TWI610389B - Polishing pad conditioning method, polishing pad conditioning device and chemical mechanical planarization system - Google Patents

Abstract

The invention discloses a polishing pad dressing method, a polishing pad dressing device and a chemical mechanical polishing device. The polishing pad dressing method includes trimming the polishing surface of the polishing pad with a dresser. Before the dresser trims the area to be finished on the polishing surface, a plurality of soft microstructures located in the area to be finished are hardened, so that the dresser can trim the soft microstructure on the polishing pad more quickly and easily.

Description

Grinding pad dressing method, polishing pad dressing device and chemical mechanical polishing device

The invention relates to a polishing pad dressing method, a polishing pad dressing device and a chemical mechanical polishing device, in particular to a polishing pad dressing method, a polishing pad dressing device and a chemical mechanical polishing device which perform surface treatment before trimming the polishing surface of the polishing pad.

In chemical mechanical polishing (CMP), the polishing pad is usually used in conjunction with the polishing slurry to planarize the wafer surface. The polishing pad is a cushion formed by polymer polymerization. At the microscopic level, the structure of the polishing pad is a porous microstructure, and the surface of the polishing pad has a lot of fluff. When the polishing pad continuously grinds several wafers, the by-products generated in the chemical mechanical polishing process will adhere to the surface of the polishing pad, even embedded in the micropores on the surface, causing the holes to be filled or adhered to the fluff. In the middle, the fluff is poured, which in turn causes the polishing ability of the polishing pad to be greatly reduced. Therefore, when the polishing pad is used for a period of time, it is necessary to use a polishing pad conditioner to swing back and forth along the radial direction on the polishing pad, and cooperate with the rotation of the polishing pad itself to generate a new pile structure and remove chemical mechanical polishing. Residue attached to the process.

In addition, as the size of the wafer shrinks, the requirements for wafer surface flatness are also increasing. If the height of the fluff on the surface of the polishing pad is too large, the raised fluff is very likely to scratch the surface of the wafer, which is called killer Asperities. Therefore, these killer cilia must also be removed by a pad conditioner.

Referring to Figure 1, in the prior art, a plurality of cutting edges 100a are provided on the working surface 100 of the dresser 10 to cut off the excessively long piles 11s on the surface of the polishing pad 11, i.e., killer cilia. However, when the blade portion 100a of the polishing pad dresser 10 comes into contact with these piles 11s, since the material of the pile 11s is soft and flexible, the pile 11s is not easily cut by the blade portion 100a, and remains on the surface of the polishing pad 11.

The technical problem to be solved by the present invention is to provide a polishing pad dressing method, a polishing pad dressing device, and a chemical mechanical polishing device using the same, which are disadvantages of the prior art.

In order to solve the above technical problems, one of the technical solutions adopted by the present invention is to provide a polishing pad dressing method. First, a plurality of soft microstructures in the area to be repaired on the polishing surface of the polishing pad are hardened, and then a plurality of hardened soft microstructures located in the area to be finished are cut by a dresser to trim the polishing surface of the polishing pad.

In order to solve the above technical problems, another technical solution adopted by the present invention is to provide a polishing pad dressing device including a dresser and a polishing surface processing unit. The dresser is used for trimming a polishing surface of the polishing pad, and the polishing surface processing unit is disposed above the polishing pad to harden the surface to be trimmed by the polishing surface treatment unit before the dresser trims a surface to be finished on the polishing surface. Multiple soft microstructures in the area.

In order to solve the above technical problems, another technical solution adopted by the present invention is to provide a chemical mechanical polishing apparatus including a stage, a polishing pad, and the above-mentioned polishing pad dressing device.

The invention has the beneficial effects of the polishing pad dressing method, the polishing pad dressing device and the chemical mechanical polishing device using the same according to the technical solution of the present invention, which can be hardened by being trimmed before being trimmed by the dresser for the dressing area. The soft microstructure in the zone makes it easier for the dresser to cut soft microstructures.

For a better understanding of the features and technical aspects of the present invention, reference should be made to the accompanying drawings.

