CN111149197A

CN111149197A - Adsorption platform

Info

Publication number: CN111149197A
Application number: CN201880062674.0A
Authority: CN
Inventors: 小林泰人; 马诘邦彦
Original assignee: Shinkawa Ltd
Current assignee: Shinkawa Ltd
Priority date: 2017-09-28
Filing date: 2018-09-18
Publication date: 2020-05-12
Anticipated expiration: 2038-09-18
Also published as: TW201921522A; JP6817658B2; CN111149197B; KR20200051815A; WO2019065355A1; SG11202004896TA; TWI684223B; JPWO2019065355A1; KR102339079B1

Abstract

The adsorption platform (20) comprises: an upper plate (21) provided with a plurality of adsorption holes (22); first to third vacuum flow paths (41 to 43) for connecting the plurality of suction holes (22) provided in the upper plate (21) to a vacuum device (45) in a plurality of groups A1 to A3 corresponding to the size of the semiconductor crystal grains; and first and second check valves (61, 62) provided in the second and third vacuum flow paths (42, 43), wherein when the adsorption hole (22) is open to the atmosphere, the first and second check valves (61, 62) are closed, and when the adsorption hole (22) is blocked by the semiconductor crystal grain, the first and second check valves (61, 62) are opened.

Description

Adsorption platform

Technical Field

The invention relates to a structure of an adsorption platform for adsorbing and holding semiconductor crystal grains.

Background

A die bonding apparatus that bonds a semiconductor die picked up from a wafer to a substrate is widely used. In a die bonding apparatus, a semiconductor die picked up from a wafer is temporarily placed on an intermediate stage, position detection of the semiconductor die is performed by a camera, and then the semiconductor die is picked up from the intermediate stage by a bonding head and bonded at a predetermined position on a substrate (for example, see patent document 1).

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open No. 2013-65711

Disclosure of Invention

Problems to be solved by the invention

In order to perform position detection of the semiconductor die on the intermediate stage by the camera, the semiconductor die must be smoothly placed on the surface of the intermediate stage. Since the semiconductor crystal grain is thin and often warps upward, the intermediate stage is formed by the suction stage and the semiconductor crystal grain is sucked and held on the surface by vacuum in order to smoothly place the semiconductor crystal grain on the surface of the intermediate stage.

In this case, in order to smoothly adsorb the semiconductor crystal grains on the surface of the adsorption stage, the whole semiconductor crystal grains must be adsorbed with good balance. Further, if there are adsorption holes around the semiconductor crystal grain that do not adsorb the semiconductor crystal grain, air enters from the adsorption holes and the adsorption force decreases, and therefore adsorption holding of the semiconductor crystal grain may not be detected. Therefore, it is necessary to prepare several suction stages having different suction hole arrangements in accordance with the size of the semiconductor die mounted on the surface, and replace the suction stages in accordance with the size of the semiconductor die. Therefore, there is a problem that the tact time (takttime) of the bonding becomes long.

Accordingly, an object of the present invention is to provide an adsorption stage capable of suitably adsorbing and holding semiconductor crystal grains of various sizes.

Means for solving the problems

The adsorption platform of the present invention is an adsorption platform for adsorbing and holding semiconductor crystal grains of different sizes, and is characterized by comprising: an upper plate provided with a plurality of adsorption holes; a plurality of vacuum flow paths for connecting the plurality of suction holes provided in the upper plate to a vacuum device in a plurality of groups corresponding to the size of the semiconductor crystal grains; and a check valve provided in at least one of the plurality of vacuum flow paths, the check valve being closed when the adsorption hole is open to the atmosphere, and being opened when the adsorption hole is blocked by the semiconductor crystal grain.

In the adsorption platform of the present invention, the check valve may have a valve seat surface with a rough surface, and a small leakage of air may occur between the valve body and the valve seat in the closed state.

In the adsorption platform of the present invention, the valve seat surface may be a circular arc surface provided with a hole through which air flows, the valve body may be a band-shaped body including an elastic body attached to one end of the valve seat surface, and the check valve may be a valve that opens and closes the hole through the valve body. In the suction stage of the present invention, the suction holes may be arranged uniformly.

ADVANTAGEOUS EFFECTS OF INVENTION

The present invention can provide an adsorption platform capable of suitably adsorbing and holding semiconductor crystal grains of various sizes.

