Patent Application
20060207872
The present invention relates generally to deposition of thin films by sputtering processes, and more specifically to the manipulation of a magnetic field within a processing chamber during thin film deposition by sputtering.
Physical vapor deposition by sputtering is a well known process that has widespread applications in the fabrication of integrated circuit semiconductor devices. In a conventional fabrication process, a large number of integrated circuit devices are formed on a thin, generally circular semiconductor substrate known as a wafer. Integrated circuit device fabrication commonly involves several processing steps. While sputtering has a wide variety of different applications in semiconductor processing, it is often used for reactive sputtering of dielectric films from conductive target materials. Such films include, but are not limited to, aluminum nitride and aluminum oxide.
A magnetron is a device that is used for depositing a film onto a wafer surface using a sputtering process. A conventional magnetron includes a processing chamber connected to a gas source and targets of sputterable material positioned within the chamber. In operation of the magnetron, a suitable DC or AC electric field is applied to the chamber, thereby causing a plasma of an inert gas in the chamber to be formed. A magnetic field is used to confine the plasma to a region near the sputterable target material. The target material is subjected to an electric potential and acts as a cathode with respect to an anode. This causes positive ions from the plasma to strike the targets which have a negative potential, thereby ejecting atoms from the targets. Ejected material from the targets is deposited as a thin film onto a wafer positioned in the chamber.
The sputterable target material is usually selected to yield a particular substance to be deposited on the wafer. For example, to deposit an aluminum nitride film onto a silicon wafer, an appropriate target material is aluminum. Aluminum nitride films are useful in the manufacture of piezoelectric acoustic resonator filters, including film bulk acoustic resonator (FBAR) filters.
It is generally advantageous for thin films produced using a sputtering process to have relatively uniform physical and electrical properties across the film. Examples of such properties include, but are not limited to, thickness, stress characteristics and coupling coefficient. For example, in the manufacture of FBAR filters, the greater the uniformity in the film thickness, the higher the yield of the resulting filters. As another example, providing a film with a uniformly higher coupling coefficient generally yields a more efficient energy transfer between electrical and acoustic domains of an FBAR filter. That is, coupling coefficient is a measure of the piezoelectricity of the film, or of the ability of the film to convert acoustic energy to electrical energy and vice versa. Thin films with enhanced uniformity in physical or electrical properties also have applications in devices other than FBAR filters. An improved sputtering deposition system has been developed that provides greater control over certain properties of a deposited thin film.
In one embodiment of the present invention, a magnetron comprises a processing chamber having an upper sputtering target and a lower sputtering target positioned therein. The magnetron further comprises an upper magnetic structure positioned adjacent to the upper sputtering target and outside the processing chamber. The magnetron further comprises a lower magnetic structure positioned adjacent to the lower sputtering target and outside the processing chamber. The magnetron further comprises a rotatable magnet that is coupled to an exterior portion of the processing chamber. The rotatable magnet is configured to rotate around the processing chamber in a region adjacent to at least one of the upper and lower sputtering targets.
In another embodiment of the present invention, a magnetron comprises a processing chamber. The magnetron further comprises a first and second concentric targets for sputtering a film onto a wafer in the processing chamber in response to the generation of a plasma in the processing chamber. The magnetron further comprises a rotatable magnet that is configured to rotate around at least a portion of the processing chamber in a region adjacent to at least one of the first and second concentric targets.
In another embodiment of the present invention, a method comprises providing a processing chamber having a wafer and a sputtering target positioned therein. The method further comprises exposing the sputtering target to a first magnetic field. The method further comprises sputter depositing material from the sputtering target onto the wafer. The method further comprises exposing the sputtering target to a second magnetic field. The second magnetic field is time-varying during a period when the first magnetic field is substantially constant.
Exemplary embodiments of an improved thin film deposition system are illustrated in the accompanying drawings, which are for illustrative purposes only. The drawings comprise the following figures, in which like numerals indicate like parts.
