# Patent

## Title
Method for fabricating multiple transistor devices on a substrate with varying threshold voltages

## Abstract

Fabrication of a first device on a substrate is performed by exposing a first device region, removing a portion of the substrate to create a trench in the first device region, forming a screen layer with a first dopant concentration in the trench on the substrate, and forming an epitaxial channel on the screen layer having a first thickness. On or more other devices are similarly formed on the substrate independent of each other with epitaxial channels of different thicknesses than the first thickness. Devices with screen layers having the same dopant concentration but with different epitaxial channel thicknesses have different threshold voltages. Thus, a wide variety of threshold voltage devices can be formed on the same substrate. Further threshold voltage setting can be achieved through variations in the dopant concentration of the screen layers.

## Background
TECHNICAL FIELD
The present disclosure relates in general to semiconductor devices and manufacturing processes and more particularly to a method for fabricating multiple transistor devices on a substrate with varying threshold voltages.
BACKGROUND
Cost effective electronic manufacturing requires transistor structures and manufacturing processes that are reliable at nanometer scales, and that do not require expensive or unavailable tools or process control conditions. While it is difficult to balance the many variables that control transistor electrical performance, finding suitable transistor dopant structures and manufacturing technique that result in acceptable electrical characteristics such as charge carrier mobility and threshold voltage levels are a key aspect of such commercially useful transistors.
SUMMARY
From the foregoing, it may be appreciated by those of skill in the art that a need has arisen for a technique to fabricate improved transistor devices that on a substrate that provide various threshold voltage control options and improved operational performance. In accordance with the following disclosure, there is provided a method for fabricating multiple transistor devices on a substrate with varying threshold voltages that substantially eliminates or greatly reduces disadvantages and problems associated with conventional transistor device fabrication and design.
According to an embodiment of the present disclosure, a method for fabricating multiple transistor devices on a substrate with varying threshold voltages includes forming a first device in a substrate having a screen layer with a first dopant concentration and an undoped epitaxial channel with a first thickness. A second device is formed in the substrate independent of the first device. The second device has a screen layer with a second dopant concentration and an undoped epitaxial channel with a second thickness where the second thickness is different than the first thickness such that the first and second devices have different threshold voltages. Other devices may be formed with varying epitaxial channel thicknesses and varying screen layer dopant concentrations to provide further threshold voltage options.
The present disclosure describes various technical advantages and features not present in conventional transistor fabrication and design. Embodiments of the present disclosure may enjoy some, all, or none of these advantages. Other technical advantages may be readily apparent to one skilled in the art from the following figures, description, and claims.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present disclosure, reference is made to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals represent like parts, in which:
FIGS. 1A to 1E illustrate a fabrication process for multiple transistor devices on a substrate with varying threshold voltage options;
FIG. 2 illustrates a chart showing threshold voltage, Ion current, and Ioff current for various screen layer dopant concentrations and undoped epitaxial channel thicknesses.