Drawings

Fig. 1 is an explanatory diagram showing a configuration of a die bonding apparatus incorporating a suction table according to an embodiment.

Fig. 2 is a perspective view of the adsorption stage of the embodiment.

Fig. 3 is a sectional view of the adsorption stage of the embodiment.

Fig. 4 is a system diagram showing the arrangement of the suction holes on the surface of the suction table and the configuration of the vacuum flow path according to the embodiment.

Fig. 5 is an exploded perspective view of a valve portion of the adsorption platform according to the embodiment.

Fig. 6 is a perspective view of the check valve incorporated in the suction table of the embodiment.

Fig. 7 is an explanatory view showing a state before the semiconductor die is mounted on the adsorption stage of the embodiment.

Fig. 8 is an explanatory view showing the suction holding operation when the semiconductor crystal grains of small size blocking the suction holes included in the first group are placed on the suction stage of the embodiment.

Fig. 9 is an explanatory view showing the suction holding operation when the semiconductor crystal grains of the middle size blocking the suction holes included in the first group and the second group are placed on the suction stage of the embodiment.

Fig. 10 is an explanatory view showing the suction holding operation when the large-sized semiconductor crystal grains blocking the suction holes included in the first group, the second group, and the third group are placed on the suction stage according to the embodiment.

Detailed Description

The adsorption stage 20 of the present embodiment will be described below with reference to the drawings. First, referring to fig. 1, a die bonding apparatus 100 assembled with the suction table 20 of the present embodiment will be described.

As shown in fig. 1, the die bonding apparatus 100 includes a pickup portion 10, an intermediate positioning portion 18, and a bonding portion 70.

The pick-up section 10 is a section that picks up the semiconductor die 13 from the surface of the wafer 12 and conveys it to the intermediate positioning section 18. As shown in fig. 1, the pickup 10 includes: a wafer holder 11 holding a wafer 12; a push-up mechanism 16 for pushing up the semiconductor crystal grain 13 from below; and a pick-up head 14 including a chuck 15 for picking up the semiconductor die 13 pushed up by the push-up mechanism 16. When the semiconductor die 13 is attached to the chuck 15, the pick-up head 14 transfers the semiconductor die 13 from the pick-up section 10 to the intermediate positioning section 18, and places the semiconductor die on the surface 21a of the attaching table 20.

The intermediate positioning portion 18 includes: an adsorption stage 20 for temporarily adsorbing and holding the semiconductor crystal grain 13 on the surface 21 a; and a camera 48 disposed above the surface 21a of the suction table 20 and configured to take an image of the semiconductor crystal grain 13 sucked and held on the surface 21 a. The adsorption platform 20 includes an upper plate 21 and a valve portion 50.

The joint 70 includes: a bonding stage 71 for vacuum-adsorbing the substrate 17 to the surface; and a bonding head 72 including a bonding tool 73 for picking up the semiconductor die 13 from the surface 21a of the suction table 20 and bonding it to the substrate 17.

Next, the structure of the adsorption platform 20 will be described with reference to fig. 2 to 6. As shown in fig. 2, the suction table 20 includes an upper plate 21 having a plurality of suction holes 22 uniformly formed in a surface 21a thereof, and a valve portion 50 for housing therein a first check valve 61 and a second check valve 62 shown in fig. 3 and 5. As shown in fig. 1, the adsorption holes 22 are divided into three groups as follows: a first group a1 including the adsorption holes 22 in the center, a third group A3 including the adsorption holes 22 in the outermost periphery, and a second group a2 including the adsorption holes 22 located in the middle between the first group a1 and the third group A3. The grouping of the adsorption holes 22 will be described later in detail.

As shown in fig. 3, the recess 23, a step 29 adjacent to the recess 23, and a step 31 are provided on the lower surface 21b side opposite to the surface 21a of the upper plate 21. As shown by the broken line in fig. 2, the concave portion 23 has a size surrounding the outside of the arrangement of the suction holes 22. Two

annular projections

24, 25 project from the bottom surface 23a of the recess 23. The annular projection 24 forms an annular third cavity 28 surrounded by the side surface and the bottom surface 23a of the recess 23. The

convex portions

24 and 25 form an annular second cavity 27 surrounded by the inner peripheral surface of the annular convex portion 24 and the outer peripheral surface of the convex portion 25. Further, the inner peripheral surface of the convex portion 25 constitutes a first cavity 26.