As described herein, it is generally advantageous for thin films to have relatively uniform physical and electrical properties across the film. Several factors can cerate non-uniformities in thin films produced by sputtering. For example, the characteristics of the sputterable target material typically change throughout the life of the targets. Particularly, the cumulative effect of material erosion from a target gradually changes the shape of the surface of the target. This often changes the angle at which material is eroded from the target, which alters the rate of material deposition onto a wafer surface, thereby degrading the uniformity in thickness of films deposited as a target ages. Other sources of nonuniformities include asymmetrical gas flows in the processing chamber and interference from other nearby magnetrons. The challenge of providing uniform properties across a thin film has increased as wafer size as increased.
To increase uniformity of thin films deposited using a sputtering process, various modifications have been made to the conventional magnetron design. For example, the magnetic field in the processing chamber—which is generally spatially symmetrical—can be modified using a secondary magnetic field generated by either electromagnets or a fixed array of permanent magnets.
The secondary magnetic field is intended to compensate for nonuniformities caused by changes in the characteristics of the sputtering target, asymmetrical gas flows, surrounding magnetrons, and other factors. Specifically, the secondary magnetic field is used to manipulate the area from which the predominant amount of material is eroded from the sputtering targets. This area is commonly referred to as the “racetrack”. In a planar magnetron, the secondary magnetic field is used to adjust the position of the racetrack, thereby affecting film deposition around the perimeter of the wafer. In a conical magnetron, the secondary magnetic field is used to adjust the depth of the racetrack: a deeper racetrack has stronger self-shadowing, thereby causing less deposition from the deepened racetrack area of the target.
While these modified designs can nominally enhance the uniformity of the thin films deposited using a magnetron, they suffer from significant disadvantages. For example, the large size of electromagnets renders their use with many conventional magnetron configurations impractical. A fixed array of permanent magnets positioned around the perimeter of a magnetron can be used to adjust deposition thickness only around the perimeter of the wafer. Furthermore, the nature of these conventional adjustments typically limits the usefulness of the magnetic improvement to only about 5% to about 10% of the life of the sputtering target. Additionally, secondary magnetic fields generated by electromagnets or a fixed array of permanent magnets are not capable of correcting nonuniformities resulting from nonuniform grain structure in the sputtering target. Specifically, each target is unique on a molecular level, and therefore will need a different degree of compensation from the secondary magnetic field. Conventional electromagnets and fixed arrays of permanent magnets are not readily capable of adjusting for these variances, which tend to be more pronounced in conical magnetrons.
In accordance with the foregoing, an improved system for adjusting the magnetic field in a magnetron system has been developed. This improved system allows thin films with greater uniformity in both physical and electrical characteristics to be deposited, as compared with films deposited using conventional systems. An exemplary embodiment of this improved system includes two distinct mechanisms for controlling the magnetic field inside the magnetron processing chamber, thereby compensating for nonuniformities caused by both systematic issues (such as nonuniform gas flows in the process chamber) and random issues (such as erosion or varying grain patterns in sputtering targets).
One mechanism allows the magnetic field to be fine tuned to provide a desired asymmetry in the processing chamber magnetic field. This asymmetry can be used to compensate for systematic nonuniformities resulting from effects such as uneven gas flows or other system components that affect film deposition is a consistent manner.
Another mechanism allows for compensation due to nonuniformities caused by more random sources, such as erosion or varying grain patterns in the sputtering targets. In an exemplary embodiment, a secondary set of movable magnets are positioned around the exterior of the processing chamber. These movable magnets are positioned on nonmagnetic rings that are driven by a computer-controlled motor. The magnets can be positioned around the perimeter of the wafer and/or above and below the wafer. This arrangement provides compensation for nonuniformities resulting from more random effects such as erosion or varying grain patterns in sputtering targets. This arrangement also provides compensation for nonuniformities that manifest themselves across the wafer, rather than only at the wafer perimeter.
A cross-sectional side view of an exemplary embodiment of a magnetron is illustrated in
Still referring to
As described herein, during a sputtering operation a gas is supplied into the chamber 18 and electrical potentials are applied to the targets 12, 14. A plasma of the gas is formed in the chamber, which causes ions to accelerate towards and impact the targets 12, 14. This causes erosion of material from the targets 12, 14; this material is subsequently deposited onto the wafer 16.
