# Patent

## Title
Integrated circuit transistor structure with high germanium concentration SiGe stressor

## Abstract

An integrated circuit transistor structure includes a semiconductor substrate, a first SiGe layer in at least one of a source area or a drain area on the semiconductor substrate, and a channel between the source area and the drain area. The first SiGe layer has a Ge concentration of 50 percent or more.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is a divisional of U.S. application Ser. No. 12/831,842, filed Jul. 7, 2010, which claims priority of U.S. Provisional Application No. 61/229,253, filed Jul. 28, 2009. The above-listed applications are incorporated herein by reference in their entireties.
The present application is related to U.S. patent application Ser. No. 12/707,788, filed on Feb. 18, 2010, titled MEMORY POWER GATING CIRCUIT AND METHODS; Ser. No. 12/758,426, filed on Apr. 12, 2010, titled FINFETS AND METHODS FOR FORMING THE SAME; Ser. No. 12/731,325, filed on Mar. 25, 2010, titled ELECTRICAL FUSE AND RELATED APPLICATIONS; Ser. No. 12/724,556, filed on Mar. 16, 2010, titled ELECTRICAL ANTI-FUSE AND RELATED APPLICATIONS; Ser. No. 12/757,203, filed on Apr. 9, 2010, titled STI STRUCTURE AND METHOD OF FORMING BOTTOM VOID IN SAME; Ser. No. 12/797,839, filed on Jun. 10, 2010, titled FIN STRUCTURE FOR HIGH MOBILITY MULTIPLE-GATE TRANSISTOR; Ser. No. 12/831,842, filed on Jul. 7, 2010, titled METHOD FOR FORMING HIGH GERMANIUM CONCENTRATION SiGe STRESSOR; Ser. No. 12/761,686, filed on Apr. 16, 2010, titled FINFETS AND METHODS FOR FORMING THE SAME; Ser. No. 12/766,233, filed on Apr. 23, 2010, titled FIN FIELD EFFECT TRANSISTOR; Ser. No. 12/757,271, filed on Apr. 9, 2010, titled ACCUMULATION TYPE FINFET, CIRCUITS AND FABRICATION METHOD THEREOF; Ser. No. 12/694,846, filed on Jan. 27, 2010, titled INTEGRATED CIRCUITS AND METHODS FOR FORMING THE SAME; Ser. No. 12/638,958, filed on Dec. 14, 2009, titled METHOD OF CONTROLLING GATE THICKNESS IN FORMING FINFET DEVICES; Ser. No. 12/768,884, filed on Apr. 28, 2010, titled METHODS FOR DOPING FIN FIELD-EFFECT TRANSISTORS; Ser. No. 12/731,411, filed on Mar. 25, 2010, titled INTEGRATED CIRCUIT INCLUDING FINFETS AND METHODS FOR FORMING THE SAME; Ser. No. 12/775,006, filed on May 6, 2010, titled METHOD FOR FABRICATING A STRAINED STRUCTURE; Ser. No. 12/886,713, filed Sep. 21, 2010, titled METHOD OF FORMING INTEGRATED CIRCUITS; Ser. No. 12/941,509, filed Nov. 8, 2010, titled MECHANISMS FOR FORMING ULTRA SHALLOW JUNCTION; Ser. No. 12/900,626, filed Oct. 8, 2010, titled TRANSISTOR HAVING NOTCHED FIN STRUCTURE AND METHOD OF MAKING THE SAME; Ser. No. 12/903,712, filed Oct. 13, 2010, titled FINFET AND METHOD OF FABRICATING THE SAME; 61/412,846, filed Nov. 12, 2010, 61/394,418, filed Oct. 19, 2010, titled METHODS OF FORMING GATE DIELECTRIC MATERIAL and 61/405,858, filed Oct. 22, 2010, titled METHODS OF FORMING SEMICONDUCTOR DEVICES.


