemplary ALD reactor suitable for system 100 is described in U.S. Pat. No. 8,152,922, the contents of which are hereby incorporated herein by reference, to the extent such contents do not conflict with the present disclosure.

(13) Substrate holder 104 is designed to hold substrate or workpiece 116, having a semiconductor surface, in place during processing. In accordance with various exemplary embodiments, holder 104 may form part of a direct plasma circuit. Additionally or alternatively, holder 104 may be heated, cooled, or be at ambient process temperature during processing.

(14) Although gas distribution system 106 is illustrated in block form, gas distribution system 106 may be relatively complex and designed to mix vapor (gas) from sulfur precursor source 108 and carrier/purge gas from one or more sources, such as gas source 110, prior to distributing the gas mixture to remainder of reactor 102. Further, system 106 may be configured to provide vertical (as illustrated) or horizontal flow of gasses to the semiconductor surface. An exemplary gas distribution system is described in U.S. Pat. No. 8,152,922.

(15) Sulfur precursor source 108 may be a liquid, solid, or gas source of sulfur-containing material suitable for passivating a semiconductor surface. If sulfur precursor source 108 is liquid or solid, the source material is vaporized prior to entering reaction chamber 103. Exemplary sulfur precursors for source 108 include (NH.sub.4).sub.2S solution ((NH.sub.4).sub.2S in aqueous solutione.g., 22% solution), H.sub.2S gas, NH.sub.4HS solid, and organosulfur compounds, such as thiourea, SC(NH.sub.2).sub.2.

(16) Sulfur precursor 108 may be used to passivate a variety of semiconductor material surfaces. For example, the precursor may be used to passivate doped or undoped high mobility semiconductors, such as germanium, and III-V semiconductors, such as GaAs, InGaAs, other III-V semiconductors including Ga and/or As, and other III-V materials. By way of example, system 100 may be used to passivate a semiconductor surface including doped or undoped InGaAs, such as In.sub.0.53Ga.sub.0.47As.

(17) Carrier or purge gas source 110 may include any suitable carrier gas suitable for mixing with the sulfur precursor from source 108 and/or any gas suitable for purging reactor 102 before and/or after sulfur passivation of the semiconductor surface. In accordance with exemplary embodiments of the disclosure, a purge gas may be nitrogen, argon, helium, or a combination thereof. The carrier gas may be nitrogen, argon, helium, or a combination thereof.

(18) System 100 may also include a cleaning source 116, which includes solid, liquid, or gas phase chemicals to clean the semiconductor surface prior to passivation. For example, source 116 may include chemicals, which are gas-phase when entering chamber 113, to remove native oxides from the semiconductor surface. Exemplary chemicals suitable for source 116 include HCl, HF, NH.sub.4OH, H.sub.2, and hydrogen plasma.

(19) As illustrated in FIG. 1, sources 108, 110, and 116 are in fluid communication with reactor 102 via valves 112, 114, and 118, which may be used to control the flow, mixing, and distribution of the respective source materials to reactor 102 using supply lines 120-124.

(20) During a semiconductor surface passivation process, wafer or workpiece 116 is placed in chamber 103 of reactor 102, and reactor 102 is brought to a desired pressure (e.g., about 1 to about 10 Torr) using vacuum pump 126. Sulfur precursor material from source 108 is introduced into the reaction chamber 103 of reactor 102. If desired, carrier gas from source 110 may be mixed with the sulfur precursore.g., using gas distribution system 106, prior to the sulfur precursor entering chamber 103.

(21) The passivation process conditions may vary according to a number of factors, including, for example, a substrate size, a substrate type, prior substrate processing steps, a type of reactor, a size of the reactor, and the sulfur precursor. Table 1 below illustrates exemplary process conditions for thermal and plasma passivation processes.

(22) TABLE-US-00001 TABLE 1 CONDITIONS FOR THERMAL TREATMENT Sulfur source (e.g., Sulfur precursor (e.g., Reaction (NH.sub.4).sub.2S solution) vapor (NH.sub.4).sub.2S solution) vapor chamber Temperature flux to reaction chamber concentration in reaction pressure Treatment C. (sccm) chamber (%) (Torr) time(s) Room 100-4000 5-90 or 5-95 0.5 to 1-600 temperature atmospheric (RT)-350 or (e.g., 760) RT-400 or 1-10 CONDITIONS FOR PLASMA TREATMENT Sulfur precursor Sulfur source (e.g., (NH.sub.4).sub.2S) (e.g., (NH.sub.4).sub.2S solution vapor Reaction solution) vapor concentration in chamber Plasma Plasma Temperature flux to reaction reaction chamber pressure Treatment Power C. chamber (sccm) (%) (Torr) time(s) (W) RT-350 or 100-4000 5-90 or 5-95 0.5-10 or 0.1-600 25-1000 RT to 400 1-10