The chemical composition of the gas in the chamber 18, together with the material comprising the targets 12, 14, are preselected for deposition of a desired substance onto the wafer 16. In one embodiment, magnetron 10 is used for sputtering thin films of highly piezoelectric aluminum nitride. In such an embodiment, the chamber contains a mixture of nitrogen and argon gas, the targets 12, 14 comprise aluminum, and the wafer 16 is a silicon wafer.
In one embodiment, the electrical potentials applied to the targets 12, 14 are reversed in polarity at a frequency of about 40 kHz. Periodic reversal of the electrical potentials causes ions to alternate between striking upper target 12 and lower target 14, thereby promoting erosion from both targets in a relatively even pattern.
As illustrated in
For example, in one embodiment, the upper magnetic structure 30 includes between 12 and 60 horizontal magnets, between 20 and 60 vertical magnets, between 20 and 180 individual pole pieces and two common pole pieces. Other numbers of magnets and pole pieces are used in other embodiments. The film deposition pattern is adjustable by changing the size and strength of the magnets, and the shape of the pole pieces, in the upper magnetic structure 30. For example, manipulating the magnets and pole pieces that comprise the upper magnetic structure 30 allows the racetrack in the upper target 12 to be manipulated. Changing the racetrack position on the upper target 12 affects the angle, energy, and amount of sputtered material deposited on the surface of the wafer 16 from the upper target 12.
Additionally, the deposition rate and the piezoelectric properties of the film deposited onto the wafer 16 are controllable based on the strength of the magnetic filed generated by the upper magnetic structure 30. A low magnetic field strength from the upper magnetic structure 30 yields a higher deposition rate. A magnetic field strength that acts on the upper target 12 that is too low or too high reduces the coupling coefficient of the film deposited on the wafer 16. Therefore, magnetic field strength affects both film thickness and coupling coefficient.
As another example, in one embodiment the lower magnetic structure 40 includes between 14 and 42 horizontal magnets and between 14 and 42 vertical magnets. The lower magnetic structure also includes between 14 and 126 individual pole pieces and two common pole pieces. Other numbers of magnets and pole pieces are used in other embodiments. The film deposition pattern is adjustable by changing the size and strength of the magnets, and the shape of the pole pieces, in the lower magnetic structure 40. Specifically, manipulating the magnets and pole pieces that comprise the lower magnetic structure 40 allows the racetrack in the lower target 14 to be manipulated. Changing the racetrack position on the lower target 14 affects the angle, energy and amount of sputtered material deposited on the surface of the wafer 16 from the lower target 14.
Additionally, the deposition rate and the piezoelectric properties of the film deposited onto the wafer 16 are controllable based on the strength of the magnetic filed generated by the lower magnetic structure 40. A low magnetic field strength from the lower magnetic structure 40 yields a higher deposition rate. A magnetic field strength that acts on the lower target 14 that is low or high reduces the coupling coefficient of the film deposited on the wafer 16. Therefore, magnetic field strength affects both film thickness and coupling coefficient.
In one embodiment, the magnetic structures 30, 40 are adjusted to compensate for nonuniform film deposition, such as that illustrated in
To compensate for a nonuniform film growth pattern, such as the growth pattern illustrated in
In an exemplary embodiment, the magnetic field is fine tuned by making small adjustments to the position and orientation of the magnetic structures 30, 40, including the stronger magnets 92. Advantageously, once the position and orientation of the magnetic structures 30, 40 have been adjusted to compensate for systematic nonuniformities of a particular magnetron 10, such as those owing to nonuniform gas flows or asymmetries in the process chamber, the need for further adjustment over the life of the magnetron may be greatly reduced.
