Patent Publication Number: US-2022216089-A1

Title: Conveying device

Description:
BACKGROUND 
     Technical Field 
     The present invention relates to a conveying device. 
     Description of Related Art 
     A semiconductor chip obtained by dicing a semiconductor wafer is a component for an electronic device. A process of manufacturing the semiconductor chip includes the work of moving the semiconductor wafer. Further, the process of manufacturing the electronic device includes the work of picking up the semiconductor chip and the work of arranging the semiconductor chip on a circuit board. The semiconductor wafer and the semiconductor chip need to be kept in a clean state. Therefore, it is desirable that the semiconductor wafer and the semiconductor chip are not directly touched in these works. For example, Patent Documents 1 to 3 disclose techniques for holding a semiconductor wafer, etc. in a non-contact manner. 
     RELATED ART 
     Patent Documents 
     [Patent Document 1] Japanese Laid-Open No. 2014-003238 
     [Patent Document 2] Japanese Laid-Open No. 2014-165470 
     [Patent Document 3] Japanese Laid-Open No. 01-127538 
     SUMMARY 
     Problems to be Solved 
     A so-called Bernoulli chuck is known as a technique for holding an object in a non-contact manner. The Bernoulli chuck utilizes a force generated by flowing a fluid between the chuck and the object. This force pulls the object to the chuck. The pulling force is called a holding force. However, the Bernoulli chuck does not exert any binding force on the object in a lateral direction that intersects the direction of the holding force. As a result, when a force in the lateral direction acts on the object in a state where the object is held by the chuck, the object may easily move. As a result, it is difficult to pick up the object stably. 
     The present invention provides a conveying device that can pick up the object stably. 
     Means for Solving the Problems 
     A conveying device according to an embodiment of the present invention includes: a chuck part holding an object, which is a semiconductor chip or a semiconductor wafer, in a non-contact manner to face a holding surface; and a guide part having a guide probe capable of abutting a side surface of the object and, for the object held by the chuck part, the guide probe limits movement of the object in a lateral direction that intersects a direction of a normal of the holding surface. The guide probe is capable of performing reciprocating movement in which a tip of the guide probe moves toward and away from the holding surface. 
     The guide probe of the guide part provided in the conveying device limits the movement of the object held by the chuck in the lateral direction. The guide probe can move to be close to and move away from the holding surface of the chuck. Then, when the object is brought close to the chuck, the guide probe moves to be close to the holding surface of the chuck. Therefore, the chuck can be easily brought close to the object regardless of the surrounding conditions of the object. Then, when the chuck holding the object is lifted, the guide probe moves away from the holding surface. As a result, the guide probe exists on the side surface of the object. Therefore, the movement of the object in the lateral direction is limited. That is, it is possible to both limit the movement of the object in the lateral direction and lift the object. Therefore, the object can be picked up stably. 
     The guide probe of the conveying device may switch between a first form in which a distance from the tip of the guide probe to the holding surface is a first distance, and a second form in which the distance from the tip of the guide probe to the holding surface is a second distance, and the second distance may be shorter than the first distance. According to this configuration, the reciprocating movement in which the tip of the guide probe moves toward and away from the holding surface can be reliably performed. 
     The guide probe of the conveying device may extend in the direction of the normal. According to this configuration, the configuration of the guide part can be simplified. 
     The guide part of the conveying device may include an urging force generating part that provides an urging force toward the direction of the normal to the guide probe. According to this configuration, the operation of moving the tip of the guide probe away from the holding surface can be reliably performed. 
     At least two guide parts of the conveying device may be arranged along a first edge of the holding surface. At least two other guide parts may be arranged along a second edge of the holding surface. According to these configurations, the two guide parts are capable of abutting the side surface of the object. As a result, the rotational movement of the object is preferably limited. Therefore, the object can be picked up more stably. 
     The guide part of the conveying device may include a first guide probe that extends in a direction inclined with respect to the direction of the normal. Another guide part may include a second guide probe that extends in a direction inclined with respect to the direction of the normal. A separation width between a tip of the first guide probe and a tip of the second guide probe may be larger than a separation width between a base end of the first guide probe and a base end of the second guide probe. According to these configurations, the object can be picked up even when the position of the chuck is displaced with respect to the object. 