(23) A temperature of the passivation process may be the same as a temperature used for subsequent processing of workpiece 116 (e.g., deposition of a dielectric material, such as a high dielectric constant material), in which case, the passivation process and dielectric material deposition process may take place in the same reactor/chamber. Performing both steps in the same reactor may be advantageous, because the subsequent deposition can be performed without breaking the vacuum conditions; thus workpiece 116 exposure to air or an oxidizing environment can be reduced. However, the passivation process can be performed in a separate chamber, and using a separate chamber may be desirable if the process temperature of the passivation process is different from the temperature used for subsequent workpiece 116 processing.

(24) The method may also include depositing dielectric material, such as high-k dielectric material onto the passivated semiconductor surface, which, as noted above, can be done in the same reactor or a different reactor as the gas-phase passivation process. If done in separate reactors, the reactors may or may not be part of the same cluster tool.

(25) Exemplary high-k materials that may be deposited onto the passivated surface include forms of metallic oxides with dielectric constants (k values) greater than about 7. Such materials include magnesium oxide (MgO), aluminum oxide (Al.sub.2O.sub.3), zirconium oxide (ZrO.sub.2), hafnium oxide (HfO.sub.2), hafnium silicon oxide (HfSiO), tantalum oxide (Ta.sub.2O.sub.5), tantalum silicon oxide (TaSiO), barium strontium titanate (BST), strontium bismuth tantalate (SBT), and lanthanide oxides, oxides of physically stable rare earth elements as scandium (Sc), yttrium (Y), lanthanum (La), cerium Ce, praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu), as well as silicon nitride (SiN).

(26) Using a gas-phase passivation process has several advantages compared to wet solution passivation processes. For example, the exposure to air between passivation and deposition of the dielectric material can be significantly reduced or eliminated or nearly eliminated using a gas-phase process. As a result, devices with much lower D.sub.it values and consequently better performance can be made using the techniques and system described herein.

SPECIFIC EXAMPLES

(27) The following non-limiting examples illustrate a process of passivating a surface of high-mobility semiconductor material and a device or structure formed using the process. These examples are merely illustrative, and it is not intended that the disclosure be limited to the examples.

Comparative Example 1

(28) An n-doped In.sub.0.53Ga.sub.0.47As semiconductor surface was epitaxially grown over a two inch InP substrate. The native oxide on the In.sub.0.53Ga.sub.0.47As surface was removed using a diluted HCl solution (37% HCl diluted10 with deionized water) at room temperature for about sixty seconds. The samples were then rinsed twice with deionized water for 15 seconds/rinse and dried with a nitrogen gun. The surface was passivated by immersing the workpiece in a liquid solution of (NH.sub.4).sub.2S.

(29) An ALD Al.sub.2O.sub.3 layer was then deposited onto the passivated workpiece surface using trimethylaluminium (TMA) and H.sub.2O as the precursors. The thickness of the Al.sub.2O.sub.3 layer was about 1 nm for XPS analysis and about 5 nm for electrical characterization samples. For the samples used for electrical characterization, the dielectric material was annealed at about 400 C. for about five minutes in a 10% hydrogen in nitrogen environment.

Example 1

(30) An n-doped In.sub.0.53Ga.sub.0.47As semiconductor surface was epitaxially grown over a two inch InP substrate. The native oxide on the In.sub.0.53Ga.sub.0.47As surface was removed using a diluted HCl solution as set forth in the Comparative Example, and the samples were loaded into a reactor chamber within five minutes of completing the clean, to control an amount of time the surface was exposed to ambient air.

(31) The surface was passivated with sulfur by exposing the surface to a mixture of carrier gas (nitrogen) and a sulfur precursor source (22% (NH.sub.4).sub.2S solution source). The reactor chamber temperature was about 300 C., the reaction time about 5 minutes, and the chamber pressure at about 4 Torr.