Additionally or alternately, one or more rotatable magnets are positioned around a circumference of the magnetron 10, as illustrated in
In one embodiment, a computer-controlled motor 102 is used to control movement of the rotatable magnets 94, 96 along the circular tracks 98, 100. For example, in one embodiment, the motor 102 is configured to control movement of a group of rotatable magnets, while in other embodiments the motor 102 is configured to control movement of one or more rotatable magnets individually. In the exemplary embodiment illustrated in
In a modified embodiment, film thickness is monitored and/or measured during deposition. One or more of these measurements are then used to adjust movement of the rotatable magnets 94, 96 in a way that enhances film uniformity, thereby providing a real time feedback system. In other embodiments, other properties of the deposited film—such as film thickness, stress characteristics, and coupling coefficient—are measured after deposition. One or more of these measurements are then used to adjust movement of the rotatable magnets 94, 96 in a way that enhances film uniformity during a subsequent deposition process.
Although the rotatable magnets 94, 96 illustrated in
In an exemplary embodiment, the rotatable magnets 94, 96 are capable of compensating for nonuniformities in the thickness of the thin film deposited on the wafer. For example, the rotatable magnets 94, 96 are used to (a) increase the magnetic field in a region of the wafer where the film growth rate is to be reduced, and/or (b) decrease the magnetic field in a region of the wafer where the film growth rate is to be increased. Whether the rotatable magnets 94, 96 increase or decrease the magnitude of the magnetic field in a particular region of the wafer depends on how the field produced by the rotating magnets interacts with the field produced by the magnetic structures 30, 40.
In one embodiment, the rotatable magnets 94, 96 are rotated around the processing chamber 18 with a variable angular velocity. This causes the rotatable magnets 94, 96 to have an increased effect in the region of the wafer 16 adjacent where the angular velocity is low, and to have a decreased effect in the region of the wafer 16 adjacent where the angular velocity is high. In a modified embodiment, the rotatable magnets 94, 96 are not rotated around the processing chamber, but rather are oscillated adjacent a region of the wafer 16 where they are to have an increased effect.
The size of the region of the wafer 16 where the rotatable magnets 94, 96 are to have an increase effect is at least partially dependent on the characteristics of the film thickness nonuniformity. For example, in one embodiment the rotatable magnets are configured to have a decreased angular velocity, or are configured to oscillate in a region comprising approximately 120° of the wafer circumference.
Another technique for reducing or eliminating nonuniformities in a sputter-deposited thin film is to position supplemental magnets above and/or below the wafer. Optionally, supplemental magnets are positioned above and/or below the wafer in addition to movable magnets placed around the circumference of the wafer, as illustrated in
The rotatable magnets 94, 96 are particularly useful for compensating for nonuniformities present around the circumference of the wafer 16, the supplemental magnets 104 illustrated in
The mechanisms described herein for manipulating the magnetic field in a magnetron processing chamber 18 allow thin films with fewer non-uniformities to be formed. The nonuniformities can be in the electrical characteristics of the film, such as film coupling coefficient, or can be in the physical characteristics of the film, such as film thickness. For example, the mechanisms described herein allow thin films with enhanced thickness uniformity to be manufactured, which is particularly advantageous in the manufacture of FBAR filters.
An FBAR filter is a series of electrically connected, air suspended membrane-type resonators of a piezoelectric material, such as aluminum nitride or zinc oxide, sandwiched between metallic electrodes, such aluminum or molybdenum. The resonant frequency of an FBAR filter is at least partially dependent on the thickness of the piezoelectric material. FBAR filters are formed on silicon wafers using sputter deposition techniques and apparatuses disclosed herein. In one embodiment, a 150 mm silicon wafer holds over 104 individual filters. Using the techniques disclosed herein, the filters on a silicon wafer have a resonant frequency that is within 0.2% of a nominal center frequency. To accomplish this, the 1σ thickness uniformity of an aluminum nitride layer deposited over the wafer surface is 0.2%, which produces a filter yield of 70%. In contrast, if the 1σ thickness uniformity falls to 1%, a yield of only 14% will result, rendering commercial fabrication problematic.
While the foregoing detailed description discloses several embodiments of the present invention, it should be understood that this disclosure is illustrative only and is not limiting of the present invention. It should be appreciated that the specific configurations and operations disclosed can differ from those described above, and that the methods described herein can be used in contexts other than deposition of thin films.
