TWI694522B - Microwave assisted parallel plate e-field applicator - Google Patents

Abstract

A system and method for annealing a target substrate such as a semiconductor using industrial microwave heating and parallel plate reaction. Using a uniform microwave field, with the target substrate located between parallel plates controls application of eddy currents to the target substrate. The system may include a uniform microwave field generator, support elements, two plates held in parallel to each other, and a turntable device configured to rotate the two plates and the target substrate within the uniform microwave field. The rotating of the plates and target substrate in the uniform microwave field creates a periodic change in polarity of the microwaves applied to the target substrate. The eddy currents in the uniform microwave field react by flowing perpendicular to the plates, and not parallel to the surface as in traditional microwave reactions of metals. This redirection of the eddy currents provides even heating of the target substrate.

Description

Microwave assisted parallel plate electric field applicator

Embodiments of the present invention can provide an annealing system and method for annealing a target substrate, such as a semiconductor, using industrial microwave heating and parallel plate reaction.

Advances in miniaturization of semiconductor devices have led to many electronic devices having better performance and increased storage capacity. Many process steps are involved in the manufacture of semiconductor devices. One step is the doping of the semiconductor substrate to form the source/drain junction. Ion implantation is used to modify the electrical characteristics of the semiconductor substrate by implanting specific dopant impurities into the semiconductor wafer surface. Commonly used dopants are boron, arsenic and phosphorus. With the use of ion implantation, a post-annealing process is desired to complete the activation process and repair any damage related to the implanted area. Depending on the implantation dose (the amount of atoms implanted in the surface) and the implantation energy (depth of atoms entering the surface), various annealing techniques can be used. For example, annealing techniques may include furnace processing, rapid thermal processing (RTP), millisecond annealing (MSA), and various other versions including laser annealing. However, there are disadvantages associated with each of these technologies, namely, as described in US Patent No. 7,928,021, which is hereby incorporated by reference in its entirety.

Experiments using microwave heating for this annealing process have been conducted in the solid-state device industry, but the use of microwave heating suffers from some shortcomings. For microwave heating, A multi-mode reaction chamber is used to heat/process a target substrate, which is relatively larger than the wavelength of the microwave used. In a multi-mode room, the microwave energy is coupled through mode excitation to manage the local microwave field, which is also known as an electric field (E-field). The electric field may also be affected by the dielectric properties of the heated target substrate in the multi-mode chamber. If the target substrate is made of a material with an appropriate dielectric, the microwave will flow to the target substrate at a higher concentration. According to the electromagnetic properties of the microwave and the skin effect of the target substrate in the multi-mode room, the target substrate can form a current flow through or on its surface according to its electrical conductivity.

However, the electric field concentration may be difficult to monitor and control. For example, if the concentration of the electric field is strong enough, it may cause undesirable thermal runaway and arcs unrelated to the dielectric reaction of microwaves, which may cause non-target substrates in the multi-mode reaction chamber. Even heating and possible damage. Stirrers and rotating plates have been used to try to make the electric field more uniform, and metal foil layers have also been used to change the local field energy of the heated target substrate. However, each of these methods faces the challenge of trying to manage, minimize, or eliminate the formation of eddy currents to avoid the uneven heating traditionally caused by the formation of eddy currents.

Another method of heating the target substrate is a parallel plate reactor, which is most commonly used in radio frequency (RF), mainly due to technical limitations in higher frequencies. Therefore, the independent parallel-plate reactor is generally limited to frequencies in the RF band, and due to the wavelength used, produces a limited response in the target substrate. RF heating in the conventional technology has only been introduced to the solid-state market as a matrix heater, and there is no real difference in heating compared to other traditional heating methods such as infrared and the like.

Embodiments of the present invention solve the aforementioned problems and provide significant advances in the technology of annealing semiconductor materials. Specifically, embodiments of the present invention can provide an annealing system and method for annealing a target substrate, such as a semiconductor, using industrial microwave heating and parallel plate reaction.

In some embodiments of the invention, the annealing system may include a uniform microwave field generator; two plates held close to and/or parallel to each other; and a turntable device, which is coupled to the uniform The two plates within the microwave field and the target substrate. The uniform microwave field generator can generate a uniform microwave field, and the two plates can be kept spaced apart from each other within the uniform microwave field generator. In particular, the plates can be spaced sufficiently close together to create a capacitive effect between the plates within the uniform microwave field. The turntable can rotate the plates and the target substrate within the uniform microwave field, which produces a periodic change in the polarity of the microwave applied from the uniform microwave field to the target substrate, thereby The eddy current flows perpendicular to the plates and the target substrate.

