Conceptual Design of Planar Retainer Mechanisms for Bottom-Opening SMIF Environment

Transcription

1 Proceedings of the th World Congress in Mechanism and Machine Science August 8~, 003, Tianjin, China China Machiner Press, edited b Tian Huang Conceptual Design of Planar Retainer Mechanisms for Bottom-Opening SMIF Enironment Wei-Ming Pai Dar-Zen Chen* dchen@ccms.ntu.edu.tw Jh-Jone Lee Department of Mechanical Engineering, National Taiwan Uniersit, Taipei, Taiwan 0660 R. O. C. Tong-Ming Wu Industrial Technolog Research Institute, Hsinchu, Taiwan 30 R. O. C. Abstract: This paper presents an efficient and sstematic methodolog for the conceptual design of planar retainer mechanism. The mechanism is used to hold wafers in a wafer container and preent wafers from collision and scrape during transportation. At the request of high cleanliness, the mechanism is required to hae least frictional motion to aoid generating particles inside the wafer container. These conceptual functions of retainer mechanism are embodied as functional requirements for the motions of mechanism s ke links. From the functional requirements, it is possible to construct the admissible ke-links chain. The ke-links chain is then assigned into feasible graphs eisting in the atlas. In addition to the ke-links chain in the feasible graphs, the remaining links and joints confining the mechanism motion to the specific mobilit and nature of motion can be identified. Admissible retainer mechanisms can be finall obtained b assigning appropriate specifications of the remaining joints. Kewords: Retainer mechanism, SMIF, Wafer container Introduction The cleanliness request for semiconductor processing equipment is unceasingl increased due to the enhancement of integrated circuit s integration. With a iew to ensuring wafers against contamination, standard mechanical interface (SMIF) enironment [, ] is constructed to interface a clean wafer transport container to the port on semiconductor processing equipment. This technique reduces the cost and difficult in maintenance and production in an integrated-circuit manufacturing factor. Two classes of interfaces are proided for the SMIF enironment, one as the bottom-opening interface [] that is arranged in the bottom of the wafer transport container, and another as the front-opening interface [] arranged in the front side of the container. The former interface is used in 00, 5, 50, 00 and 300 mm wafer sie ersions of the SMIF port, and the latter in 300 mm port. In this paper, we focus on the bottom-opening SMIF enironment, which is shown in Fig.. The wafer transport container creates a particle-free and airtight mini-enironment such that wafers can be preented from contamination b abraded particles during the transportation or storage. In the bottom-opening container, wafers are stationed in a cassette located on the container door. While operating, the container is first mated to the port of processing equipment, and the container door is Wafer Port Mini-enironment Processing equipment Wafer transport container Container door Figure : Bottom-opening SMIF enironment. then released from the container and deliers wafers into the processing equipment. For safet reason, the retainer mechanism is used to retain wafers from collision and scrape in the container. An eisting design of retainer mechanism [3] with a referenced coordinate sstem attached to the lower left corner is shown in Fig.. This retainer mechanism inside the container is constructed b a parallelogram four-bar linkage comprising the retaining link (link 3), two cranks (links, ) and the container case as ground link (link ). As the container door engages with the container in its, the retaining link seres as an input link and is drien b the upward push of the container door. Followed b the door engaging motion, the retaining link slides in direction with respect to the container door to retain the wafers in the cassette. Various designs of retainer mechanisms can be found in the industrial applications [3, -]. Since the retainer mechanism is directl eposed inside the clean mini-enironment of the container, it is possible to contaminate wafers due to the abraded particles b the mechanism motions. The wa to reduce the contamination possibilit is to adopt the mechanism design, with a reduction of relatie motions among objects for the sake of eliminating contact abrasion. For eample, the mechanism acts without relatie motions between the retaining link and wafers, or between the input link and

