Selective spacer formation on transistors of different classes on the same device

A method of selectively forming a spacer on a first class of transistors and devices formed by such methods. The method can include depositing a conformal first deposition layer on a substrate with different classes of transistors situated thereon, depositing a blocking layer to at least one class of transistors, dry etching the first deposition layer, removing the blocking layer, depositing a conformal second deposition layer on the substrate, dry etching the second deposition layer and wet etching the remaining first deposition layer. Devices may include transistors of a first class with larger spacers compared to spacers of transistors of a second class.

FIELD OF INVENTION

BACKGROUND OF INVENTION

Metal-oxide-semiconductor (MOS) transistors are the primary building blocks for modem integrated circuits. A typical highly integrated circuit, such as a microelectronic device, can contain millions of transistors on a single silicon substrate no bigger than a thumbnail. Generally, a transistor, or device and hereinafter referred to interchangeably, includes a gate structure formed on a substrate with a source region and a drain region, separated from each other by the gate structure and formed within the substrate, adjacent to the gate structure. A transistor may be thought of as an electronic switch having three nodes. When a voltage is applied to a first node of the transistor, i.e., the gate, the flow of electric current between the other two nodes, i.e., the source and the drain regions, via a channel region below the gate, is modulated. For example, to turn one type of n-channel (NMOS) transistor “ON,” a positive voltage is applied to the gate, allowing electric current to flow between the source and drain. To turn this transistor “OFF,” zero volts is applied to the gate which cuts off the flow of electric current between the source and drain.

The type of transistor on a microelectronic device varies depending on its intended function. Examples of transistors include NMOS and PMOS transistors used in Logic circuits and NMOS and PMOS transistors used in SRAM circuits. Generally, the function of Memory transistors require less power (and therefore slower current flow) while Logic transistors require more power (and therefore faster current flow). Power (represented by the formula Power equals I×V, wherein I equals current and V equals voltage) is measured by the speed of electrons moving from the source and drain regions via the channel region. One method of controlling this movement, and hence the power of a given transistor, is to control the distance from the source region to the drain region. Typically, because Memory transistors require less power, the distance from the source region to the drain region is larger when compared to that of a Logic transistor.

Distance between the source region and the drain region also affects leakage of current flow in the OFF state. “Leakage” is the amount of current flowing through the transistor when in the OFF state. Although a given transistor is in the OFF state, a small amount of current continues to flow through the channel region. The total current of a transistor is measured by the current flow in both the ON and OFF states. That is, current (I) equals ION+IOFF, where IOFFis very small compared to ION. The greater the distance between the source region and the drain region, the smaller the leakage. However, the trade-off is that the overall speed of the transistor is lessened.

DETAILED DESCRIPTION

Fabrication of transistors can involve the formation of “spacer” structures adjacent to gate structures. Spacers insulate gate stacks and provide distance between a source region and a drain region to, for example, decrease OFF state leakage, which consequentially reduces power. In some fabrication methods, a conformal layer is deposited on the substrate with a multitude of gate structures thereon. The conformal layer is then anisotropically etched leaving spacer structures adjacent to the gate structure. “Anisotropic etching” is an etch process that exhibits little or no undercutting, resulting in features whose sides are perpendicular to the underlying layer.

In some microelectronic device fabrication methods, efficiency of a device is increased by doping the source and drain regions with, for example, silicon-germanium SiGe or silicon-carbon SiC. SiGe can be introduced such that it can cause compressive strain on a channel region, which in turn increases the speed of holes traveling from the source region to the drain region of a PMOS device. SiC can be introduced such that it can cause tensile strain on a channel region, which in turn increases the speed of electrons traveling from the source region to the drain region of an NMOS device In some applications, however, conventional spacer structure fabrication methods do not allow for sufficient space between gate structures for doping of the source and drain regions alternating between gate to gate.