Another embodiment of the present invention includes a method for annealing semiconductor material, which includes placing a target substrate made of the semiconductor material between two plates in a uniform microwave field, and A periodic change occurs in the polarity of the microwave applied from the uniform microwave field to the target substrate. The periodic change provides vertical flow of eddy current relative to the target substrate and the plates.

In yet another embodiment of the present invention, a method for annealing semiconductor material includes doping parallel plates, and then disposing a target substrate made of the semiconductor material on the substrate within a uniform microwave field Between the parallel plates. The doping may be sufficient to allow the parallel plates to react to the uniform microwave field, and the parallel plates may be sufficiently close to They are spaced together to form a capacitive effect between the parallel plates within the uniform microwave field. The target substrate may contain the semiconductor material doped with impurities. The uniform microwave field may contain frequencies in the range of 900MHz to 26GHz. Next, the method may include a step of rotating the parallel plates and the target substrate within the uniform microwave field, thereby generating a period on the polarity of the microwave applied from the uniform microwave field to the target substrate Sexual changes. The periodic change can provide vertical flow of eddy currents relative to the target substrate and the parallel plates, thus providing uniform heating of the target substrate and selectively heating defects in the target substrate.

This summary of the invention is provided to introduce selected concepts in a simplified form, which are further described in the following detailed description. This summary of the invention is not intended to indicate the key features or important features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Other features and advantages of the present invention will be apparent from the following embodiments and accompanying drawings.

10‧‧‧Annealing system

12‧‧‧Uniform microwave field generator

14‧‧‧Support element

16‧‧‧ board

18‧‧‧Turntable device

20‧‧‧Target substrate

22‧‧‧Semiconductor layer

24‧‧‧Base

26‧‧‧Surface current (eddy current)

28‧‧‧Silicon lattice

30‧‧‧ Defect

32‧‧‧Microwave

34‧‧‧Internal heat

200‧‧‧Method

202, 204, 206, 208‧‧‧

The embodiments of the present invention are described in detail below with reference to the accompanying drawings, in which: FIG. 1 is a schematic diagram of an annealing system constructed according to various embodiments of the present invention; FIG. 2 is a 3D schematic view of an example target substrate heated in an annealing system; FIG. 3 is a perspective view of another example target substrate to be heated in the annealing system of FIG. 1; FIG. 4 is a variety of A flowchart of an annealing method according to an embodiment; FIG. 5 is a graph of band gaps of different semiconductor materials with respect to the substrate mobility for the target substrate; FIG. 6 is a schematic view of a target substrate subjected to conventional microwave heating of a conventional technology, in which eddy currents are concentrated in the The edge of the target substrate, and flows substantially parallel to the surface of the target substrate; FIG. 7 is a schematic view of the target substrate between the two plates using the method of FIG. 4 for mobility annealing, wherein the eddy current is vertical 8 is a schematic view of a target substrate having a silicon lattice with a defect therein; FIG. 9 is a schematic view of a conventional microwave heating of the target substrate of FIG. Eddy current flows on the surface of the target substrate and parallel to the surface; and FIG. 10 is a schematic view of the target substrate of FIG. 8 using the method of FIG. 4 to perform mobility annealing, wherein the eddy current flows into the target substrate And perpendicular to it.

The drawings do not limit the invention to the specific embodiments disclosed and described herein. This drawing is not necessarily to scale, but emphasizes clearly describing the principles of the present invention.

The following detailed description of the invention refers to the accompanying drawings, which depict specific embodiments in which the invention may be implemented. These embodiments are intended to describe the features of the present invention in sufficient detail to enable those skilled in the art to implement the present invention. Other embodiments can also be utilized, and changes can be made without departing from the scope of the present invention. Therefore, the following detailed description is not intended to be limiting. The scope of the present invention is only defined by the complete scope of the attached patent application scope and the equivalents granted by such patent application scope.