As shown in fig. 2 to 4, the adsorption holes 22 included in the first group a1 communicate with the first cavity 26, the adsorption holes 22 included in the second group a2 communicate with the second cavity 27, and the adsorption holes 22 included in the third group A3 communicate with the third cavity 28. Therefore, the

protrusions

24 and 25 are also walls that partition the suction holes 22 into the first group a1, the second group a2, and the third group A3.

An upper pad 29a is fitted into a step portion 29 adjacent to the recess 23, and a spacer 30a and a lower pad 31a are fitted into a step portion 30 adjacent to the step portion 29. The upper surface of the upper pad 29a constitutes the lower surface of the first, second, and

third cavities

26, 27, 28. Further, the upper pad 29a, the spacer 30a, and the lower pad 31a are provided with a through-hole 33, a through-hole 36, and a through-hole 39 at a first position, a through-hole 34, a through-hole 37, and a through-hole 39 at a second position, and a through-hole 32, a through-hole 35, and a through-hole 38 at a third position. The through-hole 33, the through-hole 36, and the through-hole 39 provided at the first position constitute a first vacuum flow path 41 communicating with the first cavity 26, the through-hole 34, the through-hole 37, and the through-hole 40 provided at the second position constitute a second vacuum flow path 42 communicating with the second cavity 27, and the through-hole 32, the through-hole 35, and the through-hole 38 provided at the third position constitute a third vacuum flow path 43 communicating with the third cavity 28.

As shown in fig. 3, the first vacuum channel 41 is connected to a through-hole 52 provided in a base 51 of the valve portion 50. The second and third

vacuum flow paths

42 and 43 communicate with the first and second

valve receiving recesses

55a and 55b of the base 51. As shown in fig. 5, a through-hole 53a and a through-hole 53b are provided in the bottom surfaces of the first valve accommodating recess portion 55a and the second valve accommodating recess portion 55b, respectively. The through-hole 52, the through-hole 53a, and the through-hole 53b are connected to the vacuum apparatus 45 via a pipe 54. The pipe 54 is provided with a pressure sensor 46 for detecting the pressure of the pipe.

As shown in fig. 3 to 5, a first check valve 61 is housed in a first valve housing recess 55a communicating with the second vacuum flow path 42 and the through-hole 53a, and a second check valve 62 is housed in a second valve housing recess 55b communicating with the third vacuum flow path 43 and the through-hole 53 b. As shown in fig. 4, the first check valve 61 is disposed in a direction of blocking the flow of the air from the adsorption hole 22 included in the second group a2 toward the vacuum apparatus 45, and the second check valve 62 is disposed in a direction of blocking the flow of the air from the adsorption hole 22 included in the third group A3 toward the vacuum apparatus 45.

Next, the structure of the first check valve 61 and the second check valve 62 will be described with reference to fig. 6. The first check valve 61 and the second check valve 62 have the same structure. As shown in fig. 6, the first check valve 61 and the second check valve 62 include: a substantially rectangular parallelepiped main body 63 having an arc-shaped valve seat surface 64 provided with a hole 66 through which air flows, and a valve body 65 of a band-like body including an elastic body attached to one end of the valve seat surface 64. The valve seat surface 64 is a rough surface, and the arithmetic average roughness Ra may be about 1.6, for example. The valve body 65 is a smooth-surfaced metal plate. The hole 66 communicates with a hole 69 provided in a side surface of the body 63 via a flow path 67 and a flow path 68 provided in the body. As shown in fig. 5, when the first check valve 61 and the second check valve 62 are fitted in the first valve accommodating recess 55a and the second valve accommodating recess 55b, the hole 69 provided in the side surface of the main body 63 communicates with the vacuum apparatus 45 through the through hole 53a and the through hole 53 b. Further, the gaps D between the tip of the valve element 65 on the hole 66 side and the wall surfaces of the first and second valve accommodating

recess portions

55a and 55b communicate with the second and third

vacuum flow paths

42 and 43.