10‧‧‧Finisher

100‧‧‧ work surface

100a‧‧‧blade

11‧‧‧ polishing pad

11s‧‧‧ fluff

2‧‧‧Chemical mechanical grinding equipment

20‧‧‧ stage

21‧‧‧ polishing pad

210‧‧‧ Polished surface

210s‧‧‧Soft microstructure

3‧‧‧ Ice crystal layer

22‧‧‧ Wafer Holder

22s‧‧‧Rotary axis

23‧‧‧Slurry distribution unit

24‧‧‧ polishing pad dressing device

240‧‧‧Finisher

240a‧‧‧ work surface

242‧‧‧ moving arm

243‧‧‧Rotary axis

241, 241'‧‧‧ polished surface treatment unit

241p‧‧‧Supply unit

241s, 241S‧‧ nozzle

241h‧‧‧cantilever

241L‧‧‧ bracket

25‧‧‧ shaft

26‧‧‧ cleaning unit

W1‧‧‧ wafer

R1‧‧‧Renovation area

R1’‧‧‧ hardened area to be repaired

D1‧‧‧ radial direction

D2‧‧‧Rotation direction

S100~S200‧‧‧ Process steps

1 is a partial enlarged view of a conventional polishing pad dresser trimming polishing pad.

2 is a flow chart of a polishing pad dressing method according to an embodiment of the present invention.

3 is a perspective view of a chemical mechanical polishing apparatus according to an embodiment of the present invention.

4 is a top plan view of the chemical mechanical polishing apparatus of FIG. 3.

Figure 5 is a side elevational view of the chemical mechanical polishing apparatus of Figure 3.

Fig. 6 is a partially enlarged view showing the polishing pad being trimmed by the polishing pad dressing device of the embodiment of the present invention.

Figure 7 is a top plan view of a chemical mechanical polishing apparatus according to another embodiment of the present invention.

Figure 8 is a side elevational view of the chemical mechanical polishing apparatus of Figure 7.

The following is a description of embodiments of the present invention relating to "polishing pad conditioning methods, polishing pad conditioning devices, and chemical mechanical polishing devices" by way of specific embodiments. Typically the polishing pad has a polished surface and the polishing surface has a plurality of soft microstructures. After the polishing pad is used a plurality of times, the polishing pad dressing device may be performed by the polishing pad dressing device of the embodiment of the present invention to remove the residue generated in the chemical mechanical polishing process and trim the polishing surface of the polishing pad to reduce the softness. The height difference of the microstructure. The aforementioned soft microstructures are, for example, fluff or other soft protrusions.

Please refer to Figure 2. 2 is a flow chart of a polishing pad dressing method according to an embodiment of the present invention. As shown in FIG. 2, in step S100, a plurality of soft microstructures in a region to be trimmed on the polishing surface of the polishing pad are hardened. Next, in step S200, the area to be trimmed is trimmed by a dresser. The aforementioned area to be trimmed may refer to the entire polished surface or a partially polished surface. That is to say, the polishing surface may be partially or completely hardened first to harden a plurality of soft microstructures. After that, the soft microstructure of the area to be repaired is cut by the dresser to trim the polished surface of the polishing pad.

In an embodiment of the invention, the polishing surface of the polishing pad is hardened by rapid freezing. In detail, the refrigerant may be sprayed on the surface to be finished on the polishing surface so that the surface layer of the polishing pad is rapidly frozen and hardened.

In one embodiment, the cryogen may be liquid nitrogen, liquid oxygen, dry ice or a refrigerant. The refrigerant is, for example, liquid ethyl chloride or a near-azeotropic refrigerant obtained by mixing difluoromethane (R-32) and pentafluoroethane (R-125). The cryogen can cool the polishing surface to a maximum of zone of maximum crystallization of ice, between about 0 ° C and -5 ° C, so that the polished surface hardens due to freezing.

In addition, since moisture is usually adhered to the polishing pad, after the refrigerant is sprayed, the moisture is instantaneously frozen to form an ice crystal layer covering the polishing surface. The ice crystal layer will coat and fix the soft microstructure.

Therefore, when the polishing pad is trimmed with the dresser, the soft microstructure of the polishing surface is hardened and fixed by the ice crystal layer, and can be kept upright without being poured or bent, thereby making it easier for the dresser to cut the soft microstructure. It should be noted that the object of the present invention can be achieved as long as the polishing pad is hardened to a depth slightly larger than the depth of the dresser cutting. In one embodiment, the depth of the polishing pad is hardened to be between about 0.01 mm and 0.2 mm. In other words, the bottom layer of the polishing pad is not freeze hardened while still maintaining the original material properties.