     The chuck part of the conveying device may be a Bernoulli chuck. According to this configuration, the object can be reliably held in a non-contact manner. 
     Effects 
     The conveying device according to an embodiment of the present invention can pick up the object stably. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a state where a conveying device of an embodiment picks up a semiconductor chip. 
         FIG. 2  is a perspective view showing a configuration of the conveying device. 
         FIG. 3  includes views showing an end surface of the conveying device. 
         FIG. 4  includes views showing an operation of picking up a semiconductor chip by using the conveying device of  FIG. 2 . 
         FIG. 5  includes views showing an operation of picking up the semiconductor chip by using the conveying device, following  FIG. 4 . 
         FIG. 6  is a view showing a conveying device of the first modification. 
         FIG. 7  is a view showing a conveying device of the second modification. 
         FIG. 8  includes views showing an operation of picking up a semiconductor chip by using a conveying device of a comparative example. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are assigned with the same reference numerals, and the repeated description will be omitted. 
     As shown in  FIG. 1 , a semiconductor wafer  100  that has been diced has a plurality of semiconductor chips  101 . An interval between the semiconductor chips  101  is at least equivalent to a thickness of a dicing cutter. As an example, the interval between the semiconductor chips  101  is about 100 micrometers. For the semiconductor chip  101 , it is desirable to avoid contact with a chip main surface  101   t  in order to maintain cleanness. Therefore, the conveying device  1  of the embodiment picks up the semiconductor chip  101  as an object one by one without touching the chip main surface  101   t  of the semiconductor chip  101 . When other semiconductor chips  101  are arranged all around the semiconductor chip  101  to be picked up, the conveying device  1  can still pick up the semiconductor chip  101  to be picked up. When other semiconductor chips  101  are arranged partially around the semiconductor chip  101  to be picked up, such as in a peripheral portion of the semiconductor wafer  100 , the conveying device  1  can still pick up the semiconductor chip  101  to be picked up. 
     The conveying device  1  for conveying the semiconductor chip  101  may be used, for example, in a die bond apparatus and a pickup apparatus. 
     As shown in  FIG. 2 , the conveying device  1  includes a chuck  2  (chuck part) and guides  3  (guide parts). The chuck  2  generates a holding force FK for holding the semiconductor chip  101  on a holding surface  2 B. The holding force FK is based on, for example, the effect of Bernoulli. The chuck  2  is a so-called Bernoulli chuck. The chuck  2  has an air hole  2   h . Compressed air for generating the holding force FK is supplied to the air hole  2   h . The air hole  2   h  has an opening formed on a chuck main surface  2   t  and an opening formed on the holding surface  2 B of the chuck  2 . A control plate  4  for controlling the flow of air may be provided in the vicinity of the opening on the side of the holding surface  2 B. The compressed air supplied from the chuck main surface  2   t  is ejected from the opening on the holding surface  2 B which is a lower surface of the chuck. The control plate  4  controls a direction of the compressed air. As a result, the compressed air flows in a direction along the holding surface  2 B. Then, the compressed air flows out from the side of a chuck side surface  2   s  to the surroundings. 
     The configuration of the chuck  2  is not limited to the above configuration, and a configuration capable of holding the semiconductor chip  101  in a non-contact manner may be adopted. 
     The holding force FK of the chuck  2  is along a direction of a normal N of the holding surface  2 B. The direction of the holding force FK is opposite to the direction of the normal N. That is, the direction of the holding force FK is toward the holding surface  2 B. When the semiconductor chip  101  is held by the holding force FK, the semiconductor chip  101  is not constrained in an X direction which is a lateral direction that intersects the normal N. Therefore, when a force in the X direction acts on the semiconductor chip  101 , the semiconductor chip  101  easily moves in the X direction. Similarly, the semiconductor chip  101  is not constrained in a Y direction which is another lateral direction that intersects the normal N. Therefore, when a force in the Y direction acts on the semiconductor chip  101 , the semiconductor chip  101  easily moves in the Y direction. 