Current complementary metal-oxide-semiconductor (CMOS) fabrication processes for microelectronic devices incorporate both PMOS and NMOS multileg (isolated and somewhat randomly oriented) layout transistor devices and SRAM array devices on the same substrate. Due to the large number of SRAM devices in an array layout, the gate-to-gate space between SRAM devices is generally smaller compared to the gate-to-gate space between Logic transistors, which are fewer in number and randomly situated. In some applications, a first class of transistors situated on the same substrate as a second class of transistors can have a decrease in OFF state leakage at the expense of decreased power. In some embodiments, a first class of transistors can include transistors with a first predetermined sized spacer and a second class of transistors can include transistors with a second different predetermined sized spacer. Such an embodiment can be useful in, for example, a laptop computer battery in which speed of the computer may be compromised in return for a longer battery lifetime. In some embodiments, a method to accomplish this is to increase the size of the spacer. However, fabrication methods can involve depositing a conformal layer on a die with different classes of transistors situated thereon, which deposition does not discriminate between the different classes of transistors. As a result, the spacers formed thereafter are substantially the same thickness with respect to the different classes of transistors. Thus, while accomplishing reduction of OFF state leakage in one class of transistors, e.g., PMOS Logic transistors, this can result in largely degraded performance on certain transistors in which there is a small gate-to-gate space, such as in a SRAM transistor array or stacked devices, leading to degraded performance and, eventually, function failure.

In some applications, a microelectronic device, representatively shown inFIG. 1A, can include both Logic transistors102and other types of transistors104on the same die100. Other types of transistors can include, but are not limited to, SRAM Memory, hereinafter collectively referred to as “non-Logic transistors.” Logic transistors generally require more power relative to non-Logic transistors. Thus, the distance between the source and drain regions can be smaller in Logic transistors when compared to non-Logic transistors. As a consequence, the IOFFcan be higher in Logic transistors when compared to non-Logic transistors. In some applications, such as those applications requiring a slower efficiency yet a longer life, the Logic transistors can be configured to have a low IOFF.

FIG. 1Brepresents an embodiment of MOS transistor108. MOS transistor includes a gate structure110, source region112and drain region114formed on substrate124. The gate structure110can include spacers118located adjacent thereto. In the ON state, i.e., when negative voltage is applied, holes flow from source region112to drain region114via channel region116, representatively shown by arrow120. In the OFF state, i.e., when no voltage is applied, a small of amount of current, or leakage, continues to flow from source region112to drain region114via channel region116. Leakage is a direct function of the distance between source region112and drain region114, representatively shown by arrow122. That is, the smaller gate structure110provides a smaller distance between source and drain regions110and112, respectively. Such configuration generally allows for relatively increased speed at the cost of high leakage.

FIG. 1Crepresents an embodiment of an SRAM transistor130. SRAM transistor includes gate structure126a gate structure128, source region130and drain region132formed on substrate138. Similar to the embodiment inFIG. 1B, a channel region134and spacers136are also provided. The distance between source130and drain132is representatively shown by arrow136. The larger gate structure128provides a greater distance between source region130and drain region132. Such configuration generally allows for relatively slower speed with low leakage.

On a die, MOS Logic transistors can be situated at random, while non-Logic transistors can be situated in an array. In some embodiments, an array takes up a larger space relative to the randomly situated Logic transistors on a given die. Thus, the gate-to-gate space, i.e., the pitch, should be as small as possible for array of non-Logic transistors, such as an SRAM array. For Logic transistors, the pitch can be approximately 180 nanometers (nm). For SRAM transistors, the pitch can be approximately 160 nm.

