The present invention generally relates to semiconductor device fabrication, and particularly to semiconductor surface treatment processes and equipment using gas cluster ion beams.
Complementary Metal-oxide-semiconductor (CMOS) technology is commonly used for fabricating field effect transistors (FETs) onto a semiconductor wafer. At the core of planar FETs, a channel region is formed in a n-doped or p-doped semiconductor substrate on which a gate structure is formed. The gates are serially connected to form discrete functional modules that together form advanced integrated circuits, such as CPUs, memory units, storage devices, and the like.
The processes employed to fabricate FETs are well known in the art, although there is a continuing trend towards ever smaller and more efficient devices, requiring making improvements to known techniques and devising new ones. In various steps of the fabrication process, it is necessary to deposit material onto the semiconductor wafer, to etch material already formed on the wafer, or otherwise treat the wafer surface. Together, these techniques sculpt the surface of the wafer to form functioning structures. Each technique has its own advantages and side effects that make it suitable for a particular step in the fabrication process, and plays a role in determining a manufacturer's cost structure.
A relatively new technique used in the fabrication process is a gas cluster ion beam (“GCIB”) system. GCIB allows for nano-scale modification of surfaces at a faster rate compared to some other known techniques, and without causing the substantial sub-surface damage that other techniques with comparable speed may cause. Generally, a GCIB system comprises a module that expands a high pressure gas into a vacuum. As the gas cools, it condenses into nano-sized crystalline clusters that are then emitted out of a nozzle. The clusters pass through differential pumping apertures into a high vacuum region where they are ionized through collisions with energetic electrons. The ionized clusters are accelerated and focused into a tight beam.
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GCIB is a very versatile technique because it can be used with virtually any gas, with varying intensity. Among other applications, it can be used for deposition, etching, and doping steps in the microelectronics fabrication process. These and other properties of the beam 50, such as beam size, the number of times it passes over an area of the substrate 44, and related factors depend on the particular application. In one typical micro fabrication process, the substrate 44 is a semiconductor wafer having, for example, a 300 mm diameter. The beam 50 spot size, i.e. the point where it is intercepted by the wafer, may be 1 cm in diameter. The wafer may be scanned approximately 100 times from left to right, using an approximately 3 mm step, beginning at a top side of the wafer. In typical GCIB applications, the platen 42 is movable along a vertical axis 70 and a second horizontal axis 80 (corresponding to the Y-axis and Z-axis).
Despite their advantages, current GCIB techniques are limited because they are applied to the target surface at a perpendicular angle, leading to substantially symmetric structures. For example, a GCIB system can be used to form spacer film layers onto a gate electrode and its flanking source and drain regions, and then again to etch the spacer film layers to form final spacer structures. However, without implementing hard-to-perfect, time consuming and costly masking steps, the final spacer structures are substantially identical both in their constituent material, thicknesses, and shapes.
While forming substantially identical structures is not necessarily disadvantageous, there are circumstances in which it would be desirable to form asymmetrical structures. For example, as transistors become smaller, the inherent parasitic capacitance of the gate-spacer-source/drain regions of a transistor have an increasing effect on the transistor's reliable operability. One way of decreasing the parasitic capacitance in these structures is to form the spacers asymmetrically on the source and drain sides of the gate electrode, by using different materials, different thicknesses, or both. The limitation in existing GCIB systems to facilitate this aim is due, in large part, to the fact that the beam emitting from a GCIB device collides with a semiconductor wafer at a right angle, resulting in nearly uniform changes to the wafer surface and creating symmetrical structures. Therefore, it would be desirable to develop a method and a system for using GCIB to manipulate a target surface asymmetrically.
According to one embodiment of the disclosed invention, an angled gas cluster ion beam system includes a first module for forming a collection of gas clusters, a second module for generating a gas cluster beam by using the collection of gas clusters formed by the first module, the second module communicating with the first module along a first axis extending through the modules; a third module for ionizing the gas cluster beam generated by the second module and for directing the gas cluster beam along the first axis, the third module communicating with the second module along the first axis; and a fourth module for housing a target surface, the fourth module configured to position the target surface at an acute angle from at least a second axis being substantially perpendicular to the first axis to intercept the gas cluster beam directed by the third module, the fourth module communicating with the third module along the first axis.