DETAILED DESCRIPTION
FIGS. 1A-1E show a process for manufacturing multiple transistor devices on a substrate 10 and some with varying threshold voltage settings. Devices with varying voltage threshold settings are generally categorized as Low Threshold Voltage (LVT), Super Low Threshold Voltage (SLVT), Standard Threshold Voltage (SVT), High Threshold Voltage (HVT), and Super High Threshold Voltage (SHVT).
In FIG. 1A, shallow trench isolation regions 12 are formed in substrate 10. Shallow trench isolation regions 12 are formed through use of any conventional fabrication techniques known in the industry. Shallow trench isolation regions 12 define device regions where separate devices are to be formed.
In FIG. 1B, formation of a transistor device begins by placing a photoresist layer 14 over substrate 10 to expose a first device region 100. At first device region 100, a first predetermined thickness of substrate is etched away or otherwise removed. Within the trench left by the removal process, a screen layer 20 is formed on the remaining substrate 10 preferably by way of ion implantation of dopants of the same type (n versus p) as the well, using implant dose and energy conditions selected to form a pre-defined thickness and concentration of dopants for screen layer 20. An undoped epitaxial layer 22 representing a channel for the transistor device is then formed on screen layer 20. By “undoped”, the inventors are referring to a preference to use an epitaxial growth process recipe selected to result in an intrinsic semiconductor. Undoped may refer to a degree of dopants inevitably formed in the semiconductor material, preferably in concentration of less than 1×1017 atoms per cm3, thereby enabling the semiconductor material to be formed in a substantially undoped state. The thickness preferences for the trench and epitaxial layer 22 are further discussed below. The thicknesses are selected to result in the targeted threshold voltage for the device. Generally, the thinner the epitaxial layer 22, the higher the threshold voltage. Therefore, the thinnest epitaxial layer 22 would be used for the SHVT devices, with a thicker epitaxial layer 22 used for HVT devices, a still thicker epitaxial layer 22 used for SVT devices, an even thicker epitaxial layer 22 used for LVT devices, and the thickest epitaxial layer 22 used for SLVT devices. The threshold voltage may be further controlled by way of the dose of the screen layer 20 implant, wherein the higher the dose of screen layer 20 results in a higher threshold voltage. Further tuning of the threshold voltage may be achieved by way of a controlled out-diffusion of material using thermal cycling of anneals, so that a limited diffusion upward of implanted dopants from screen layer 20 results in a further control of the resulting thickness of epitaxial layer 22 and therefore tuning of the threshold voltage. Though not shown, still another option to tune threshold voltage is by way of a separate threshold voltage control layer that may be formed on screen layer 20 prior to formation of epitaxial layer 22. Such separate threshold voltage control layer may preferably be achieved by out-diffusion using thermal cycling as set forth above, or by way of an additional implant step or steps after the screen layer 20 is formed, using the same dopant material or including an additional or different material. For instance, if the screen layer 20 is comprised of antimony of a concentration of 5×1019 atoms per cm3, the subsequent implant steps may result in a threshold voltage control layer above the screen layer 20 having a concentration of 1×1018 atoms per cm3, comprising antimony or a combination of antimony and arsenic. In other embodiments, a well region and, if desired, an anti-punch through region may be formed that lies below screen layer 20. Both the well region and anti-punch through region, if present, are doped using the same dopant type (n or p) as the screen layer 20 but having a concentration of at least to one order of magnitude less than the dopant concentration of screen layer 20.
For improved operation, screen layer 20 is preferably a highly uniform screen. In operation, a gate induced electric field and consequent depletion zone extends to the screening layer. Whether the channel above the screen is formed by an undoped blanket epitaxial layer that extends across multiple transistors or a selective epitaxial channel layer formed under each transistor, efforts are made to maintain the channel layer as undoped. Since there is minimal dopant presence in the channel, there is substantially no variation in dopant positioning or concentration in the channel, resulting in channels having high mobility and devices that are well matched. To further improve device matching, the screen layer 20 is preferably maintained as an atomically uniform layer that extends a selected distance from a gate dielectric.
The precise depth and thickness of the undoped epitaxial channel layer is preferably maintained over at least 80% of the gate dielectric area and depth and thickness may slightly increase or decrease along the edge of the gate dielectric due to well proximity or etch effects. Typically, adjacent transistors will have a gate dielectric to screen layer thickness that only varies within a one-half nanometer range, while more distant transistors on the same die will still have a channel layer thickness that varies within one nanometer. Additionally, the screen layer may have a dopant concentration between 1×1018 and 5×1020 atoms per cubic centimeter, and further may have a defined thickness of between five (5) and twenty (20) nanometers that preferably varies no more than three (3) nanometers. Maintaining a controlled thickness of screen layer 20 additionally helps to match leakage current and body bias related performance factors. Having a tight control of screen layer 20 positioning leads to a tight control of the depletion zone when the transistor gate is activated, which in turn enables better device matching than without a screen layer 20 and undoped epitaxial channel. For comparison, an epitaxial transistor device having a retrograde dopant profile that is conventionally formed using multiple implants has an irregular depletion zone. Similarly sized transistor devices with an atomically uniform screening layer 20 will have a uniform depletion zone set by screen layer 20 with minimal edge effects.
In FIG. 1C, similar processing steps may be performed to form additional transistor devices such as a second device region 200 and a third device region 300. Selective use of subsequent photoresist layers allow for separate exposure and formation of second device region 200 and third device region 300 as similarly achieved with first device region 100. The differences between first device region 100, second device region 200, and third device region 300 may lie in the thickness of the trench formed in each region and the resulting thickness of the undoped epitaxial layer formed therein. The difference in thickness of the undoped epitaxial layers provide devices with different threshold voltages. In this manner, devices with similar screen layers but different threshold voltages may be formed on a same wafer.
In FIG. 1D, a legacy device may also be formed in a fourth device region 400 through selective exposure and fabrication as performed with the device regions discussed above. Thus, a wafer may include devices with differing threshold voltages as well as devices of differing types. Thus, logic, analog, SRAM, and legacy devices may be fabricated on the same substrate. Gate, source, and drain regions may be formed at each device region using conventional techniques. Halo implants are not required for formation of each device.