In this description, the reference frame for "one embodiment" or "multiple embodiments" The one or more features discussed are included in at least one embodiment of the technology. Unless so stated and/or except for those skilled in the art will be quite obvious from the description, individual references to "one embodiment" or "multiple embodiments" in this description It does not necessarily refer to the same embodiment, and it is not mutually exclusive. For example, a feature, structure, action, etc. described in one embodiment may also be included in other embodiments, but it is not necessary to be included. Therefore, the current technology may include various combinations and/or integrations of the embodiments described herein.

The embodiment of the present invention relates to an annealing system. A specific embodiment of the present invention relates to industrial microwave heating, which utilizes a uniform microwave field and parallel plates to control the application of eddy current to a target substrate 20.

As depicted in FIG. 1, an annealing system 10 of the present invention may include a uniform microwave field generator 12, a support element 14, two plates 16 maintained in a spaced relationship with each other, and a turntable device 18, It is configured to rotate the two plates 16 and the target substrate 20 within a uniform microwave field in one of the uniform microwave field generators 12. Although a set of parallel plates 16 is described here, it is noted that many additional plates can also be stacked vertically to form a batch reaction (ie, multiple target substrates are operated at once).

The target substrate 20 may be a substrate material having any geometry known in the art, such as a semiconductor device, an ion-implanted wafer, and/or a silicon wafer, and may have a flat plate Or wafer geometry. For example, the target substrate 20 may be a semiconductor substrate doped with specific dopant impurities (eg, boron, arsenic, phosphorous) to form a source/drain junction. In some embodiments of the present invention, the target substrate 20 may be a transistor, as outlined in FIGS. 2 and 3. Additionally or alternatively, the target substrate 20 may include a plurality of target substrates located between the plates 16 described herein, such as a plurality of ion-implanted wafers. The An annealing treatment of the target substrate 20 can be used to complete its activation and repair any related damage to the implanted area, as described in detail below.

The uniform microwave field generator 12 may be a single-mode or multi-mode chamber, or may alternatively include a waveguide port configured to form around and/or between the plates 16 described below A microwave field. In some embodiments of the present invention, the microwave frequency range generated by the uniform microwave field generator 12 may be in a range of approximately 900 MHz to 26 GHz. For example, the frequency generated by the microwave field generator 12 may be about 915 MHz, about 2.45 GHz, or about 5.8 GHz or 24 GHz. However, the uniform microwave field generator 12 can be configured to generate any desired microwave frequency without departing from the scope of the present invention. In some embodiments of the present invention, the heat generated by the uniform microwave field generator 12 may be in a range of about 400°C to 800°C. However, other temperatures can be used without departing from the scope of the present invention.

The support elements 14 may be made of an insulator material such as quartz, and may be configured to hold and/or support the plates 16 and target substrate 20. For example, the supporting elements 14 may be fixed to a rotating element of the turntable device 18. Alternatively, the support elements 14 may be attached to the inner wall or other parts of the uniform microwave field generator 12. Furthermore, the supporting elements 14 may include grooves, clamps, or other configurations for fixing the plates 16 and the target substrate 20 at a predetermined distance from each other. In some embodiments of the invention, the support elements 14 may be selectively adjustable, so that different spacings can be used for different plates 16 and/or different geometries and/or different materials Different target substrate 20.

The plates 16 may be substantially parallel to each other, and each may include half of the guide The body layer 22 and a susceptor layer 24. The base layer 24 may be disposed closest to the target substrate 20, wherein each of the semiconductor layers 22 faces outward from the two base layers 24. However, in some embodiments of the present invention, the base layer 24 may be omitted.

The semiconductor layer 22 can be configured to function as a dielectric at lower temperatures and as a metal at higher temperatures. Therefore, the semiconductor layer 22 increases in conductivity as the temperature increases. This generates a capacitive field to generate a capacitive electric field plane between the two semiconductor layers 22. Therefore, the plates 16 cooperatively function as a parallel plate capacitor. However, in some embodiments of the present invention, the semiconductor layer 22 may be replaced by a conductive layer made of metal or other conductive material that becomes conductive when the temperature increases, as long as such Metals and other materials may be within a range of conductivity that can carry a surface current flow when heated as described herein.