FIGS. 2A-2Hillustrate an embodiment of a method for selectively forming a spacer on a gate structure of a first class of transistor.FIG. 2Ashows a portion of a microelectronic device100, representatively shown as die200, including a substrate202with an embodiment of a transistor204of a first class and an embodiment of a transistor214of a second class situated thereon. Transistor204can include an etch stop206, a gate electrode208and a dielectric210, collectively, a gate structure212. Etch stop portion206can be, for example, silicon nitride (Si3N4), oxynitride (SiOyNx) and the like; gate electrode208can be, for example, a polycrystalline semiconductor, such as polycrystalline silicon (polysilicon), polysilicon germanium (poly-SiGe) or a metal having, for example, a work function appropriate for a p-type or n-type semiconductor; and dielectric210can be a non-conductive material, such as silicon dioxide, silicon nitride and the like. Transistor214can include an etch stop216, a gate electrode218and a dielectric220, collectively, a gate structure232. The materials of gate structure222can be similar to those of gate structure212. In some embodiments, transistor204can be an NMOS or PMOS within an SRAM or NMOS Logic transistor and transistor214can be a PMOS Logic transistor.

FIG. 2Bshows an embodiment of the formation of first deposition layer224on microelectronic device100ofFIG. 2A. In some embodiments, first deposition layer224can be a dielectric material. In some embodiments, first deposition layer224can be conformal. First deposition layer224can be in a range of approximately 50 Angstroms (Å) to 1500 Å. In some embodiments, first deposition layer224can be in a range from approximately 200 Å to 600 Å. First deposition layer227can be applied by processes known in the art. Examples of such processes include, but are not limited to, physical vapor deposition (PVD), atomic layer deposition (ALD), chemical vapor deposition (CVD), low pressure CVD, plasma-enhanced CVD or any other suitable process.

FIG. 2Cshows an embodiment of the selective formation of a blocking layer226on microelectronic device100ofFIG. 2B. In some embodiments, blocking layer226can be a photo-imaging material, such as a photoresist. Photoresists can be applied by a process known as photolithography, also known as photomasking. “Photolithography” is a process used to selectively create patterns on a substrate surface. “Patterning” is the basic operation that removes specific portions of the uppermost layer at a given fabrication step on the substrate surface. Photoresists can be either negative or positive. In both forms, photoresists are three-component materials including a matrix, a photoactive compound and a solvent. For positive photoresists, the matrix may be a low-molecular weight novolac resin, the photoactive component may be a diazonaphthaquinone compound and the solvent system may be a mixture of n-butyl acetate, xylene and cellosolve acetate. For negative photoresists, the matrix may be cyclized synthetic rubber resin, the photoactive component may be a bis-arylazide compound and the solvent system may be an aromatic solvent. In some embodiments, blocking layer226can be selectively deposited on, or applied to, transistor204of the first class. In some embodiments, blocking layer226can be applied to an array of transistors.

FIG. 2Dshows an embodiment ofFIG. 2Cfollowing selective removal of first deposition layer224. In some embodiments, first deposition layer224can be dry etched from gate structure222while blocking layer226remains on gate structure212. Dry etching can be performed by such processes including, but not limited to, reactive ion etching, sputter etching and vapor phase etching. Dry etching can result in isotropic etching. “Isotropic etching” is a process in which etching occurs in all directions causing undercutting. After the dry etching is performed on the exposed portion of first deposition layer224, blocking layer226can be removed from gate structure212by a process known as “ashing.” “Ashing” is a method of stripping photoresist that utilizes high energy gas, usually an oxygen plasma or ozone, to burn off photoresist. The result is gate structure222with a first spacer layer228adjacent thereto and gate structure212covered with first deposition layer224substantially or completely intact.

FIG. 2Eshows an embodiment ofFIG. 2Dfollowing the formation of a second deposition layer thereon. In some embodiments, second deposition layer230can be a dielectric material, which, in some applications, can be a different material than that of first deposition layer227. Examples of dielectric materials comprising second deposition layer include, but are not limited to, nitrides such as (Si3N4), (SiOyNx) and the like. In some embodiments, the second deposition layer230can be conformal. Second deposition layer230can be in a range of approximately 100 Å to 1000 Å. In some embodiments, second deposition layer230can be in a range from approximately 200 Å to 600 Å. Second deposition layer230can be applied by processes known in the art, including, but not limited to, PVD, ALD, CVD, low pressure CVD, plasma-enhanced CVD or any other suitable process.