It should be noted that after the dresser is trimmed, the contact between the dresser and the polishing pad will generate thermal energy, and the polished surface hardened by freezing and the ice crystal layer will be thawed and melted, so that the polishing pad can be restored after being trimmed. Material properties. In an embodiment, during the dresser trimming of the polishing pad, a cleaning liquid such as water is sprayed on the polishing pad in synchronization. Thus, in addition to cleaning the residue generated during the dressing process, the freeze-hardened polished surface can be promptly thawed.

It should be noted that the present invention does not limit the time point at which the refrigerant is sprayed, the spray range per spray, and the path of the spray as long as the polished surface can be freeze-hardened before the finisher trims the polished surface.

For example, in the step of hardening a plurality of soft microstructures on the polishing surface of the polishing pad, the entire polishing pad may be sprayed with a coolant so that the polishing surface of the entire polishing pad is hardened, and then the dresser is used for dressing and polishing. pad. That is to say, the step of spraying the refrigerant and the trimming step are not performed simultaneously.

In this case, each spray range of the cryogen may cover the polished surface of the entire or partial polishing pad. In addition, when the refrigerant is sprayed, the polishing pad can be stationary or rotated by the rotating shaft.

However, in another embodiment, the step of spraying the cryogen and the step of trimming the polishing pad with the dresser can be performed simultaneously. In detail, when the polishing surface is trimmed by the dresser, the polishing pad is rotated by a rotating shaft. Therefore, the polishing surface can be sequentially divided into a plurality of areas to be trimmed along a rotation direction of the polishing pad. The step of spraying the refrigerant and the step of trimming the polishing pad with the dresser are performed in the same direction.

Further, when the polishing pad rotates, the dresser moves along a radial direction of the polishing pad and sequentially trims the plurality of areas to be trimmed. In one embodiment, the refrigerant is also sprayed in a radial direction over the area that the dresser is about to pass to first harden the soft microstructure of the polishing surface.

The cryogen may be sprayed continuously or intermittently on the polishing surface, depending on the size of the spray range. The refrigerant may be intermittently sprayed when the polishing surface is hardened each time the coolant is sprayed to a much larger extent than the working surface of the dresser. When the polishing surface is hardened by a range close to or less than the working surface of the dresser after each spraying of the refrigerant, the refrigerant can be continuously sprayed.

In another embodiment, when the polishing pad is rotated, the refrigerant is sprayed continuously or intermittently in each of the areas to be trimmed in the radial direction of the polishing pad. In this embodiment, each spray range of the cryogen covers only one of the areas to be trimmed, wherein the area to be trimmed extends from the center of the polishing pad to the edge of the polishing pad.

Referring to FIG. 3, a schematic perspective view of a chemical mechanical polishing apparatus according to an embodiment of the present invention is shown. The chemical mechanical polishing apparatus of FIG. 3 can be used to perform a chemical mechanical polishing process on the wafer, or the polishing pad conditioning method shown in FIG. 2 can be performed after the chemical mechanical polishing process.

As shown in FIG. 3, the chemical mechanical polishing apparatus 2 includes a stage 20, a polishing pad 21, a wafer holder 22, a slurry dispensing unit 23, and a polishing pad finishing device 24. The stage 20 is used to carry the polishing pad 21 and is coupled to a rotating shaft 25 to generate rotation.

The polishing pad 21 has a polishing surface 210 for polishing or grinding the wafer. The wafer holder 22 is used to grab the wafer W1 and place the wafer W1 on the polishing pad 21 and press against the polishing surface 210. The wafer holder 22 can be rotated by a rotating shaft 22s. When the CMP process is performed, the polishing pad 21 and the wafer W1 are relatively rotatable through the rotating shaft 25 and the rotating shaft 22s, respectively.

The slurry dispensing unit 23 is disposed above the polishing pad 21 for distributing the polishing slurry (not numbered) to the polishing surface 210 of the polishing pad 21. The abrasive slurry contains a solution of etched and/or abrasive particles that can chemically interact with the surface of the wafer W1 or mechanically grind the surface of the wafer W1.