The pedestal layers 24 can be used to pre-heat the target substrate 20 between the two. In particular, the pedestal layers 24 may be made of materials configured to absorb the microwave, and thus cooperatively generate a uniform microwave field between the two. However, in some alternative embodiments of the present invention, if the uniform microwave field is otherwise generated between and/or around the two semiconductor layers 22, the pedestal layers 24 may be Omitted.

The plates 16 can have any size and geometry known in the art. In some embodiments of the present invention, the plates 16 may be dish-shaped, square, or rectangular. Furthermore, the plates 16 may be thin flat discs, which have a thickness roughly related to a plate or disc in the solid-state industry. The plates 16 may have an interval of about 0.5 mm to about 5 mm from each other. The plates 16 may preferably be as thin as possible, as long as they are not so thin as to be sacrificed to when they are mounted on the support elements 14 and/or when they are heated within the uniform microwave field generator 12 The structural integrity is sufficient. In some embodiments of the invention, the plates 16 may be spaced apart by approximately 1 mm to 10 mm. However, other separation distances can be used without departing from the scope of the present invention. Specifically, the plates 16 should be spaced close enough together to form a capacitive effect, and therefore close enough to allow the surface current (ie, eddy current) to react, as described herein .

In some embodiments of the present invention, the plates 16 may be arranged in any orientation within the uniform microwave field, such as horizontal, vertical, or other orientations. The plates 16 can generally be arranged parallel to each other. However, in some alternative embodiments of the present invention, the plates 16 may be arranged in a non-parallel relationship with each other and/or with the target substrate 20, as long as the plates 16 are close enough to be formed here The capacitance effect described above is sufficient.

The carousel device 18 may be any mechanism known in the art for generating rotation of items attached to it. For example, the turntable device 18 may include a rotary motor external to the uniform microwave field generator. Furthermore, one of the support elements 14 described above can be attached to a rotating shaft of the rotary motor and can extend into the uniform microwave field generator 12 to allow the plates 16 and/or target substrate 20 to It is rotatably supported at the desired position and at the desired interval from each other. Within the uniform microwave field, the rotation of the two plates 16 and the target substrate 20 can change the polarity of the microwave applied to it, which is an RF switch that simulates a method of conventional technology. For example, the annealing system 10 may be configured such that the rotation of the target substrate 20 may change the polarity of the microwave applied thereto every 15°. Other methods of switching the microwave polarity can be used instead without departing from the scope of the present invention.

The turntable device 18 can be configured for any use that does not cause the plates 16 and /Or the separation rate of the target substrate 20. In some embodiments of the present invention, the turntable device 18 can rotate the plates 16 and/or target substrate 20 at a minimum speed of one revolution per minute (rpm) and a maximum speed of 10 rpm. For example, the turntable device 18 can rotate the plates 16 and/or the target substrate 20 at a rate of about 2 rpm. However, other rates can be used without departing from the scope of the present invention.

In use, the target substrate 20 can be placed between the plates 16 within the uniform microwave field generator 12 and rotated by the turntable device 18 within the uniform microwave field, Therefore, a periodic change occurs in the polarity of the microwave applied to the target substrate 20. The target substrate 20 will be heated mainly according to its own dielectric properties, which converts the microwave into heat, and/or generates eddy currents on the surface of the target substrate 20. As described below, the eddy current system reacts by flowing perpendicular to the plates 16, which uniformly heats the target substrate 20. In some embodiments of the invention, the plates 16 may need to be doped in order to react to the uniform microwave field.

FIG. 4 depicts the steps in a method 200 for annealing semiconductor materials using a uniform microwave field and parallel plate reaction according to various embodiments of the present invention. The steps of the method 200 may be performed in the order as shown in FIG. 4, or they may be performed in a different order. Furthermore, as opposed to sequential execution, certain steps can be performed simultaneously. In addition, certain steps may not be performed. Some of these steps may represent code segments or executable instructions of the aforementioned computer program or application program.

In some embodiments of the invention, as depicted in block 202, the method 200 may include a step of doping the plates 16 to react to the uniform microwave field. For example, intrinsic silicon at room temperature may be mainly microwave transparent, so it can be doped to reaction. Doping the silicon material can change the electrical conductivity at room temperature, thus allowing microwaves to be heated at room temperature/reacting with these silicone parallel plates. In this example, once the silicone plates are heated, their conductivity can be reduced according to a band gap of the foreign silicon material. A graph depicting the band gaps of various materials and their matrix mobility is provided in FIG. 5. This reduction in conductivity can achieve a microwave response/through, and when the temperature or conductivity is within the range, it generates a parallel plate electric field in a microwave field. In the parallel plate electric field described here, the eddy current generated from a microwave reaction will flow perpendicular to the target substrate 20 (as depicted in FIG. 10), rather than parallel to the microwave reaction of a conventional metal The surface flows (as depicted in Figure 9).