When the orifice 69 is vacuumed by the vacuum device 45 connected to the orifice 69, the valve element 65 is attracted to the valve seat surface 64 located in the orifice 66 and seals the orifice 66 as indicated by an arrow 95 in fig. 6. Thereby, the first check valve 61 and the second check valve 62 are closed. When the orifice 69 becomes atmospheric pressure, the valve element 65 is separated from the valve seat surface 64 by its own elastic force to open the orifice 66, and the first check valve 61 and the second check valve 62 are closed. However, since the seat surface 64 is roughened, even if the first check valve 61 and the second check valve 62 are closed, a slight leakage of air occurs between the seat surface 64 and the valve body 65.

Next, the operation of each part when the semiconductor crystal grain 13 is adsorbed to the adsorption platform 20 will be described with reference to fig. 7 to 10.

First, referring to fig. 7, a state in which the vacuum apparatus 45 is activated before the semiconductor die 13 is sucked will be described. In this state, since the surface 21a of the upper plate 21 of the adsorption platform 20 is at atmospheric pressure, when the vacuum device 45 is activated, the

flow paths

67 and 68 communicating with the orifice 69 of the first check valve 61 and the orifice 69 of the second check valve 62, which are communicated with the vacuum device 45, become vacuum, and the valve element 65 is adsorbed on the seating surface 64 to seal the orifice 66 of the seating surface 64. Therefore, the first check valve 61 and the second check valve 62 are in the closed state, and the second vacuum flow path 42 and the third vacuum flow path 43 are also in the closed state. On the other hand, the first vacuum flow path 41 that has communicated with the adsorption holes 22 included in the first group a1 is opened to the atmosphere, and therefore air is drawn from the first vacuum flow path 41 into the vacuum apparatus 45. At this time, the pressure sensor 46 at the inlet of the vacuum apparatus 45 becomes a pressure higher than-60 to-70 Pa, for example, about-10 Pa, of the ultimate pressure (ultimate pressure) at which the semiconductor crystal grain 13 is adsorbed on the surface 21 a.

Next, as shown in fig. 8, a case where the small-sized semiconductor crystal grain 13a is mounted on the front surface 21a will be described. The small-sized semiconductor die 13a has the same size as the dotted line representing the outer periphery of the first group a1 shown in fig. 4, and has a size that covers the surfaces of the adsorption holes 22 included in the first group a1, and does not cover the adsorption holes 22 included in the second group a2 and the third group A3.

As shown in fig. 8, when the small-sized semiconductor die 13a is placed on the surface 21a, the surface of the suction holes 22 included in the first group a1 is covered. Thereby, the first vacuum flow path 41 becomes vacuum, and the semiconductor crystal grain 13a is adsorbed on the surface 21 a. Since the suction holes 22 are uniformly arranged, the entire semiconductor crystal grain 13a is vacuum-held on the surface 21a with good balance. Since the second and third

vacuum flow paths

42 and 43 are closed by the first and

second check valves

61 and 62, when the air having entered the first vacuum flow path 41 is sucked by the vacuum device 45, the pressure of the first vacuum flow path 41 reaches-60 to-70 Pa which is the limit pressure at which the semiconductor crystal grain 13 is adsorbed on the surface 21 a. Accordingly, the pressure sensor 46 provided at the inlet pressure of the vacuum apparatus 45 also detects pressures of-60 to-70 Pa, and the control apparatus, not shown, detects that the semiconductor die 13a is sucked and held. The control device acquires an image of the semiconductor die 13a by the camera 48 and performs position detection.

Next, a case where the middle-sized semiconductor crystal grain 13b is placed on the front surface 21a will be described with reference to fig. 9. The middle-sized semiconductor die 13b is the same size as the broken line representing the outer periphery of the second group a2 shown in fig. 4, and is a size covering the surfaces of the adsorption holes 22 included in the first group a1, the second group a 2. However, the middle-sized semiconductor die 13b is not covered on the adsorption holes 22 included in the region of the third group a 3.

As described above with reference to fig. 7, in a state where the vacuum apparatus 45 is activated before the semiconductor crystal grain 13 is adsorbed, the first and

second check valves

61 and 62 are closed, the second and third

vacuum flow paths

42 and 43 are closed, the first vacuum flow path 41 is opened to the atmosphere, and air flows from the first vacuum flow path 41 to the vacuum apparatus 45. At this time, the inlet pressure of the vacuum device 45 is, for example, about-10 Pa.