     Thus, the guides  3  regulate the movement in the lateral direction. The lateral direction includes the X direction and the Y direction. The conveying device  1  includes eight guides  3 . When a planar shape of the chuck  2  is rectangular as viewed from the direction of the normal N, two guides  3  are arranged on each chuck side surface  2   s . In other words, two guides  3  are arranged along a first edge  2 Ba of the holding surface  2 B. Further, two other guides  3  are arranged along a second edge  2 Bb of the holding surface  2 B. The plurality of guides  3  differ from each other only in the positions where they are arranged, and the individual configurations are the same. Hereinafter, the detailed configuration thereof will be described with reference to the guide  3 A as an example, and the description of the other guides  3  will be omitted. 
     As shown in the part (a) of  FIG. 3 , the guide  3 A has a guide main body  7 , a spring  8 , and a guide probe  9 .  FIG. 3 ,  FIG. 4 , and  FIG. 5  show only a pair of guides  3 A and  3 B, and the other guides  3  are omitted. The guide main body  7  is fixed to the chuck side surface  2   s . The guide main body  7  accommodates the spring  8  and a probe base end  9   b . The position of a guide lower surface  7   b  of the guide main body  7  is above the position of the holding surface  2 B of the chuck  2 . In other words, the guide lower surface  7   b  does not protrude from the holding surface  2 B. The guide lower surface  7   b  may be on the same plane as (flush with) the holding surface  2 B. Further, the guide lower surface  7   b  does not protrude from at least a holding force acting region SA. 
     The guide probe  9  is capable of performing reciprocating movement with respect to the guide main body  7  in an axial direction of the guide probe  9 . That is, the guide probe  9  is capable of performing reciprocating movement with respect to the chuck  2  along the normal N of the holding surface  2 B. The spring  8  is accommodated in the guide main body  7 . The spring  8  applies an urging force FS along the normal N to the guide probe  9 . The spring  8  is a compression spring. The upper end of the spring  8  is fixed to the guide main body  7 . The lower end of the spring  8  is fixed to the guide probe  9 . In the initial state, the spring  8  is slightly shorter than the natural length. According to the state of being slightly shorter than the natural length, in the initial state, the spring  8  applies the urging force FS that presses the guide probe  9  downward to the guide probe  9 . Therefore, the guide probe  9  can be reliably protruded. The distance from the holding surface  2 B to a probe tip  9   a  in this state is a first distance. 
     As shown in the part (b) of  FIG. 3 , when the probe tip  9   a  comes into contact with a chip main surface  104   t , the guide probe  9  receives an upward reaction force FU. When the upward reaction force FU acts on the guide probe  9 , the spring  8  is compressed. As a result, the spring  8  becomes shorter. That is, the guide probe  9  is allowed to move upward. Then, as shown in the part (a) of  FIG. 3 , the probe tip  9   a  is separated from the chip main surface  104   t . As a result, the upward reaction force FU acting on the guide probe  9  disappears. Therefore, the guide probe  9  is pressed downward by the urging force FS of the spring  8 . 
     The guide  3  may adopt a configuration capable of exerting the urging force FS as an urging force generating part. For example, the guide  3  may use an air cylinder as the urging force generating part. Further, the guide  3  may omit the urging force generating part if necessary. 
     Based on the above operation, the guide probe  9  may take a first form and a second form that can be switched. The first form refers to a state in which the movement of the held semiconductor chip  101  in the lateral direction can be regulated (see the part (a) of  FIG. 3 ). In the first form, the position of the probe tip  9   a  of the guide probe  9  is at least below the holding force acting region SA. It suffices if the held semiconductor chip  101  is regulated in the lateral direction. At least, the position of the probe tip  9   a  is below the chip main surface  103   t  of the semiconductor chip  101  in the held state. That is, the guide probe  9  overlaps with the chip side surface  103   s  of the semiconductor chip  103 . The first form can be taken in a process of approaching the semiconductor chip  101  (for example, the part (c) of  FIG. 4 ). Further, the first form can be taken in a process of holding the semiconductor chip  101  and then moving it above the semiconductor wafer  100  (for example, the part (a) of  FIG. 5 ). In other words, the first form does not include a moment when the chuck  2  switches from the state in which the holding force FK is not applied to the semiconductor chip  103  to the state in which the holding force FK is applied (see, for example, the part (b) of  FIG. 3 ). In this period, the guide probe  9  takes the second form described below. 