In some embodiments of the present invention, as depicted in block 204, the method 200 may optionally include adjusting the geometry and material for at least one of the plates and the target substrate One step at a distance between the plates 16. For example, as described above, the support elements 14 may be selectively adjustable so that different spacings can be used for different plates 16 and/or different target substrates with different geometries and/or different materials 20.

As depicted in block 206, the method 200 may further include a step of placing the target substrate 20 between the plates 16 within the uniform microwave field (eg, a multi-mode chamber). As described above, the intervals between the plates 16 may be about 0.5 mm to about 5 mm or 10 mm. As mentioned above, the target substrate 20 and the plates 16 can be suspended and supported by a support element 14 made of an insulator material such as quartz.

Then, as depicted in block 208, the method 200 may include a step of using the turntable device 18 to rotate the plates 16 and/or the target substrate 20 within the uniform microwave field. The polarity of the microwave applied to the target substrate 20 generates a periodic Variety. The target substrate 20 will be heated mainly according to its own dielectric properties, which converts the microwave into heat, and/or generates eddy currents on the surface of the target substrate 20. Conventionally, as depicted in FIG. 6, the surface current or eddy current 26 is formed in a flat plate within a microwave field and/or at the edge or boundary of the target substrate 20. However, as depicted in FIG. 7, the configuration of the rotating plate disclosed herein provides vertical flow of eddy current 26 relative to the target substrate 20, which results in its uniform heating and as shown in FIG. 10 The painted selectively heats the defects in the silicon lattice of the target substrate 20. On the contrary, the conventional heating method (for example, the simple microwave heating depicted in FIG. 9) does not selectively heat the defects within the silicon lattices.

Specifically, FIG. 8 is a schematic depiction of the target substrate 20, in which a silicon lattice 28 has a defect 30, which is also known as a region with reduced mobility. FIG. 9 depicts that the same silicon lattice 28 is heated only by the microwave 32, wherein the generated eddy current 26 flows parallel to the target substrate 20. FIG. 9 also depicts the internal heat 34 generated by the microwave 32 in the silicon lattice 28.

FIG. 10 depicts that the silicon lattice 28 is heated via the method 200 described above and depicted in FIG. 4, wherein the generated internal heat 34 is further increased in volume by the eddy current 26 flowing vertically into the target substrate 20. The above is aimed at the defect 30 therein. The changing direction of the eddy current 26 allows interface polarization to occur at selected points in the target substrate 20 where the eddy current 26 is prohibited (eg, grain defects, impurities, and other defects 30). The polarization of the defect 30 may not cause a substantial reaction in itself, but now the polarized defect 30 also suffers from the already existing microwave field (for example, microwave 32), which allows the defect 30 to be "selective" The ground is heated. This means that the temperature of the defect 30 will be much higher than the rest of the target substrate 20 (also referred to herein as the base material). This base material can act as a heat sink, This is heat dissipated in the base material of the target substrate 20.

Therefore, the heating method 200 described herein completes an activation process and repairs any damage related to the implanted or doped regions of the target substrate 20, which is an undesirable heat for microwave annealing methods without prior art Runaway and arc. Advantageously, it is not an attempt to manage, minimize, or eliminate the formation of eddy currents as in conventional microwave methods, but the present invention changes the direction in which the eddy currents flow, thereby avoiding unevenness Heating and effectively repair the defects in the target substrate 20.

Although the present invention has been described with reference to the embodiments depicted in the accompanying drawings, it should be noted that equivalents and substitutions can be used here without departing from the invention as set forth in the scope of the patent application Category.