The pad dressing device 24 is used to provide the polishing surface 210 of the polishing pad 21 with a predetermined roughness. However, when the chemical mechanical polishing apparatus 2 operates, the polishing surface 210 gradually becomes smooth. A pad dressing device can be used to maintain the polishing surface 210 at a predetermined roughness and remove residue from the polishing surface 210. In one embodiment, the polishing surface 210 is trimmed by the polishing pad conditioning device 24 when the surface of the wafer W1 is polished or after polishing.

The pad dressing device 24 includes a trimmer 240 and a polishing surface processing unit 241. The trimmer 240 is disposed on the polishing pad 21 for scraping and trimming the polishing surface 210. Further, the polishing pad dressing device 24 further includes a moving arm 242 and a rotating shaft 243 rotatably coupled to the moving arm 242. The trimmer 240 is disposed on the moving arm 242 through the rotating shaft 243, and can be rotated. The shaft 243 produces a rotation.

Please refer to FIG. 4 and FIG. 5 , which are respectively a top view and a side view of the chemical mechanical polishing apparatus of FIG. 3 . The moving arm 242 can drive the trimmer 240 to move along the radial direction of the polishing pad 21 on the polishing surface 210. When the dresser 240 moves on the polishing surface 210, it also rotates through the rotation shaft 243 to trim the polishing surface 210.

In addition, the polishing surface processing unit 241 is disposed above the polishing pad 21 to harden a plurality of soft microstructures located in the region to be trimmed R1 before the trimmer 240 trims a region to be trimmed R1 on the polishing surface 210. In the present embodiment, the polishing surface processing unit 241 is disposed on the moving arm 242 together with the trimmer 240 to make the polishing surface processing list Both the element 241 and the trimmer 240 are synchronously movable by the moving arm 242 along the radial direction D1 of the polishing pad 21.

As shown in FIG. 5, the polishing surface processing unit 241 includes a supply unit 241p that can supply a refrigerant and a nozzle 241s that communicates with the supply unit 241p, and the refrigerant supplied from the supply unit 241p is sprayed through the nozzle 241s in the area to be trimmed R1. . As mentioned previously, the cryogen can be liquid nitrogen, liquid oxygen, dry ice or a refrigerant. When the refrigerant is dry ice, the ultra-low temperature dry ice is passed through the nozzle 241s to the polishing pad 21, and the polishing surface 210 is frozen to harden. In addition, the communication between the supply unit 241p and the nozzle 241s may be achieved by piping or other known means, and the invention is not limited thereto.

In the present embodiment, the nozzle 241s is fixed to the moving arm 242 by a cantilever 241h, and is located on the side of the dresser 240 near the center of the polishing pad 21 to spray the refrigerant to the to-be-trimmed area R1 to be passed by the dresser 240. . Before the to-be-trimmed area R1 is thawed, the hardened area to be trimmed R1 is trimmed by the dresser 240 to return the polished surface 210 to the original material characteristics. In other embodiments, the relative position of the nozzle 241s and the trimmer 240 is not limited as long as the area to be passed by the dresser 240 can be sprayed with the refrigerant before the dresser 240 is trimmed.

Referring to FIG. 5 and FIG. 6, a partial enlarged view of FIG. 5 in the area A is shown. As shown in FIG. 6, since the polishing surface 210 is sprayed with the refrigerant, the plurality of soft microstructures 210s of the polishing surface 210 are hardened. In addition, the moisture on the polishing surface 210 also freezes instantaneously to form an ice crystal layer 3 covering the soft microstructure 210s.

A plurality of diamond particles (not numbered) are distributed on the working surface 240a of the finisher 240. Thus, when the working surface 240a of the trimmer 240 contacts the polishing surface 210 for trimming, the soft microstructure 210s can remain upright without being overwhelmed or bent by the dresser 240, thereby making it easier for the trimmer 240 to cut the soft microstructure 210s. Thus, after trimming by the trimmer 240, the unevenness of the polishing surface 210 can be reduced to avoid damage to the surface of the wafer W1 by the protruding killer cilia during the chemical mechanical polishing process.

Referring to FIG. 3 again, the chemical mechanical polishing apparatus 2 of the embodiment of the present invention further includes a cleaning unit 26 for cleaning the trimmed polishing pad 21. Cleaning unit 26 can be sprayed Sprinkle a cleaning solution, such as water to the polishing surface 210, to remove the residue remaining on the polishing surface 210 after trimming, and to help the soft microstructure 210s of the polishing pad 21 to be repaired to be frozen to be thawed to recover The original material properties of the surface of the polishing pad 21.