     The second form is a state in which the guide probe  9  is contracted most. In other words, it can be said that the distance from the probe tip  9   a  to the holding surface  2 B is the shortest. The distance from the holding surface  2 B to the probe tip  9   a  in this state is a second distance. In this state, the probe tip  9   a  overlaps with the holding force acting region SA. That is, in the state where the guide probe  9  is contracted most, the position of the probe tip  9   a  is on the side of the holding surface  2 B with respect to the lower end of the holding force acting region SA. 
     Hereinafter, an operation of picking up the semiconductor chip  103  by using the conveying device  1  will be described with reference to  FIG. 4  and  FIG. 5 . 
     First, as shown in the part (a) of  FIG. 4 , the conveying device  1  is moved to be directly above the semiconductor chip  103  to be picked up. At this time, the position of the guide probe  9  is outside the chip side surface  103   s  of the semiconductor chip  103 . 
     Next, as shown in the part (b) of  FIG. 4 , the conveying device  1  is brought close to the semiconductor chip  103 . As a result, the probe tip  9   a  comes into contact with the chip main surface  104   t  of another semiconductor chip  104  adjacent to the semiconductor chip  103 , respectively. In this state, the holding force acting region SA of the chuck  2  does not overlap with the semiconductor chip  103 . Therefore, the chuck  2  cannot hold the semiconductor chip  103 . 
     Next, as shown in the part (c) of  FIG. 4 , the conveying device  1  is further brought close to the semiconductor chip  103 . More specifically, the holding surface  2 B is brought close to the chip main surface  103   t . Then, the probe tip  9   a  receives the reaction force FU from the semiconductor chip  104 . Therefore, as the chuck  2  approaches the semiconductor chip  103 , the spring  8  is contracted. In parallel with this operation, the probe base end  9   b  is accommodated inside the guide main body  7 . In other words, the guide probe  9  does not move, but the chuck  2  and the guide main body  7  move downward with respect to the stationary guide probe  9 . Even in this state, the holding force acting region SA of the chuck  2  does not overlap with the semiconductor chip  103 . Therefore, the chuck  2  cannot hold the semiconductor chip  103 . 
     Next, as shown in the part (d) of  FIG. 4 , the conveying device  1  is further brought close to the semiconductor chip  103 . More specifically, the holding surface  2 B is brought close to the chip main surface  103   t . As a result, the holding force acting region SA overlaps with the chip main surface  103   t . In this state, the holding force FK of the chuck  2  acts on the semiconductor chip  103 . Then, compressed air is supplied to the chuck  2 . As a result, the holding force FK is generated. Therefore, the semiconductor chip  103  is held by the chuck  2 . 
     Next, as shown in the part (a) of  FIG. 5 , the conveying device  1  is moved away from a dicing tape  201 . In other words, the chuck  2  of the conveying device  1  is moved upward (Z direction). Along with this movement, the semiconductor chip  103  moves upward. Then, the guide probe  9  keeps being pressed against the semiconductor chip  104  by the urging force FS of the spring  8 . That is, the chuck  2  and the guide main body  7  move upward, but the guide probe  9  keeps its position. Then, as the chuck  2  rises, the guide probe  9  accommodated in the guide main body  7  seems to extend. Therefore, in this period, as the semiconductor chip  103  rises, an overlapping portion between the chip side surface  103   s  of the semiconductor chip  103  and the guide probe  9  is formed. That is, as the semiconductor chip  103  rises, the function of suppressing the displacement of the semiconductor chip  103  in the lateral direction is exhibited. In particular, while a chip back surface  103   b  of the semiconductor chip  103  is peeled from the dicing tape  201 , partial peeling occurs irregularly. As a result, an unintended external force tends to act on the semiconductor chip  103 . However, the guide probe  9  already exists on the lateral side of the semiconductor chip  103 . That is, the displacement of the semiconductor chip  103  in the lateral direction can be suppressed. 
     The semiconductor chip  103  may be pushed up by using a pickup pin  202  arranged on the back surface side of the dicing tape  201 . Further, the chip main surface  103   t  of the semiconductor chip  103  may reach the holding force acting region SA by pushing up the pickup pin  202 . 