So far, various embodiments of the present invention have been described. Those who claim to be novel and wish to be protected by patent certificates include the following:

10‧‧‧Annealing system

12‧‧‧Uniform microwave field generator

14‧‧‧Support element

16‧‧‧ board

18‧‧‧Turntable device

20‧‧‧Target substrate

22‧‧‧Semiconductor layer

24‧‧‧Base

Claims (20)

An annealing system for annealing a target substrate, the system includes: a uniform microwave field generator configured to generate a uniform microwave field; two spaced-apart plates are arranged on the uniform Within the microwave field generator, and is oriented to form a capacitive effect between the two plates within the uniform microwave field; and a turntable device, which is configured to surround the diameter along the target substrate The axis of the wire rotates the two plates and the target substrate within the uniform microwave field, thereby generating a periodic change in the polarity of the microwave applied from the uniform microwave field to the target substrate . As in the annealing system of claim 1, the two plates are doped to react to a microwave electric field generated by the uniform microwave field generator. The annealing system as claimed in item 1 of the patent scope further includes a support element composed of an insulator material, which attaches the turntable device to the two plates and keeps the two plates parallel to each other. As in the annealing system of claim 3, at least one of the support elements is configured to fix the target substrate between the two plates and parallel to the two plates. As in the annealing system of claim 3, where the support elements are selectively adjustable, such that a distance between the two plates is adjustable. For example, the annealing system of claim 1 of the patent application, wherein the two plates are separated by 0.5mm to 10mm. As in the annealing system of claim 1, the eddy current in the uniform microwave field flows perpendicular to the two plates to react to the periodic change in polarity. Provides uniform heating of the target substrate. An annealing system as claimed in item 1 of the patent application, in which the two plates have a sufficiently high electrical conductivity to form an electric field in the uniform microwave field and withstand a heat of approximately 400 to 800 degrees Celsius. As in the annealing system of claim 1, the uniform microwave field generator is a single-mode or multi-mode chamber, or a waveguide port, which is configured to form the uniform microwave around the two plates field. For example, in the annealing system of claim 1, the uniform microwave field generator generates a frequency in the range of 900 MHz to 26 GHz. As in the annealing system of claim 1, the two plates respectively include a semiconductor layer and a pedestal layer, wherein the two plates are oriented such that the pedestal layers face each other. As in the annealing system of claim 1, the two plates respectively include semiconductor material or conductor material, which is configured to increase in conductivity with increasing temperature. A method for annealing semiconductor materials, the method comprising the steps of: placing a target substrate composed of the semiconductor material between two plates in a uniform microwave field, wherein the two plates are sufficient Spaced close together to form a capacitive effect between the two plates within the uniform microwave field; and to generate a period on the polarity of the microwave applied from the uniform microwave field to the target substrate Change in nature, where the periodic change provides vertical flow of eddy current relative to the target substrate and the two plates. A method as claimed in item 13 of the patent application, wherein the step of generating the periodic change includes rotating the two around the axis along the diameter line of the target substrate within the uniform microwave field The board and the target substrate produce a periodic change in the polarity of the microwave applied to the target substrate. As in the method of claim 13, the method further includes doping the two substrates before placing the target substrate between the two plates, wherein the doping is sufficient to allow the two plates to apply uniform microwaves The field reacted. As in the method of claim 13, the target substrate includes a semiconductor substrate doped with impurities. As in the method of claim 13, the method further includes adjusting the distance between the two plates according to the geometry and material used for at least one of the two plates and the target substrate. For example, in the method of claim 13, the uniform microwave field contains a frequency in the range of 900MHz to 26GHz. A method for annealing semiconductor materials, the method comprising the steps of: doping parallel plates, wherein the doping is sufficient to allow the parallel plates to react to a uniform microwave field; within the uniform microwave field A target substrate is disposed between the parallel plates, wherein the target substrate includes the semiconductor material doped with impurities, wherein the uniform microwave field includes a frequency in the range of 900 MHz to 26 GHz, wherein the parallel The plates are spaced close enough together to form a capacitive effect between the parallel plates within the uniform microwave field; and rotating around the axis of the target substrate along the diameter line of the uniform The parallel plates and the target substrate within the microwave field, thereby generating a periodic change in the polarity of the microwave applied from the uniform microwave field to the target substrate, wherein the periodic change provides vortex Current phase For the vertical flow of the target substrate and the parallel plates, this provides uniform heating of the target substrate and selective heating of defects in the target substrate. For example, the method of claim 19, wherein the rotation is performed at a rate within a range of 1 revolution per minute to 10 revolutions per minute.

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