Then, as shown in fig. 9, when the middle-sized semiconductor die 13b is mounted on the surface 21a, the semiconductor die 13b covers the surfaces of the suction holes 22 included in the first group a1 and the second group a 2. Thereby, first, the first vacuum flow path 41 is evacuated, and the center portion of the semiconductor crystal grain 13b is adsorbed to the surface 21 a. Since the first check valve 61 is in a closed state and there is a slight leakage of air, if the second vacuum flow path 42 is covered with the semiconductor die 13b, the air having entered the second vacuum flow path 42 flows through the first check valve 61 toward the vacuum apparatus 45 as shown by an arrow 92 in fig. 9. Therefore, the pressure in the second vacuum flow path 42 gradually decreases toward the vacuum. When the differential pressure between the pressure in the second vacuum flow path 42 and the pressures in the

flow paths

67 and 68 inside the main body 63 becomes small, the spring force of the valve element 65 to separate from the valve seat surface 64 becomes larger than the force pressing the valve element 65 against the valve seat surface 64, and therefore, the valve element 65 moves upward as indicated by an arrow 91 shown in fig. 9, and the first check valve 61 is opened.

Thereby, the second vacuum flow path 42 is in the same vacuum state as the first vacuum flow path 41, and the peripheral portion of the semiconductor crystal grain 13b is adsorbed on the surface 21 a. Since the suction holes 22 are arranged uniformly, the entire semiconductor die 13b is vacuum-held on the surface 21a with good balance by the suction holes 22 included in the first group a1 and the suction holes 22 included in the second group a 2.

As described above with reference to fig. 8, since the third vacuum flow path 43 is closed by the second check valve 62, when the air having entered the first vacuum flow path 41 and the second vacuum flow path 42 is sucked by the vacuum device 45, the pressures of the first vacuum flow path 41 and the second vacuum flow path 42 reach-60 Pa to-70 Pa which is the limit pressure at which the semiconductor crystal grain 13b is adsorbed on the surface 21 a. Accordingly, the pressure sensor 46 provided at the inlet pressure of the vacuum apparatus 45 also detects pressures of-60 to-70 Pa, and the control apparatus, not shown, detects that the semiconductor die 13b is sucked and held. The control device acquires an image of the semiconductor die 13b by the camera 48 and then performs position detection.

Next, referring to fig. 10, a case where the large-sized semiconductor die 13c is mounted on the surface 21a will be described. The large-sized semiconductor crystal grain 13c is of a size covering all the adsorption holes 22 shown in fig. 4.

As shown in fig. 10, when the large-sized semiconductor crystal grain 13c is placed on the surface 21a, the semiconductor crystal grain 13c covers the surfaces of all the suction holes 22. Thereby, first, the first vacuum flow path 41 is evacuated, and the semiconductor crystal grain 13c is adsorbed on the surface 21 a. Since the first check valve 61 and the second check valve 62 have a slight air leak in the closed state, the first check valve 61 and the second check valve 62 are in the open state when the second vacuum flow path 42 and the third vacuum flow path 43 are covered with the semiconductor crystal grain 13c, as described above.

Thereby, the second vacuum flow path 42 and the third vacuum flow path 43 are in the same vacuum state as the first vacuum flow path 41, and the peripheral portion of the semiconductor crystal grain 13c is adsorbed on the surface 21 a. Since the suction holes 22 are uniformly arranged, the entire semiconductor crystal grain 13c is vacuum-held on the surface 21a with good balance by the suction holes 22 included in the first group a1 to the third group A3.

When the air having entered the first vacuum flow path 41, the second vacuum flow path 42, and the third vacuum flow path 43 is sucked by the vacuum device 45, the pressure of the first vacuum flow path 41, the second vacuum flow path 42, and the third vacuum flow path 43 reaches-60 to-70 Pa which is the limit pressure at which the semiconductor crystal grain 13c is adsorbed on the surface 21 a. Accordingly, the pressure sensor 46 provided at the inlet pressure of the vacuum apparatus 45 also detects pressures of-60 to-70 Pa, and the control apparatus, not shown, detects that the semiconductor die 13c is adsorbed and fixed. The control device acquires an image of the semiconductor die 13c by the camera 48 and then performs position detection.