TECHNICAL FIELD
This disclosure relates generally to integrated circuits, and more particularly, to a SiGe source-drain stressor in a semiconductor transistor structure.
BACKGROUND
Strain engineering is employed in semiconductor manufacturing to enhance device performance. Performance benefits are achieved by modulating strain in the transistor channel, which enhances electron mobility (or hole mobility) and thereby conductivity through the channel.
In CMOS technologies, PMOS and NMOS respond differently to different types of strain. Specifically, PMOS performance is best served by applying compressive strain to the channel, whereas NMOS receives benefit from tensile strain. SiGe (Si1-xGex), consisting of any molar ratio of silicon and germanium, is commonly used as a semiconductor material in integrated circuits (ICs) as a strain-inducing layer for strained silicon in CMOS transistors.
Strained silicon is a layer of silicon in which the silicon atoms are stretched beyond their normal inter atomic distance. This can be accomplished by putting the layer of silicon over a substrate of silicon germanium (SiGe), for example. As the atoms in the silicon layer align with the atoms of the underlying silicon germanium layer, which are arranged farther apart with respect to those of a bulk silicon crystal, the links between the silicon atoms become stretched—thereby leading to strained silicon.
Currently, the PMOS strain is realized by undercutting the source/drain area and epitaxially growing SiGe film in the undercut region. The larger lattice constant of the SiGe film provides the uniaxial strain to the Si channel. The higher the Ge concentration, the larger the strain and thus better performance. However, the Ge incorporation into the SiGe film is limited by the epitaxial process. Very high Ge concentration SiGe film is difficult to realize using the conventional epitaxial method, which is extremely sensitive to surface preparation, pre-cursors used and growth conditions. It is challenging to meet the ever-increasing Ge concentration requirement and maintain proper control of the SiGe profile for the SiGe source/drain (S/D) in PMOS with epitaxial growth.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of exemplary embodiments of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIG. 1A-FIG. 1C illustrate an exemplary process for SiGe stressor with high Ge concentration according to one embodiment of this invention;
FIG. 2A-FIG. 2F illustrate one embodiment of the process for a planar device with strained source drain (SSD);
FIG. 3A-FIG. 3F illustrate another embodiment of the process for a planar device without SSD;
FIG. 4A-FIG. 4F illustrate one embodiment of the process for a FinFET or Trigate device with strained source drain (SSD); and
FIG. 5A-FIG. 5F illustrate another embodiment of the process for a FinFET or Trigate device without SSD.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.
A method for producing a SiGe stressor with high Ge concentration is provided. This method converts a SiGe film with low Ge concentration into a SiGe film with very high Ge concentration, without the need to epitaxially form a high Ge concentration source/drain (S/D). By having a separate oxidation step and diffusion step for the condensation process, high Ge concentration SiGe profile can be formed and properly controlled. This method is applicable to both planar and FinFET devices on either bulk Si or SOI (silicon on insulator) substrate. Throughout the various views and illustrative embodiments of the present invention, like reference numbers are used to designate like elements.
FIGS. 1A-1C illustrate an exemplary process for a SiGe stressor with high Ge concentration. In FIG. 1A, a SiGe film layer 106 and a Si cap layer 108 are deposited on the Si substrate 102 (the substrate 102 can also be SiGe substrate or relaxed SiGe film on Si substrate) beside spacer layer 116 in the S/D area. The channel area 104 below the gate 118 can comprise Si or SiGe channel. The Si cap layer 108 serves as the starting point for oxidation. The oxide quality of a SiGe film in general can be poor and its oxidation front can be hard to control without the Si cap layer 108. Having the Si cap layer 108 helps to form a uniform oxide layer 112 to begin with and makes the overall condensation profile easier to control.
In FIG. 1B, a low temperature thermal oxidation (dry or wet, preferably wet to lower the temperature) is performed to form a localized high Ge percentage SiGe film 110 at the bottom interface with little or completely without Ge diffusion through SiGe/Si interface into the substrate 102. This thermal oxidation step can also comprise at least two stages each with different temperatures and durations to ensure little to no Ge diffusion during the process. In FIG. 1C, a thermal diffusion is performed to form SiGe stressor 114 with the desired profile and depth. Later, the top oxide layer 112 is removed and regular processing can continue. The oxidation and thermal diffusion steps can be combined and performed simultaneously if an optimal temperature can be identified to simultaneously achieve oxidation and desired diffusion profile.
The SiGe film layer 106 has a composition ratio of Si:Ge=1−x: x (i.e., Si1−xGex), while the condensed SiGe stressor film 114 has a composition ratio of Si:Ge=1−y:y (i.e. Si1−yGey), where x and y represent the Ge percentage in each SiGe film (y>x). The SiGe stressor layer 114 has a higher Ge concentration than deposited SiGe film layer 106, and applies uniaxial compressive strain to the channel 104.
By having a separate (low-temperature) oxidation step and a Ge diffusion step for the condensation process, a higher Ge concentration profile in SiGe S/D stressor film 114 can be formed and properly controlled from the originally deposited SiGe film 106 having a low Ge concentration percentage. Low temperature thermal oxidation (e.g., below 800° C. for x<0.5 and below 600° C. for x approaching 1, where x is the number in Si1−xGex) that does not incur Ge diffusion is to obtain a localized, non-diffused high Ge percentage SiGe film 110. This can be achieved through high water vapor pressure wet oxidation, for example.