2 container door. In recent decades, the application of graph theor has plaed an important role in the pursuit of sstematic approach of mechanism design [-8]. From that point on, the mechanism design was able to eole in a relatiel more sstematic manner and be applied to a ariet of industrial applications [9-3]. The objectie of this paper is to conceie a sstematic methodolog for the design of planar retainer mechanisms for the bottom-opening SMIF enironment [] with an aid of graph theor. It will be shown that the ke-links chain in the retainer mechanism performs the desired functions, and the remaining links and joints constrain the mechanism to fulfill the structural requirements for the specific mobilit and nature of motion. In accordance with the functional requirements embodied from the desired functions of mechanism, admissible ke-links chain is constructed. Then, feasible graphs are determined b appling the structural requirements. The design of mechanisms can then be performed b assigning the admissible ke-links chain into feasible graphs followed b determining the remaining joints. Wafers Container door Wafer transport container. Container case 3. Retaining link (input link) Figure : An eisting retainer mechanism design. Functional Requirements of Retainer Mechanism It is shown that the primar function of retainer mechanism is to hold and support wafers, upon condition that relatie motions of contacting links to wafers and the container door are eliminated. In this paper, the input link and retaining link are designated as different links. In iew of Fig., the input link of retainer mechanism is actuated b the upward motion of the container door in direction. In order to aoid the abrasion between the input link and the actuating container door, it is desirable to hae no relatie motions in direction. Hence, the input link is epected to moe along the same path with the door engaging motion, i.e. a linear motion along direction. Therefore, the first functional requirement of retainer mechanism can be characteried as follows, alwas assuming the of the container door is oriented in direction : R. The input link must moe linearl in direction with respect to the ground link to aoid abrading with the actuating container door. As shown in Fig., wafers are to be retained b the retaining link moing along direction. Similarl, it is desirable to aoid the abrasion between the retaining link and wafers. Hence, relatie motions between the retaining link and wafers in direction are epected to be eliminated. In the SEMI standards for the bottom-opening container [], wafers are placed in a cassette located on the container door, and hence the can be regarded as the same bod of the container door. As a result, another functional requirement of retainer mechanism can be characteried as follows. R. The retaining link must moe linearl in direction with respect to the container door in order not to abrade wafers. 3 Construction of Ke-links Chain From R and R, it can be seen that the functional requirements characterie the motions of the input link and retaining link. The input link, retaining link and ground link are designated as the ke links of the retainer mechanism that perform required motions to achiee the functions. In order to achiee the required motions in R and R in a conenient manner, the input link is set adjacent to the ground link and the retaining link is set adjacent to the input link. Accordingl, the ke-links chain of the retainer mechanism is sequentiall formed b the ground link, input link and retaining link as the graph representation [] shown in Fig. 3(a) where links are denoted b ertices and joints b edges. In this figure, the ground link is denoted b double circles, the input link b a gra erte and the retaining link b a solid erte. The joint between the ground link and input link is called the input joint, while the joint between the input link and retaining link is called the retaining joint. The functional requirements R and R can be realied b assigning the joint tpes and orientations of the input and retaining joints into the ke-links chain. In iew of R, the required linear motion of input link in direction with respect to the ground link can be produced through assigning the input joint as a prismatic joint in direction. Thus, we hae the following functional characteristic: C. The input joint of the ke-links chain can be designated as a prismatic joint (P) in direction to aoid the abrasion between the input link and the actuating container door. When characteried b C, there will be no relatie motion between the input link and the container door while operating. As for the functional requirement R, the required linear motion of retaining link in direction with respect to the container door can be thought of a motion with respect to the input link. Thus, it can be produced through assigning the retaining joint as a prismatic joint in direction. We hae the other functional characteristic as follows: C. The retaining joint of the ke-links chain can be designated as a prismatic joint (P) in direction to aoid the abrasion between the retaining link and wafers.