     Next, as shown in the part (b) of  FIG. 5 , the chuck  2  of the conveying device  1  is moved upward. Along with this movement, the semiconductor chip  103  moves further upward. While the semiconductor chip  103  is moving, the overlapping region between the chip side surface  103   s  and the guide probe  9  increases as described above. Then, the positional relationship between the chuck  2  and the guide probe  9  returns to the first form. In this state, the length of the guide probe  9  is the maximum. 
     Then, as shown in the part (c) of  FIG. 5 , the chuck  2  of the conveying device  1  is further moved upward. Along with this movement, the semiconductor chip  103  moves further upward. Further, the length of the guide probe  9  is the maximum. That is, the relative position of the guide probe  9  with respect to the chuck  2  does not change. Therefore, the guide probe  9  moves upward along with the movement of the chuck  2 . As a result, the probe tip  9   a  is separated from the chip main surface  104   t  of the semiconductor chip  104 . 
     Through the above operation, the conveying device  1  picks up the semiconductor chip  103 . 
     The conveying device  1  includes the chuck  2  and the guides  3 . The chuck  2  holds the semiconductor chip  103  in a non-contact manner to face the holding surface  2 B. The guide  3  has the guide probe  9  capable of abutting the chip side surface  103   s  of the semiconductor chip  103 . For the semiconductor chip  103  held by the chuck  2 , the guide probe  9  limits the movement of the semiconductor chip  103  in the lateral direction that intersects the direction of the normal N of the holding surface  2 B. The guide probe  9  is capable of performing reciprocating movement in which the probe tip  9   a  moves toward and away from the holding surface  2 B. 
     The guide probe  9  of the guide  3  included in the conveying device  1  limits the movement of the semiconductor chip  103  held by the chuck  2  in the lateral direction. The guide probe  9  is capable of moving toward and moving away from the holding surface  2 B of the chuck  2 . Then, when the chuck  2  is brought close to the semiconductor chip  103 , the guide probe  9  can move to be close to the holding surface  2 B. Therefore, even when other semiconductor chips  104  exist around the semiconductor chip  103 , the chuck  2  can be easily brought close to the semiconductor chip  103 . Then, when the chuck  2  holding the semiconductor chip  103  is lifted, the guide probe  9  moves to be away from the holding surface  2 B of the chuck  2 . As a result, the guide probe  9  exists on the chip side surface  103   s  of the semiconductor chip  103 . Therefore, the movement of the semiconductor chip  103  in the lateral direction is limited. That is, the conveying device  1  can both limit the movement of the semiconductor chip  103  in the lateral direction and lift the semiconductor chip  103 . Therefore, the semiconductor chip  103  can be picked up stably. 
     For example, unlike the conveying device  1 , the guide probe  309  of the conveying device  300  according to the comparative example shown in the part (a) of  FIG. 8  is fixed to the guide main body  307 . According to this configuration, when the holding surface  302 B of the chuck  302  is brought close to the semiconductor chip  103 , the probe tip  309   a  is pressed against the chip main surface  104   t  of another semiconductor chip  104  adjacent to the semiconductor chip  103 . In the conveying device  300  of the comparative example, the guide probe  309  does not expand and contract. Therefore, when the chuck  302  is further brought close to the semiconductor chip  103 , the force with which the probe tip  309   a  presses the chip main surface  104   t  further increases, as shown in the part (b) of  FIG. 8 . As a result, the semiconductor chip  104  may be damaged. 
     On the other hand, when the chuck  2  is brought close to the semiconductor chip  103 , the conveying device  1  moves so that the probe tip  9   a  approaches the holding surface  2 B. Therefore, unlike the conveying device  300  of the comparative example, the pressing force on the chip main surface  104   t  is suppressed from increasing with the approach of the chuck  2 . Therefore, the conveying device  1  does not damage the semiconductor chip  104 . 
     The guide probe  9  of the conveying device  1  switches between the first form in which the distance from the probe tip  9   a  to the holding surface  2 B is the first distance and the second form in which the distance from the probe tip  9   a  to the holding surface  2 B is the second distance. The second distance is shorter than the first distance. According to this configuration, the reciprocating movement in which the probe tip  9   a  moves toward and away from the holding surface  2 B can be reliably performed. 
     The guide probe  9  of the conveying device  1  extends in the direction of the normal N. According to this configuration, the configuration of the guide  3  can be simplified. 