The two-step process of oxidation and diffusion eliminates the need for silicon on insulator (SOI) substrate in the conventional condensation process, which relies on the presence of oxide to block uncontrolled Ge diffusion. Disclosed embodiments of the present invention are also applicable to three-dimensional (3D) structures. The condensed high Ge concentration SiGe stressor area 114 is naturally close to the surface channel 104, resulting in effective strain on the device channel. Also, disclosed embodiments of the present invention work on high Ge percentage SiGe channel (e.g., Si0.5Ge0.5) devices, which by itself alone cannot compete performance-wise against a typical uniaxial strained Si device without further stress from the S/D.
FIGS. 2A-2F illustrate one embodiment of the process for a planar device with strained source drain (SSD). In FIG. 2A, the process starts with SSD and SiGe epi layer 202. The substrate 102, channel 104, dielectric layer 204, gate 206, and sidewall spacer 208 are also shown. In FIG. 2B, a SiGe film 106 (Ge concentration can be either higher or lower than that of 202) is deposited on top of the SiGe epi layer 202. Optionally, a Si cap layer 108 is deposited on top of the SiGe film 106 to help uniform oxidation as shown in FIG. 2C. After low temperature oxidation in FIG. 2D, high Ge percentage SiGe film 110 and oxidation layer 112 are formed. In FIG. 2E, thermal diffusion is performed to form a condensed high Ge concentration SiGe stressor film 114. The Ge concentration of the SiGe film 114 is higher than the original SiGe epi layer 202. In FIG. 2F, the oxide layer 112 can be removed.
FIGS. 3A-3F illustrate another embodiment of the process for a planar device without SSD according to one aspect of this invention. In FIG. 3A, the process starts without SSD. The substrate 102, channel layer 104, dielectric layer 204, gate 206, and sidewall spacer 208 are shown. In FIG. 3B, a SiGe film 106 is deposited on top of the channel layer 104. Optionally, a Si cap layer 108 is deposited on top of the SiGe film 106 to help uniform oxidation as shown in FIG. 3C. After low temperature oxidation in FIG. 3D, high Ge percentage SiGe film 110 and oxidation layer 112 are formed. In FIG. 3E thermal diffusion is performed to form a condensed high Ge concentration SiGe stressor 114. In FIG. 3F, the oxide layer 112 can be removed.
FIGS. 4A-4F illustrate one embodiment of the process for a FinFET device with strained source drain (SSD). FIG. 4A shows a cross section of Si S/D fin area 402 on substrate 102 and shallow trench isolation 406 along a gate direction. In FIG. 4B, anisotropic SSD etch and SiGe epi growth processes are performed on the Si S/D fin area 402 on top of Si substrate 102 to form SiGe film layer 106. In FIG. 4C, a Si cap layer 108 can be optionally deposited on top of the SiGe film 106 to help uniform oxidation. After low temperature oxidation in FIG. 4D, high Ge percentage SiGe film 110 and oxidation layer 112 are formed over the fin area 404. In FIG. 4E, thermal diffusion is performed to form a condensed high Ge concentration SiGe stressor 114. The Ge concentration of the SiGe stressor 114 is higher than the SiGe film 106. It is also possible that the entire S/D area, 404, is uniformly converted to higher Ge concentration SiGe (i.e., 404=114). In FIG. 4F, the oxide layer 112 can be removed.
FIGS. 5A-5F illustrate another embodiment of the process for a FinFET device without SSD. FIG. 5A shows a cross section of Si S/D fin area 402 on substrate 102 and shallow trench isolation 406 along a gate direction. In FIG. 5B, a SiGe film layer 106 is grown over the Si S/D Fin area 402 on top of Si substrate 102. In FIG. 5C, a Si cap layer 108 can be optionally deposited on top of the SiGe film 106 to help uniform oxidation. After low temperature oxidation in FIG. 5D, high Ge percentage SiGe film 110 and oxidation layer 112 are formed over the fin area 402. In FIG. 5E, thermal diffusion is performed to form a condensed high Ge concentration SiGe stressor 114. The Ge concentration of the SiGe stressor 114 is higher than the SiGe film 106. Again, it is possible that the entire S/D area, 404, is uniformly converted to higher Ge concentration SiGe. In FIG. 5F, the oxide layer 112 can be removed.
The advantageous features of disclosed embodiments of the present invention include much higher uniaxial compressive stress on the channel achieved without redeveloping a SiGe process that relieves the pressure of forming higher and higher Ge concentration SiGe film with epitaxy, and the elimination of critical thickness constraint, i.e., strained source drain (SSD) depth. Also, the present methods can provide additional uniaxial strain from higher Ge concentration SiGe S/D on SiGe channel on Si substrate structure that already has biaxial strain (e.g., Si0.5Ge0.5 channel with SiGe S/D having Ge greater than 50%). A skilled person in the art will appreciate that there can be many embodiment variations.
In some embodiments, an integrated circuit transistor structure comprises a semiconductor substrate, a first SiGe layer in at least one of a source area or a drain area on the semiconductor substrate, and a channel between the source area and the drain area. The first SiGe layer has a Ge concentration of 50 percent or more.
In some embodiments, an integrated circuit transistor structure comprises a semiconductor substrate, a fin structure over the semiconductor substrate, and a first SiGe layer over top and side surfaces of the fin structure.
In some embodiments, an integrated circuit transistor structure comprises a semiconductor substrate, a first SiGe layer over an active region of the semiconductor substrate, and a second SiGe layer under the first SiGe layer. The second SiGe layer has a lower Ge concentration than the first SiGe layer.
Although exemplary embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure herein, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