FIG. 2Fshows an embodiment ofFIG. 2Efollowing removal of second deposition layer230. In some embodiments, second deposition layer230can be dry etched from gate structures212and222of both transistors204and214. Dry etching can be performed by such processes which include, but are not limited to, reactive ion etching, sputter etching and vapor phase etching. Dry etching can result in isotropic etching. After the etching, a bi-layer spacer236including first spacer layer228and second spacer layer232remains adjacent to gate structure222of transistor214. Gate structure212of transistor204, on the other hand, includes remaining first deposition layer224with removable spacer layer234adjacent thereto.

FIG. 2Gshows an embodiment ofFIG. 2Fduring a selective etching process of remaining first deposition layer224from gate structure212. In some embodiments, remaining first deposition layer224can be wet etched from gate structure212. Wet etching can be performed by dipping, spraying or otherwise applying a chemical solution to the substrate. Wet etching can result in isotropic etching which will etch at the same rate in both vertical and horizontal direction. In some embodiments, after the wet etching process, remaining second deposition layer230will be removed from gate structure212automatically. That is, because the remaining first deposition layer224has been removed by the wet etching process, removable spacer234has nothing with which to adhere (both at the bottom and at the side) and will automatically be effaced.

FIG. 2Hshows an embodiment ofFIG. 2Gfollowing the selective etching process described with respect toFIG. 2G. Gate structure222of transistor214will include bi-layer spacer236adjacent thereto and gate structure212of transistor204will not include any spacer as a result of embodiments of the method described with respect toFIGS. 2A-2G. In some embodiments, the bi-layer spacer236can be in the range of approximately 5 nm to 10 nm. It should be appreciated that the method embodied inFIGS. 2A-2Hmay be repeated on the same die to form more spacers.

In some embodiments, subsequent to the method embodied inFIGS. 2A-2H, a conventional spacer deposition process may be performed on the substrate. Such process may include depositing a conformal first deposition layer, dry etching the first deposition layer, depositing a conformal second deposition layer and dry etching the second deposition layer, resulting in spacer formed adjacent to a multitude of transistors. Thus, in some embodiments, a die which was subjected to a selective spacer deposition process may be subjected to subsequent selective spacer deposition processes or conventional spacer deposition processes to form spacers of varying sizes on transistors of varying classes (seeFIG. 2I). For example, in some embodiments, a combination of at least one selective spacer deposition process and at least one conventional spacer deposition process may result in a first class of transistors having a spacer of from approximately 10 nm to 50 nm and a second class of transistors having a spacer of from approximately 50 nm to 100 nm. In some embodiments, the first class of transistors may be Logic transistors and the second class of transistors may be non-Logic transistors.

FIG. 3represents a schematic of an embodiment of a selective spacer deposition process. A die can be formed with both Logic transistors and non-Logic transistors (300). In some embodiments, Logic transistors are situated randomly and Memory transistors are arranged in an array. A first deposition layer can be conformally deposited on the die (310). Then, a blocking layer can be selectively deposited on at least one non-PMOS transistor (320). A dry etch process can be performed on the first deposition layer (330). Subsequently, the blocking layer can be removed by ashing or any other suitable method (340). Thereafter, a second deposition layer can be conformally deposited on the die (350). A dry etch process can be performed on the second deposition layer (360). Any remaining first deposition layer may then be removed by a wet etching process or any other suitable process (370). Subsequent selective or non-selective deposition processes may then be optionally performed on the die (380).