     The guide  3  of the conveying device  1  has the spring  8 . The spring  8  provides the guide probe  9  with the urging force FS in the direction of the normal N. According to this configuration, the operation of moving the tip of the guide probe  9  away from the holding surface  2 B can be reliably performed. 
     Two guides  3  of the conveying device  1  are arranged along the first edge  2 Ba of the holding surface  2 B. At least two other guides  3  of the conveying device  1  are arranged along the second edge  2 Bb of the holding surface  2 B. According to this configuration, the two guides  3  are capable of abutting the chip side surface  103   s  of the semiconductor chip  103 . As a result, the rotational movement of the semiconductor chip  103  is preferably limited. Therefore, the semiconductor chip  103  can be picked up more stably. 
     The conveying device  1  of the present disclosure has been described above. However, the conveying device  1  of the present disclosure is not limited to the above-described embodiment and may be implemented in various forms. 
     For example, the conveying device  1 A shown in  FIG. 6  has guides  3 A and  3 B. The guide probes  9 A and  9 B (first guide probe and second guide probe) of the guides  3 A and  3 B may be inclined with respect to the normal N of the holding surface  2 B. The guide probes  9 A and  9 B expand and contract with respect to the guide main body  7  in a direction inclined with respect to the normal N. The guide probes  9 A and  9 B may move along the direction of the normal N. That is, it suffices if the relative positional relationship of the probe tip  9   a  with respect to the chuck  2  is variable. Although the spring  8  is not shown in  FIG. 6 , the guides  3 A and  3 B may or may not have the spring  8 . 
     Here, a separation width H 1  from the probe tip  9   a  of the guide probe  9 A to the probe tip  9   a  of the guide probe  9 B is larger than a separation width H 2  from the probe base end  9   b  of the guide probe  9 A to the probe base end  9   b  of the guide probe  9 B. In other words, the separation width from the guide probe  9 A to the guide probe  9 B increases as they go from the holding surface  2 B. 
     In short, the guides  3 A and  3 B of the conveying device  1 A have the guide probes  9 A and  9 B that extend in the direction inclined with respect to the direction of the normal N. The separation width H 1  from the probe tip  9   a  of the guide probe  9 A to the probe tip  9   a  of the guide probe  9 B is larger than the separation width H 2  from the probe base end  9   b  of the guide probe  9 A to the probe base end  9   b  of the guide probe  9 B. According to this configuration, the semiconductor chip  103  can be picked up even when the position of the chuck  2  is displaced with respect to the semiconductor chip  103 . 
     That is, even if the positions of the conveying device  1 A and the semiconductor chip  103  to be picked up are slightly displaced, the conveying device  1 A can still guide the semiconductor chip  103  to a predetermined holding position. Therefore, the conditions of the position where the semiconductor chip  103  can be held can be relaxed. As a result, the conveying device  1 A can hold the semiconductor chip  103  more stably. 
     The mechanism that allows the horizontal displacement of the semiconductor chip with respect to the conveying device can also be realized by a mechanism different from the mechanism shown in  FIG. 6 . For example, as shown in  FIG. 7 , the guide probes  9 C and  9 D of the guides  3 C and  3 D have inclined probe side surfaces  9   s . In other words, the shapes of the guide probes  9 C and  9 D are tapered. With such a configuration, even if the position of the semiconductor chip  103  to be picked up is slightly displaced with respect to the conveying device  1 B, the semiconductor chip  103  can still be guided to the predetermined holding position. 
     The conveying device  1  of the embodiment is applied to convey the semiconductor chip  103 . The conveying device may also be applied to convey a semiconductor wafer. 
     DESCRIPTIONS OF REFERENCE NUMERALS 
       1 ,  1 A,  1 B . . . conveying device;  2  . . . chuck (chuck part);  2 B . . . holding surface;  3 ,  3 A,  3 B,  3 C,  3 D . . . guide (guide part);  4  . . . control plate;  7  . . . guide main body;  8  . . . spring;  9 ,  9 A,  9 B,  9 C,  9 D . . . guide probe;  100  . . . semiconductor wafer;  101  . . . semiconductor chip;  101 ,  103 ,  104  . . . semiconductor chip;  201  . . . dicing tape;  202  . . . pickup pin; SA . . . holding force acting region; FK . . . holding force; FS . . . urging force; N . . . normal.