(32) An in situ ALD Al.sub.2O.sub.3 layer was then deposited onto the passivated workpiece surface using TMA and H.sub.2O as the precursors, without breaking vacuum between the passivation and dielectric material deposition steps. The thickness of the Al.sub.2O.sub.3 layer was about 1 nm for XPS analysis and about 5 nm for electrical characterization samples. For the samples used for electrical characterization, the dielectric material was then annealed at about 400 C. for about five minutes in a 10% hydrogen in nitrogen environment.

(33) FIG. 2 illustrates XPS analysis of the samples, showing that a greater amount of sulfur was detected on the semiconductor surfaces of workpieces processed using the in situ vapor passivation process, compared to semiconductor surfaces treated with an aqueous immersion passivation process.

(34) FIGS. 3(a)-(c) illustrate XPS analysis results of 1 nm of Al.sub.2O.sub.3 deposited on In.sub.0.53Ga.sub.0.47As using the in situ sulfur vapor treatment of Example 1. In particular, FIG. 3(a) illustrates a single peak indium 3d; FIG. 3(b) illustrates a single peak for arsenic 3d; and FIG. 3(c) illustrates multiple XPS peaks for gallium 3p, all of which indicate a lack of oxygen at the interface between the semiconductor surface and the dielectric layer.

Example 2

(35) An n-doped In.sub.0.53Ga.sub.0.47As semiconductor surface was epitaxially grown over a two inch InP substrate. The native oxide on the In.sub.0.53Ga.sub.0.47As surface was removed using a diluted HCl solution as set forth in the Example 1. The surface was passivated by ex situ (NH.sub.4).sub.2S vapor. An ALD Al.sub.2O.sub.3 layer was then deposited onto the passivated workpiece surface using TMA and H.sub.2O as the precursors. The thickness of the Al.sub.2O.sub.3 layer was about 1 nm for XPS analysis and about 5 nm for electrical characterization samples. For the samples used for electrical characterization, the dielectric material was then annealed at about 400 C. for about five minutes in a 10% hydrogen in nitrogen environment.

(36) FIGS. 4(a)-4(c) illustrate improved frequency dispersion capacitance-voltage (C-V) characteristic of the devices formed in accordance with Examples 1 and 2, compared to the Comparative Example and to samples with no passivation. The C-V dispersion measurements were performed over frequencies in the range of about 100 Hz to about 1 MHz at room temperature and at 77 K to extract trap densities.

(37) Table 2 lists the capacitance dispersion at the depletion and accumulation regions of structures/devices formed using no passivation and the processes of the Comparative Example and Examples 1 and 2. All passivated samples exhibit improved properties compared to the samples with no passivation. The ex-situ vapor passivation treatment exhibits the lowest dispersion while the in-situ vapor and solution-based passivation treatments yield slightly higher values. In order to map the interface state across the bandgap, CV curves were generated at temperatures down to 77 K. Table 3 shows the D.sub.it values at the mid gap and conduction band edge. The D.sub.it values listed in Table 3 were extracted by using conduce method at 300 K (near mid-gap) to 77 K (close to band edge). The D.sub.it values of passivated samples are reduced to a level on the order of 1E12/cm.sup.2 eV. These low values of D.sub.it are expected to yield high mobility transistor devices with better performance.

(38) TABLE-US-00002 TABLE 2 Capacitance Dispersion (% Decade) Treatment Strong Accumulation Depletion No Passivation 3.19 7.1 Comparative Example 1 3.09 4.86 Example 2 2.8 2.76 Example 1 3.41 4.57

(39) TABLE-US-00003 TABLE 3 Samples D.sub.it at midgap D.sub.it near band edge No Passivation 2.1 e.sup.12(/cm.sup.2eV) 2 e.sup.12(/cm.sup.2eV) Example 2 1.5 e.sup.12(/cm.sup.2eV) 2.5 e.sup.11(/cm.sup.2eV) Comparative Example 1 1.5 e.sup.12(/cm.sup.2eV) 5 e.sup.11(/cm.sup.2e4) Example 1 1.8 e.sup.12(/cm.sup.2eV) 1 e.sup.12(/cm.sup.2eV)

Example 3

(40) An n-doped In.sub.0.53Ga.sub.0.47As semiconductor surface was epitaxially grown over a two inch InP substrate. The native oxide on the In.sub.0.53Ga.sub.0.47As surface was removed using a diluted HCl solution as set forth in the Comparative Example, and the samples were loaded into a reactor chamber within five minutes of completing the clean, to control an amount of time the surface was exposed to ambient air.