Please refer to FIG. 7 and FIG. 8. Figure 7 is a top plan view of a chemical mechanical polishing apparatus according to another embodiment of the present invention. Figure 8 is a side elevational view of the chemical mechanical polishing apparatus of Figure 7. Unlike the previous embodiment, the polishing surface processing unit 241' of the present embodiment is not disposed on the moving arm 242, but is disposed adjacent to the dresser 240. Further, when the polishing pad 21 is rotated by the rotating shaft 25 in a rotation direction D2, the polishing surface processing unit 241' and the finisher 240 are sequentially disposed on the polishing pad 21 in the rotation direction D2.

As shown in FIG. 7, the polishing pad 21 is rotated in the counterclockwise direction, so the polishing surface processing unit 241' and the trimmer 240 are sequentially disposed on the polishing surface 210 in the counterclockwise direction, so that the polishing surface processing unit 241 can be Before being trimmed by the dresser 240, the refrigerant is sprayed into a region to be trimmed R1 to harden a plurality of soft microstructures in the region R1 to be trimmed. The hardened area to be repaired R1' is rotated to the position where the dresser 240 is set because of the rotation of the polishing pad 240. The finisher 240 can trim the hardened area R1' to be cured. That is, the step of spraying the refrigerant is performed in synchronization with the trimming step.

In detail, in the present embodiment, the polishing surface processing unit 241' includes a holder 241L provided on the polishing pad 21, and a plurality of nozzles 241S provided on the holder 241L. These nozzles 241S are arranged in the radial direction of the polishing pad 21 and communicate with a supply unit 241p that supplies a refrigerant. In one embodiment, the spray range of the nozzle 241S sprayed with the cryogen may extend from the center of the polishing pad 21 to the edge to cover the entire area to be trimmed R1.

When the polishing surface 210 is trimmed by the polishing pad dressing device 24 of the present embodiment, the polishing pad 21 is rotated by the rotating shaft 25. Further, the step of spraying the refrigerant through the nozzle 241s is performed in synchronization with the step of trimming the polishing surface 210 by the dresser 240, wherein the refrigerant may be continuously or intermittently sprayed through the nozzle 241s to the area to be trimmed R1, which has been hardened to be trimmed. Soft microstructure in zone R1.

In another embodiment, if the polishing pad 21 is rotated in a clockwise direction, the polishing surface processing unit 241' and the finisher 240 may be sequentially disposed on the polishing surface 210 in a clockwise direction. In fact, the relative position of the polishing surface treatment unit 241' and the dresser 240 is not limited in the present invention as long as the portion R1 to be trimmed can be hardened before the dresser 240 trims the area R1 to be trimmed on the polishing surface 210.

In other embodiments, the step of spraying the cryogen is not necessarily synchronized with the trimming step. In detail, the polishing pad 240 may be rotated first, and the refrigerant is sprayed on the entire polishing surface 210 through the nozzle 241S, so that the entire polishing surface 210 is frozen, and the spraying of the refrigerant is stopped. Next, the finisher 240 is used to trim the freeze-hardened polished surface 210. In this case, the relative position of the finisher 240 to the polishing surface treating unit 241' is not particularly limited.

In summary, the beneficial effects of the present invention are that the chemical mechanical polishing device, the polishing pad dressing device and the method provided by the embodiments of the present invention can be hardened before being trimmed by the dresser. The technical characteristics of the soft microstructure inside can make the dresser easier to cut the hardened soft microstructure, improve the efficiency of dressing the polished surface of the dresser, and reduce the unevenness of the polished surface after trimming. When the wafer is polished or polished by the polishing pad, the polishing surface of the polishing pad can be further prevented from being uneven and damage to the wafer surface.

In addition, since only the surface layer of the polishing pad is freeze-hardened, the frozen and hardened area can be quickly thawed to restore the original material characteristics after being trimmed, and can continue to be used for polishing or grinding the wafer.

The above disclosure is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Therefore, equivalent technical changes made by using the present specification and the contents of the drawings are included in the application of the present invention. Within the scope of the patent.