According to a further embodiment of the disclosed invention, a method for forming asymmetric structures using a angled gas cluster ion beam system includes the steps of forming a collection of gas clusters, using a first module; generating a gas cluster beam from the collection of gas clusters, using a second module; ionizing the gas cluster beam and directing the gas cluster beam along a first axis, using a third module; and intercepting the ionized gas cluster beam, using a fourth module, by positioning a target surface at an acute angle from a second axis being substantially perpendicular to the first axis.
Embodiments will now be described herein with reference to the accompanying drawings, in which exemplary embodiments are shown. This disclosure may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of this disclosure to those skilled in the art. In the description, details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the presented embodiments.
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According to a further embodiment of the disclosed invention, the platen 42 and/or the substrate 44 are additionally configured to move along the axis 60, towards or away from the beam 50 source (i.e., the third module 30), such that the center of the beam 50 collides with the surface of the substrate 44 at a configurable distance, preferably at a substantially equal distance for each portion of the substrate 44 that is scanned by the beam 50. Angularly positioning, i.e., tilting, the platen 42 and the substrate 44 results in one portion of the substrate 44 being closer to the beam 50 than an opposing end. With the beam 50 source being stationary, moving the platen 42 along the axes 70 or 80 (depending on the direction of the tilt) during the GCIB scan process results in an increased distance between an emission point of the beam 50 from the third module 30 and the surface of the substrate 44 as the length of the substrate 44 is scanned. By allowing the platen 42 to move along the axis 60, therefore, the distance between the emission point of the beam 50 from the third module 30 and the surface of the substrate 44, is configurable, and can be maintained at a substantially constant value. For example, as the beam 50 is projected onto successively farther portions of the substrate 44, the platen 42 may be stepped forward, i.e. moved closer to the beam 50 source, to cancel out the added distance between the two. The reference point for this distance may be, for example, the center of the beam 50 at the point where it collides with the substrate 44.
In a related embodiment, the emission point 36 of the third module 30 is mechanically movable along the axis 60, such that it allows adjustment of the distance travelled by the beam 50 before it is intercepted by the substrate 44. This feature allows the angled GCIB device to maintain an equal distance between the emission point 36 of the beam 50 and the point of contact on the substrate 44 (measured, for example, at the center of the beam at the point of contact), or to vary the distance in a controlled way.
A given portions(s) of the substrate 44 may be scanned multiple times with varying intensity to achieve a desired level of surface manipulation, such as substantially uniform manipulation. Additionally, the speed at which the platen 42 is moved may be changed to further facilitate this objective.
Using the angled GCIB system 200, it is possible to treat the surface of the substrate 44 asymmetrically by adjusting the angle at which the substrate 44 intercepts the beam 50. By tilting the substrate 44, the ionized gas clusters in the beam 50 collide with surface features of the substrate 44 from an exposed side, leaving surface features of the other side virtually unexposed. GCIB deposition, etching, implantation, and related processes, therefore, may be performed asymmetrically, as claimed, and as illustrated by embodiments described below.
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In other embodiments, the gate 304 may be formed using a gate-last process, in which case the gate 304 may include a dummy gate layer made of, for example, silicon, and a dummy gate dielectric made of, for example, silicon oxide, intended to serve as a placeholder for the replacement gate formed after later processing steps. The gate 304 is replaced with a true gate dielectric and a gate conductor during subsequent processes.
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In a related embodiment, the spacer film layer 402 may be formed on the top side, the first side and the second opposite side of the gate structure 304 using angled GCIB deposition, and thereafter selectively removed by any known means (including angled GCIB etching) so that the spacer film layer 402 is removed almost entirely from the second side of the gate 304, or is removed partially so that the spacer film layer 402 is thicker on the first side of the gate 304 than it is on the second side.