3 The functional characteristics C and C ield feasible designs for the input and retaining joints. Through the combination of these two joints, admissible ke-links chain for the retainer mechanism can be obtained. The result is shown in Fig. 3(b) where the input and retaining joints are labeled according to joint tpe with the arrowed suffi indicating the direction of motion. The input joint is labeled as P and the retaining joint as P. The admissible ke-links chain constitutes the possible module in the retainer mechanism that can fulfill the functional requirements. ground input (a) retaining (b) Figure 3: (a) Ke-links chain of the retainer mechanism. (b) Admissible ke-links chain. Feasible Graphs with Assigned Ke-links Chain In this phase, the admissible ke-links chain deried from the preious section is assigned into feasible graphs of kinematic structures. To determine the kinematic structure of mechanism, the first step is to find its numbers of links and joints. In this paper, the retainer mechanism is designated as a planar mechanism. Hence, the numbers of links and joints must follow the general degree-of-freedom (DOF) equations for the planar mechanism: and F = 3 ( n ) j j () j = j + j () where F denotes the DOF of mechanism, n the number of links, j i the number of i-dof joints and j the number of joints. In this paper, we intend to design the retainer mechanism in the simplest form. Therefore, the mechanism is assumed to hae -DOF and up to links. B substituting the number of links n = 3 or, and the DOF of mechanism F = into Eq. (), possible sets of integer j and j can be soled as shown in Table. Feasible numbers of links and joints (n, j) are then obtained accordingl. For the gien sets of (n, j), feasible graphs of kinematic structures can be searched from the eisting atlas [, 5] and are shown in Table where the lower pairs are denoted b thin edges and the higher pairs b hea edges. According to the compatibilit of the joint DOF between the ke-links chain and the feasible graphs, the assignment can be eecuted through placing the thin edges of ke-links chain coincident with the thin edges of feasible graph, and hea edges of ke-links chain with those of feasible graph. Since the DOF of mechanism is set as one in this work, the -DOF loop of a multi-loops graph containing all the links of the ke-links chain forms a functionall complete retainer mechanism. The remaining links in other loops shall become redundant. Therefore, the following rule for assigning the ke-links chain can be made: All the links of the ke-links chain cannot be assigned into a -DOF loop of a multi-loop kinematic structure. B appling the aboe rule, the admissible ke-links chain in Fig. 3(b) can then be assigned into the feasible graphs in Table. Let us take the No. 3 graph in Table as an eample. This graph is tagged as shown in Fig. (a). The two thin edges of the admissible ke-links chain can be assigned coincident with two of the thin edges among e, e, e 3 of the graph. Howeer, to let the input and retaining links sta in two separate loops, the two thin edges of the ke-links chain can onl be assigned into e and e 3 of the graph in the path -e - 3 -e 3 - or the reerse, as shown in Fig. (b). The other assignments arranging the ke-links chain in the path -e - -e - 3 or the reerse make the input and retaining links in the same -DOF loop and thus iolate the rule. Following this procedure, feasible graphs with assigned ke-links chain, RM- to RM-5 can be enumerated as shown in Table. Table : Feasible graphs of kinematic structures. n j j (n, j) Graphs 3 (3, 3) 0 (, ) 3 (, 5) No. No. No. 3 No. No. 5 5 Admissible Retainer Mechanisms As the results shown in Table, feasible graphs with assigned ke-links chain are determined according to the functional requirements. Ecluding the ke-links chain from these graphs, the remaining links and joints proide the oerall mechanism the specified DOF and nature of motion. The DOF of mechanism has been determined in the preious section b the numbers of links and joints. The nature of motion depends on the tpes and orientations of the remaining joints. In this phase, these specifications of the remaining joints are finall determined to ield admissible retainer mechanisms. -DOF loop e e 5 e e 3 e 3 (a) (b) -DOF loop Figure : (a) No. 3 graph. (b) Feasible graphs with assigned ke-links chain.