S100~S200‧‧‧ Process steps

Claims (18)

A polishing pad finishing method comprising: first hardening a plurality of soft microstructures in a region to be trimmed on a polishing surface of a polishing pad; and cutting a hardened portion located in the to-be-trimmed region by using a dresser A plurality of the soft microstructures to trim the polished surface of the polishing pad. The polishing pad dressing method of claim 1, wherein the step of hardening the plurality of the soft microstructures located in the to-be-trimmed region is performed by rapid freezing. The polishing pad dressing method of claim 2, wherein the rapid freezing is spraying a refrigerant in the area to be trimmed. The polishing pad dressing method according to claim 3, wherein the refrigerant is liquid nitrogen, liquid oxygen, dry ice or a refrigerant. The polishing pad dressing method according to claim 3, wherein, when the polishing surface of the polishing pad is trimmed by the dresser, the polishing pad passes through a rotating shaft to generate rotation, and the step of spraying the refrigerant The step of trimming the polishing surface with the dresser is performed synchronously in the same direction. The polishing pad dressing method of claim 5, wherein, in the step of spraying the refrigerant, further comprising: moving along a radial direction of the polishing pad, and spraying the continuous or intermittently A refrigerant is on the polishing surface of the polishing pad. The polishing pad dressing method of claim 5, wherein, in the step of spraying the refrigerant, the method further comprises: spraying the refrigerant continuously or intermittently in a radial direction of the polishing pad; The polishing surface of the polishing pad. A polishing pad dressing device comprising: a trimmer for trimming a polishing surface of a polishing pad; and a polishing surface processing unit disposed above the polishing pad, wherein the trimmer is trimmed The polishing surface treatment unit is used to harden a plurality of soft microstructures located in the to-be-trimmed region before the surface to be trimmed on the polishing surface. The polishing pad finishing device of claim 8, wherein the polishing surface processing unit includes a supply unit for supplying a refrigerant and a nozzle communicating with the supply unit, and the supply unit is supplied The refrigerant passes through the nozzle to be sprayed on the area to be trimmed. The polishing pad dressing device of claim 8, further comprising a moving arm and a rotating shaft rotatably coupled to the moving arm. The polishing pad finishing device of claim 10, wherein the polishing surface processing unit is disposed on the moving arm, and the trimmer is disposed on the moving arm through the rotating shaft to cause the polishing Both the face processing unit and the trimmer move synchronously through the moving arm. The polishing pad finishing device of claim 8, wherein the polishing surface processing unit is adjacent to the trimmer, and the polishing surface processing unit is configured when the polishing pad is rotated by a rotating shaft toward a rotation direction. The trimmers are sequentially disposed along the rotation direction. A chemical mechanical polishing apparatus comprising: a stage connected to a rotating shaft to generate rotation; a polishing pad disposed on the stage, the polishing pad having a polishing surface; and a polishing pad finishing device comprising: a trimmer disposed on the polishing pad for trimming the polishing And a polishing surface processing unit disposed above the polishing pad, wherein the polishing surface treatment unit is hardened by the polishing surface treatment unit before the trimmer passes through a to-be-prepared area on the polishing surface A number of soft microstructures in the trimmed area are described. The chemical mechanical polishing apparatus according to claim 13, wherein the polishing surface processing unit includes a supply unit for supplying a refrigerant and a nozzle communicating with the supply unit, and the supply unit supplies The refrigerant passes through the nozzle to be sprayed on the area to be trimmed. The chemical mechanical polishing apparatus according to claim 14, further comprising a cleaning unit that sprays a cleaning liquid to clean the polishing pad and causes the plurality of polishing chambers to be hardened in the area to be trimmed The soft microstructure is thawed. The chemical mechanical polishing apparatus of claim 13, wherein the polishing pad finishing device further comprises a moving arm and a rotating shaft rotatably coupled to the moving arm. The chemical mechanical polishing apparatus according to claim 16, wherein the polishing surface processing unit is disposed on the moving arm, and the trimmer is disposed on the moving arm through the rotating shaft to cause the polishing Both the surface processing unit and the trimmer move synchronously through the moving arm. The chemical mechanical polishing apparatus according to claim 13, wherein the polishing surface processing unit and the dresser are adjacent to each other, and the polishing surface is processed when the polishing pad is rotated by the rotating shaft in a rotation direction. The unit and the trimmer are sequentially disposed along the rotation direction.