In FIG. 1E, additional devices may be formed on the same wafer where threshold voltage differences are not only established through differing thicknesses in the respective undoped epitaxial layers but also through differences in dopant concentrations in the screen layers of the devices. For example, device regions 500 and 600 have screen layers with different dopant concentrations and undoped epitaxial layers with the same thickness. Device regions 700 and 800 have different dopant concentrations in the respective screen layers and different undoped epitaxial layer thicknesses. Device regions 500 and 700 have the same dopant concentrations in the respective screen layers and different undoped epitaxial layer thicknesses. Device regions 600 and 800 have the same dopant concentrations in the respective screen layers and different undoped epitaxial layer thicknesses. Thus, multiple devices may be fabricated in a substrate with various combinations of screen layer dopant concentrations and undoped epitaxial layer thicknesses. For those device regions where the undoped epitaxial layer is of the same thickness, formation of the undoped epitaxial layer in each region may be formed individually as discussed above or simultaneously through group exposure by selective photoresist placement.
FIG. 2 shows an example of a range of devices possible on a single wafer. By varying the dopant concentration of the screen layer and the thickness of the undoped epitaxial layer, varying threshold voltages and Ion/Ioff currents can be achieved. As shown, increases in the dopant concentration in the screen layer provides increases in the threshold voltage of the device. Increases in the thickness of the undoped epitaxial layer provides decreases in the threshold voltage of the device. Thus, the threshold voltage of any device on the wafer can be fine tuned during fabrication through appropriate selection of screen layer dopant concentration and undoped epitaxial layer thickness.
Formation of the screen layer may be performed in different ways in the technique provided above. The screen layers may be formed through ion implantation into the substrate or a well region, through in-situ deposition or growth of doped material, or through intrinsic silicon epitaxial growth followed by ion implantation. The undoped epitaxial layers may be formed through intrinsic silicon epitaxial growth.
Differing types of transistor devices may be formed on substrate 10. For example, both NMOS and PMOS transistor devices may be fabricated on substrate 10. Materials used for the screen layers for the NMOS transistor devices may include boron or other p-type material, with anti-diffusion additives if needed, such as carbon, and/or germanium. For the NMOS transistor devices, ion implantation to establish the screen layers using boron are preferably performed prior to epitaxial growth of the channel layer (and also prior to formation of a threshold voltage control layer where this process step is performed). If desired, a separate threshold voltage layer may be formed after formation of the screen layer by way of a dopant implant onto the screen layer a species of the same type (n or p) as the screen layer, using a dosage and energy selected to result in a threshold voltage layer having a dopant concentration of approximately an order of magnitude less than the concentration of the screen layer.
Thermal cycles in the fabrication process lead to diffusion of the screen layer materials. For proper transistor device operation, it is necessary to prevent screen layer materials from diffusing into the undoped epitaxial layer. The effect of diffusion can be avoided by performing certain annealing processes or performing more thermal cycles prior to epitaxial growth of the channel layer.
Implanted boron is widely known to have poor diffusion performance during device fabrication, especially in thermal cycles of 900° C. or more. To improve the diffusion performance of boron, carbon may be added to protect the implanted boron profiles and germanium may also be added through pre-amorphous implant and solid phase epitaxy for best overall performance. Boron diffusion performance can also be improved through growing fully substitutional boron films by epitaxy in-situ doping. Fully substitutional boron provides superior resistance to anneals as compared to traditional implanted boron. Fully substitutional boron may avoid the use of carbon, germanium, or other stopgaps to protect against downstream thermal processes. Improved transistor performance and reduced fabrication costs may also be achieved from the elimination of extra materials such as carbon and germanium.
Boron diffusion performance can also be improved by a high temperature anneal, for example a 1000° C. spike anneal, to activate the implanted boron. The spike anneal causes the implanted boron to move into substitutional sites. Special anneals can be performed after boron implantation to fully activate the boron prior to subsequent thermal processes that normally cause the poor boron profile diffusion. Subjecting the implanted boron to a laser anneal of 1250° C. or 1300° C. for 400 μsec provides full boron activation in order for the implanted boron to behave like an in-situ doped epitaxial boron film for resistance to diffusion.
Materials used for the screen layers for the PMOS transistor devices in each fabrication process may include arsenic, phosphorous, and/or antimony. When arsenic is used for the PMOS transistor screen, ion implantation of the arsenic is preferably performed prior to epitaxial growth of the channel layer (and also prior to epitaxial growth of any threshold voltage control layer where this process step is performed). To prevent diffusion of screen layer material, a material that has a lower diffusion characteristic may be used. For instance, antimony diffuses less than arsenic in the thermal cycles of the fabrication process. Though antimony has lower diffusion than arsenic, the screen doping profile is sharper with antimony as compared to arsenic at the same doping energy and dopant concentration. This sharper doping profile of antimony can cause higher leakage currents (Id-off) than would be achieved with arsenic as the screen implant for the same epitaxial channel layer thickness. Arsenic tends to provide a lower leakage current than antimony. Leakage current for antimony gets worse at higher implant energies. However, an improvement in leakage current may be achieved by adding arsenic into the antimony implant.
Although the present disclosure has been described in detail with reference to a particular embodiment, it should be understood that various other changes, substitutions, and alterations may be made hereto without departing from the spirit and scope of the appended claims. For example, though not shown, a body tap to at least certain ones of the transistor devices may be formed in order to provide further control of threshold voltage. Although the present disclosure includes a description with reference to a specific ordering of processes, other process sequencing may be followed and other incidental process steps may be performed to achieve the end result discussed herein. Moreover, process steps shown in one set of figures may also be incorporated into another set of figures as desired.
Numerous other changes, substitutions, variations, alterations, and modifications may be ascertained by those skilled in the art and it is intended that the present disclosure encompass all such changes, substitutions, variations, alterations, and modifications as falling within the spirit and scope of the appended claims. Moreover, the present disclosure is not intended to be limited in any way by any statement in the specification that is not otherwise reflected in the appended claims.