FIG. 4represents an alternative schematic of an embodiment of a selective spacer deposition process. A die can be formed with both Logic transistors and non-Logic transistors (400). In some embodiments, the non-Logic transistors can be Memory (SRAM) and Logic transistors. In some embodiments, Logic transistors are situated randomly and Memory transistors are arranged in an array. A first deposition layer can be conformally deposited on the die (410). A dry etch process can be performed on the first deposition layer leaving spacers on both Logic transistors and non-Logic transistors (420). Then, a blocking layer can be selectively deposited on at least one non-Logic transistor (430). A dry etch process can be performed on any unblocked spacer (440). In this manner, the size of any exposed unblocked spacer can be selectively partially or completely removed. Subsequently, the blocking layer can be removed by ashing or any other suitable method (450). Subsequent selective or non-selective deposition processes may then be optionally performed on the die (460).

According to embodiments of the above-described methods, a thicker spacer can be formed on gate structures of Logic transistors than on co-situated gate structures of non-Logic transistors. “Co-situated” means both Logic transistors and non-Logic transistors are situated on the same die. The result can be to decrease OFF state leakage in Logic transistors without closing the spacer-to-spacer gap between gate structures in stacked devices while simultaneously keeping a thinner spacer on SRAM transistors to prevent closing the spacer-to-spacer gap between gate structures in these types of arrays. Doping of the source and drain regions with respect to each class of transistors can be accomplished without blocking thereof.

It should be appreciated that the embodiments described above can apply to any combination of classes of devices depending on a designer's needs and the power/performance trade-off. That is, a first spacer of a first size can be formed on a first class of devices and a second spacer of a second size can be formed on a second class of devices wherein the classes can be different. Examples include, but are not limited to, a first class including NMOS devices and a second class including PMOS devices (or vice-versa) inside a Logic circuit; a first class including NMOS devices and a second class including PMOS devices (or vice-versa) inside a SRAM memory array circuit; a first class including both NMOS and PMOS devices inside an SRAM memory array circuit and a second class including both NMOS and PMOS devices inside a Logic circuit; or a first class including all PMOS devices inside SRAM and Logic circuits and a second class including all NMOS devices inside SRAM and Logic circuits. The combinations are virtually limitless.

FIG. 5shows a cross-sectional side view of an integrated circuit package that is physically and electrically connected to a printed wiring board or printed circuit board (PCB) to form an electronic assembly. The electronic assembly can be part of an electronic system such as a computer (e.g., desktop, laptop, handheld, server, etc.), wireless communication device (e.g., cellular phone, cordless phone, pager, etc.), computer-related peripheral (e.g., printer, scanner, monitor, etc.), entertainment device (e.g., television, radio, stereo, tapes and compact disc player, video cassette recorder, motion picture expert group audio layer 3 player (MP3), etc.), and the like.FIG. 5illustrates the electronic assembly as part of a desktop computer.FIG. 5shows electronic assembly500including die502, physically and electrically connected to package substrate504. Die502is an integrated circuit die, such as a microprocessor die, having, for example, transistor structures interconnected or connected to power/ground or input/output signals external to the die through interconnect lines to contacts506on an external surface of die502. The die may be formed in accordance with known wafer processing techniques using as the substrate described with reference toFIGS. 2A-2H. Contacts506of die502may be aligned with contacts508making up, for example, a die bump layer on an external surface of package substrate504. On a surface of package substrate504opposite a surface including contacts508are land contacts510. Connected to each of land contacts510are solder bumps512that may be used to connect package514to circuit board516, such as a motherboard or other circuit board.

Although the foregoing description has specified certain steps and materials that may be used in the method of the present invention, those skilled in the art will appreciate that many modifications and substitutions may be made. Accordingly, it is intended that all such modifications, alterations, substitutions and additions be considered to fall within the spirit and scope of the invention as defined by the appended claims. In addition, it is appreciated that the fabrication of a multiple metal layer structure atop a substrate, such as a silicon substrate, to manufacture silicon device is well known in the art. Therefore, it is appreciated that the figures provided herein illustrate only portions of an exemplary microelectronic device that pertains to the practice of the present invention. Thus the present invention is not limited to the structures described herein.