## Claims

1. An integrated circuit transistor structure, comprising:
a semiconductor substrate;
a gate disposed on the semiconductor substrate and sidewall spacers abutting the gate;
a first SiGe layer in at least one of a source area or a drain area on the semiconductor substrate, wherein the first SiGe layer has a Ge concentration greater than 50 percent, and a Si concentration of greater than 0%, and the first SiGe layer is continuous across the source area or the drain area and extends under a portion of the gate and under the sidewall spacers;
a second SiGe layer under the first SiGe layer, wherein the second SiGe layer has a lower Ge concentration than the first SiGe layer, wherein the second SiGe layer extends under the gate; and
a channel between the source area and the drain area, wherein the channel comprises SiGe and the Ge concentration of the second SiGe layer is higher than a Ge concentration of SiGe in the channel
wherein the integrated circuit transistor structure is a planar transistor structure.

2. The structure of claim 1, wherein the semiconductor substrate comprises one selected from the group consisting of Si, SiGe, and a relaxed SiGe film on Si.

3. The structure of claim 1, wherein the first SiGe layer is in both the source area and the drain area.

4. The structure of claim 3, further comprising:
a second SiGe layer under the first SiGe layer in each of the source area and the drain area,
wherein the second SiGe layer has a lower Ge concentration than the first SiGe layer.

5. An integrated circuit transistor structure, comprising:
a semiconductor substrate;
a fin structure of silicon over the semiconductor substrate;
a shallow trench isolation feature abutting the fin structure; and
a first SiGe layer over top and side surfaces of the fin structure, wherein the first SiGe layer interfaces two opposing sidewalls and a top surface of the silicon of the fin structure,
wherein the first SiGe layer over the side surfaces of the fin structure further extends such that a top surface of the shallow trench isolation feature is higher than a bottommost edge of the first SiGe layer.

6. The structure of claim 5, wherein the semiconductor substrate is silicon.

7. An integrated circuit transistor structure, comprising:
a semiconductor substrate;
a fin structure over the semiconductor substrate; and
a first SiGe layer over top and side surfaces of the fin structure, wherein the fin structure comprises a second SiGe layer and a portion of the fin structure is free of germanium, and
wherein the second SiGe layer has a lower Ge concentration than the first SiGe layer.

8. The structure of claim 7, wherein the second SiGe layer is the middle portion of the fin structure.

9. The structure of claim 7, wherein the semiconductor substrate comprises one selected from the group consisting of Si, SiGe, and a relaxed SiGe film on Si.

10. The structure of claim 7, wherein the first SiGe layer extends into the semiconductor substrate.

11. The structure of claim 5, wherein
the first SiGe layer has a first portion lower than a second portion of the fin structure, and
the second portion has a lower Ge concentration than the first portion.

12. The structure of claim 5, wherein
the fin structure extends upwardly from a portion of the semiconductor substrate,
the isolation structure is co-elevational with and contacts the portion of the semiconductor substrate, and
a portion of the first SiGe layer is disposed on a top surface of the isolation structure.

13. The structure of claim 7, wherein
the first SiGe layer has a first portion lower than a second portion of the second SiGe layer, and
the second portion of the second SiGe layer has a lower Ge concentration than the first portion of the first SiGe layer.

14. The structure of claim 7, wherein the second SiGe layer comprises a further portion under the fin structure.

15. The structure of claim 14, wherein the first SiGe layer extends along the side faces of the fin structure and into the further portion of the second SiGe layer.

16. The structure of claim 14, further comprising:
an isolation structure over the semiconductor substrate,
wherein
the isolation structure is co-elevational with and contacts the further portion of the second SiGe layer, and
the first SiGe layer overlaps a boundary between the isolation structure and the further portion of the second SiGe layer.