## Claims

1. A method for fabricating multiple transistor devices on a substrate with a plurality of threshold voltages, comprising:
forming a first device in a substrate, the first device including a screen layer with a first dopant concentration, a threshold voltage layer with a second dopant concentration less than the first dopant concentration, and a substantially undoped epitaxial channel with a first thickness thereon, the screen layer, threshold voltage layer, and undoped epitaxial channel of the first device underlying a first gate;
forming a second device in the substrate electrically isolated from the first device, the second device having a screen layer with a third dopant concentration, a threshold voltage layer with a fourth dopant concentration less than the third dopant concentration, and a substantially undoped epitaxial channel with a second thickness thereon, the screen layer, threshold voltage layer, and undoped epitaxial channel of the second device underlying a second gate, the second thickness being different than the first thickness such that the first and second devices have different threshold voltages;
wherein a top surface of the substantially undoped epitaxial channel of the first device is substantially coplanar with a top surface of the substantially undoped epitaxial channel of the second device adjacent respective gates of the first and second devices.

2. The method of claim 1, further comprising:
forming a third device in the substrate, the third device having a screen layer with a third dopant concentration and a substantially undoped epitaxial channel with a third thickness thereon, the third thickness being different than the first and second thicknesses.

3. The method of claim 2, wherein the first, second, and third dopant concentrations are substantially the same.

4. The method of claim 2, further comprising:
forming a fourth device in the substrate, the fourth device having a screen layer with a fourth dopant concentration and a substantially undoped epitaxial channel with a fourth thickness thereon.

5. The method of claim 4, wherein the third dopant concentration is different than the fourth dopant concentration and wherein the third thickness and the fourth thickness are substantially the same.

6. The method of claim 5, further comprising:
simultaneously forming the substantially undoped epitaxial channel of the third and fourth devices.

7. The method of claim 1, wherein the first and third dopant concentrations are the same.

8. The method of claim 1, wherein the screen layer of the first device and the screen layer of the second device have substantially the same thickness.

9. The method of claim 1, further comprising:
forming shallow trench isolation regions in the substrate, the shallow trench isolating regions defining electrically separate device regions for formation of the first and second devices.

10. A method for fabricating multiple transistor devices on a substrate with a plurality of threshold voltages, comprising:
forming a first device in a substrate, the first device including a screen layer with a first dopant concentration, a threshold voltage layer with a second dopant concentration less than the first dopant concentration, and a substantially undoped epitaxial channel with a first thickness thereon, the screen layer, threshold voltage layer, and undoped epitaxial channel of the first device underlying a first gate;
forming a second device in the substrate electrically isolated from the first device, the second device having a screen layer with a third dopant concentration, a threshold voltage layer with a fourth dopant concentration less than the third dopant concentration, and a substantially undoped epitaxial channel with a second thickness thereon, the second thickness being different than the first thickness such that the first and second devices have different threshold voltages, the screen layer, threshold voltage layer, and undoped epitaxial channel of the second device underlying a second gate, wherein a top surface of the substantially undoped epitaxial channel of the first device is substantially coplanar with a top surface of the substantially undoped epitaxial channel of the second device adjacent respective gates of the first and second devices; and
forming a third device in the substrate electrically isolated from the first device, the third device having a threshold voltage that is set using a method that does not include a screen layer.

11. A method for fabricating multiple transistor devices on a substrate with a plurality of threshold voltages, comprising:
forming a first device in a substrate, the first device including a screen layer with a first dopant concentration and a substantially undoped epitaxial channel with a first thickness thereon;
forming a second device in the substrate electrically isolated from the first device, the second device having a screen layer with a first dopant concentration and a substantially undoped epitaxial channel with a second thickness thereon, the second thickness being different than the first thickness such that the first and second devices have different threshold voltages; and
forming a third device in the substrate electrically isolated from the first device, the third device having a threshold voltage that is set using a method that does not include a substantially undoped epitaxial channel.

12. The method of claim 9, wherein the forming shallow trench isolation regions in the substrate is done before forming the first device and forming the second device.