(41) The surface was passivated with sulfur by exposing the surface to a mixture of carrier gas (nitrogen) and a sulfur precursor source (22% (NH.sub.4).sub.2S solution source). The reactor chamber temperature was about 300 C., the reaction time about 5 minutes, and the chamber pressure at about 4 Torr.

(42) An in situ ALD Al.sub.2O.sub.3 layer was then deposited onto the passivated workpiece surface using TMA and H.sub.20 as the precursors, without breaking vacuum between the passivation and dielectric material deposition steps. An in-situ layer of hafnium oxide was then formed overlying the aluminum oxide layer using HfCl.sub.4 and water as precursors. The thickness of the Al.sub.2O.sub.3 layer was about 1 nm and the hafnium oxide layer thickness was about 3 nm. The samples were annealed at about 400 C. for about five minutes in a 10% hydrogen in nitrogen environment.

Example 4

(43) Structures were prepared according to Example 3, except H.sub.2S in a nitrogen carrier gas was used to passivate the HCl-cleaned surface prior to the aluminum oxide and hafnium oxide deposition steps.

(44) Table 4 below and FIGS. 5(a)-5(c) illustrate electrical characteristics of samples formed in accordance with Examples 3 and 4 and samples with an HCl-cleaned surface and no passivation treatment prior to the aluminum oxide and hafnium oxide deposition. As illustrated in FIGS. 5(b)-5(c), structures formed in accordance with Examples 3 and 4 exhibit improved frequency dispersion capacitance-voltage (C-V) characteristic, compared to sample with no passivation (FIG. 5(a)). The C-V dispersion measurements were performed and the D.sub.it values were extracted using the techniques described above. The D.sub.it values of passivated samples are reduced to a level about 1.8E12/cm.sup.2 eV (Example 3) and 1.6E12/cm.sup.2 eV (Example 4) or to about 60 percent (Example 3) and 53 percent (Example 4) of the D.sub.it of similarly formed structures without a passivation step. These low values of D.sub.it are expected to yield high mobility transistor devices with better performance.

(45) TABLE-US-00004 TABLE 4 D.sub.it@midgap Dispersion in strong CV Treatment CET (/eVcm.sup.2) accumulation hysteresis HCI only 1.7 nm 3e12 4.9%/dec 100 mV Example 3 1.6 nm 1.8e12 5%/dec 60 mV Example 4 1.6 nm 1.6e12 5%/dec 50 mV

Example 5

(46) Structures in accordance with Example 5 were prepared using a method similar to the method used to form the structures of Example 4, except about 4 nm of aluminum oxide, rather than a combination of aluminum oxide and hafnium oxide, is deposited onto a cleaned surface.

Example 6

(47) Similarly, structures in accordance with Example 6 were prepared using a method similar to the method used to form the structures of Example 3, except about 4 nm of aluminum oxide, rather than a combination of aluminum oxide and hafnium oxide, is deposited onto a cleaned surface.

(48) Table 5 below and FIGS. 6(a)-6(c) illustrate electrical characteristics of structures formed in accordance with Examples 5 and 6 and structures with an HCl-cleaned surface and no passivation treatment prior to the aluminum oxide deposition. As illustrated in FIGS. 6(b)-6(c), structures formed in accordance with Examples 5 and 6 exhibit improved frequency dispersion capacitance-voltage (C-V) characteristic, compared to a sample with no passivation (FIG. 6(c)). The C-V dispersion measurements were performed and D.sub.it values were obtained using the techniques described above. Table 5 shows the D.sub.it values at the mid gap were significantly reduced. The D.sub.it values are reduced to a level about 6.0E11/cm.sup.2 eV for structures formed in accordance with both Example 5 and Example 6, or to about 20 percent of the D.sub.it value of similarly formed structures without a passivation step. These low values of D.sub.it are expected to yield high mobility transistor devices with better performance

(49) TABLE-US-00005 TABLE 5 D.sub.it@midgap Dispersion in strong CV Treatment CET (/eVcm.sup.2) accumulation hysteresis HCI only 2.9 <3.0e12 0.8%/dec 10 mV Example 5 3.1 6.0e11 1.1%/dec 20 mV Example 6 3.1 6.0e11 1.0%/dec 10 mV

(50) Although exemplary embodiments of the present disclosure are set forth herein, it should be appreciated that the disclosure is not so limited. For example, although the systems, methods, devices, and structures are described in connection with various process parameters, the disclosure is not so limited. Various modifications, variations, and enhancements of the system and method set forth herein may be made without departing from the spirit and scope of the present disclosure as set forth in the following claims and their equivalents.