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Although the spacer film layer 402 forms a thicker layer than the spacer film layer 404 in the depicted embodiment, the spacer film layer 402 may in fact be the thinner layer of the two in other embodiments. In other words, it is not necessary that the thicker layer of the two spacer films be deposited first.
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In a related embodiment, the spacer 504 may be formed using an oxide compound (having, for example, a dielectric constant of 3.9 k), and the spacer 502 may be formed using a second compound having a lower dielectric constant, such as carbon doped-silicon oxide.
Although angled GCIB is applied to the substrate 44 during both the deposition and etching steps of the disclosed embodiments, other embodiments may employ angled GCIB for only the deposition or only the etching step without departing from the scope and spirit of the disclosed invention. Moreover, it is not necessary that angled GCIB deposition or etching be used in creating both of the spacers 502 and 504, or in forming or shaping both of the spacer film layers 402 and 404. For example, the spacer film layer 404 may be etched using angled GCIB to form the spacer 502, and the spacer 504 may be formed using any known etching technique in the art, such as reactive ion etching (RIE), without any angularity. In other words, using angled GCIB deposition/etching is not necessary in every deposition/etching step in order to create asymmetric structures.
Furthermore, although preferred embodiments of the disclosed invention comprise the application of a GCIB system (including both for deposition and etching steps) at a non-perpendicular angle relative to the substrate 44, such a system may nevertheless apply a beam 50 to the substrate 44 at a right angle. Among having other benefits, this feature of the disclosed invention preserves the functionality of existing GCIB systems without creating the need to process the substrate 44 in a different GCIB system for applications requiring such treatment.
Additionally, while the recited exemplary embodiments of the disclosed invention involve using an angled GCIB system for deposition and etching steps in the semiconductor fabrication processes, such uses are non-limiting. An angled GCIB system may be used for any other treatment of a surface, including a semiconductor wafer, where asymmetric application of a GCIB is required or useful.
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In related embodiments, the angled GCIB system may be used to perform only a part of the deposition or etching processes described above, without departing from the scope or spirit of the disclosed invention. For example, a process may aim to form a first and a second nitride spacer having final thicknesses of 10 nm and 5 nm respectively, the first and second spacers being formed adjacent to a gate electrode. The process may include depositing a uniform nitride layer having a thickness of 20 nm on a semiconductor wafer having a plurality of gate electrodes on its surface. Forming the first and second spacers, then, requires removing approximately 10 nm and 15 nm of the 20 nm nitride layer, from respective sides of the plurality of gate electrodes. Since forming both spacers requires removing at least a 10 nm top layer of nitride, a conventional etching process may be used to remove the top layer with substantial uniformity; the two spacers may be formed, at this stage, at a thickness of 10 nm. In other words, it is not necessary to use angled GCIB etch to perform this step. It may be desirable to use a conventional etching process due, for example, to time and cost constraints.
Subsequently, angled GCIB etch may be used to etch the second spacer in order to etch an additional 5 nm layer, without etching the first spacer. The resulting structure comprises the first spacer having a thickness of 10 nm, and the second spacer having a thickness of 5 nm.
Similarly, in deposition steps, it may not be necessary to use angled GCIB to deposit all the material needed to form a first and a second spacer film layer on respective sides of a gate electrode. For example, if the first spacer film layer is to be formed at 10 nm, and the second spacer film layer is to be formed at 5 nm, it is possible to use a conventional deposition technique to form a base layer of 5 nm on both sides of the gate electrode. Thereafter, an additional layer having a thickness of 5 nm may be formed only on the side of the first spacer film layer to achieve the desired 10 nm thickness.
The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiment, the practical application or technical improvement over technologies found in the marketplace, or to enable other of ordinary skill in the art to understand the embodiments disclosed herein. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated but fall within the scope of the appended claims.
This application is related to the following commonly-owned, co-pending U.S. patent application filed on even date herewith, the contents and disclosure of which is expressly incorporated by reference herein in its entirety: U.S. patent application Ser. No. ______ (FIS920120304US1), for “ASYMMETRIC SPACERS”.