4 Table : Planar retainer mechanisms with up to links. No. of Graph Feasible Retainer Mechanisms RM- R, P R, P RM- R, P 3 RM-3 R, P RM- RM-5 R, P RM-5- RM-5- K 5 None K RM-5-3 K P RM-5- K For the planar mechanism, onl the tpes of joints producing planar motions between the connected links can be adopted. The reolute joint (R), prismatic joint (P) and planar cam pair (K) are used in this paper. Furthermore, the tpes of joints are determined according to the joint DOF condition, that is, reolute or prismatic joint can be assigned for the -DOF joint (thin edge), and planar cam pair for the -DOF joint (hea edge). As for the orientations of the remaining joints, the can be further determined according to the motion characteristics of the ke-links chain. Since links in the same loop of a planar mechanism are constrained to moe on the same plane, the orientations of joints in the mechanism can be arranged accordingl. As shown in Fig. 3(b), the input link of the ke-links chain slides in direction with respect to the ground link, while the retaining link slides in direction with respect to the input link. These two links can be considered as moing on the same plane. Among the feasible graphs with assigned ke-links chain in Table, as the input and retaining links are placed in the same loop such as RM- and RM-, the motion plane of the loop is therefore arranged on plane. Hence, the remaining joints are oriented with the rotating ais about -ais. As a result, the thin edges are labeled as R or P, and the hea edges as where the suffi indicates the rotating ais. The prismatic joint with suffi, P represents that it can slide in arbitrar directions on plane, as it were to rotate about -ais with an infinite radius of curature. Note the difference between P and P, where the latter arrowed suffi indicates the direction of motion. As the input and retaining links are placed in distinct loops such as RM-3 to RM-5, the can be considered as moing on distinct planes. Howeer, the common link connecting the two loops shall moe linearl along the intersection of the two different planes. Since linear motions are produced b the prismatic joint, the common joint connecting the common links should be designated

5 as a prismatic joint aligned in this intersection. Hence, onl the graphs with the common joint as thin edge (prismatic joint) are feasible for the case of input and retaining links moing on distinct planes, such as RM-5. Hence, all links in RM-3 and RM- should also be constrained to moe on the same plane, and the remaining joints are accordingl oriented with the rotating ais about -ais. As for RM-5, the left-side loop containing the input joint P can be considered on or plane, and the remaining joints in this loop are oriented with the rotating ais about or -ais respectiel. The right-side loop containing the retaining joint P can be considered on or plane, and the remaining joints in this loop are oriented with the rotating ais about or -ais. As a result, the hea edge in the left-side loop can be or K, and that in the right-side loop can be or K, Through the combination of feasible options for the two hea edges, four situations RM-5- to RM-5- are obtained as shown in Table. The mechanism RM-5- contains the same plane of motion, and the common joint is designated as R or P. The mechanisms from RM-5- to RM-5- contain two different planes of motion, and the common joint is designated as a prismatic joint aligned the intersection of the two planes. Feasible planar retainer mechanisms with up to links are shown in Table. Wafers 3 R Container door R Figure 5: Functional schematic of RM- with all reolute remaining joints. Figure 5 shows the functional schematic of RM- with all remaining joints selected as R. In the mechanism, all links moe on the same plane. The input link, link, is actuated b the upward push of the container door in its. Following the door engaging motion, link moes linearl along -ais, since it is jointed with the ground link, link b a prismatic joint P. Because there is no relatie motion between the container door and link while operating, the abrasion between them is therefore aoided. Actuated b link, the retaining link, link 3 slides along direction with respect to link, since it is jointed with link b a prismatic joint P. For this reason, link 3 can proide wafers a pure retaining action without generating relatie motions between the two objects. The abrasion between link 3 and wafers is therefore aoided. In RM-, link 3 is also paired with ground link b a planar cam pair to constrain the mechanism mobilit as one. 6. Conclusion This paper describes a modular design methodolog to snthesie planar retainer mechanisms in a sstematic manner. The retainer mechanism is functionall and structurall decomposed into two modules, one as the ke-links chain and another as the remaining links and joints. The design of retainer mechanisms is treated as an integration of the construction of admissible ke-links chain, the search of feasible graphs with assigned ke-links chain, and the determination of the remaining links and joints. 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