As described above, the adsorption stage 20 according to the embodiment divides the plurality of adsorption holes 22 into the first group a1 to the third group A3 corresponding to the size of the semiconductor die 13, and provides the first vacuum flow path 41 to the third vacuum flow path 43 connecting the plurality of adsorption holes 22 to the vacuum apparatus 45 in the form of the first group a1 to the third group a, respectively, and the second vacuum flow path 42 and the third vacuum flow path 43 are provided with the first check valve 61 and the second check valve 62, respectively, such that the first check valve 61 and the second check valve 62 are closed when the adsorption holes 22 are open to the atmosphere, and such that the first check valve 61 and the second check valve 62 are opened when the adsorption holes 22 are blocked by the semiconductor die 13. The suction holes 22 are arranged uniformly. Thus, regardless of the size of the semiconductor crystal grain 13, the semiconductor crystal grain 13 can be smoothly sucked and held with good balance as a whole through the suction holes 22. Also, the position of the semiconductor die 13 can be suitably detected using the camera 48.

Further, the suction table 20 of the present embodiment can change the suction holes 22 blocked by the semiconductor crystal grain 13 according to the size of the semiconductor crystal grain 13 to vacuum and block the other suction holes 22 from the vacuum device 45 by the above configuration, so that the vacuum pressure when the semiconductor crystal grain 13 is vacuum-sucked can be changed to a pressure at which the suction holding of the semiconductor crystal grain 13 can be judged regardless of the size of the semiconductor crystal grain 13, and the confirmation of the suction holding of the semiconductor crystal grain 13 can be performed by the control device.

Thus, the suction table 20 according to the embodiment can appropriately suck and hold the semiconductor dies 13 of a plurality of sizes by one suction table 20 without replacing the suction table 20 with respect to the sizes of the semiconductor dies, so that the tact time for bonding can be shortened, and efficient bonding can be performed.

In the embodiment, the description has been given on the assumption that the first vacuum flow path 41 is not provided with the check valve, but the present invention is not limited thereto, and a check valve having the same configuration as the first check valve 61 may be provided. The check valve may be provided outside without being incorporated in the suction table 20. In this case, for example, other forms of check valves such as a swing check valve may be used.

Description of the symbols

10: pickup part

11: wafer holder

12: wafer with a plurality of chips

13. 13a, 13b, 13 c: semiconductor die

14: pick-up head

15: clamping head

16: push-up mechanism

17: substrate

18: middle positioning part

20: adsorption platform

21: upper plate

21 a: surface of

21 b: lower surface

22: adsorption hole

23: concave part

23 a: bottom surface

24. 25: convex part

26: a first cavity

27: second cavity

28: third cavity

29. 30: segment part

29 a: upper liner

30 a: spacer member

31 a: lower liner

32-40, 52, 53a, 53 b: through hole

41: a first vacuum flow path

42: second vacuum flow path

43: third vacuum flow path

45: vacuum device

46: pressure sensor

48: camera with a camera module

50: valve section

51: base seat

54: piping

55a, 55 b: valve accommodating recess

61: first check valve

62: second check valve

63: body

64: valve seat surface

65: valve body

66. 69: hole(s)

67. 68: flow path

70: joint part

71: joint platform

72: joint head

73: joining tool

100: a die bonding apparatus.

Claims

1. An adsorption platform for adsorbing and holding semiconductor crystal grains with different sizes, comprising:

an upper plate provided with a plurality of adsorption holes;

a plurality of vacuum flow paths for connecting the plurality of suction holes provided in the upper plate to a vacuum apparatus in a plurality of groups corresponding to the size of the semiconductor crystal grains, respectively; and

a check valve provided in at least one of the plurality of vacuum flow paths and

the check valve becomes closed when the adsorption hole is open to the atmosphere, and becomes open when the adsorption hole is blocked by the semiconductor crystal grain.

2. The adsorption platform of claim 1, wherein:

the check valve is a valve in which a valve seat surface is rough and a small leakage of air is generated between a valve body and the valve seat surface in a closed state.

3. The adsorption platform of claim 2, wherein:

the valve seat surface is a circular arc surface provided with a hole through which air flows, the valve body is a band-shaped body including an elastic body attached to one end of the valve seat surface, and the check valve is a valve that opens and closes the hole with the valve body.

4. The adsorption platform of any one of claims 1 to 3, wherein:

the adsorption holes are arranged uniformly.