Patent Publication Number: US-2023148357-A1

Title: Pattern transfer printing systems and methods

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Chinese Patent Applications Nos. 02111321391.3 and 202122732445.7, filed on Nov. 9, 2021, which are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to the field of pattern transfer printing, and more particularly, to producing photovoltaic cells. 
     2. Discussion of Related Art 
     U.S. Pat. Application Publication No. 2017/013724, which is incorporated herein by reference in its entirety, teaches an apparatus for generating a transfer pattern to be used in a transfer printing process. The pattern is generated in a substrate that could be a web substrate and that bears one or more trenches. A filler, e.g., high viscosity metal paste, to be transferred is made to fill the trenches within the web substrate. Upon completion of the trench of the substrate filled with filler, the filling head, which may include a scraper and a squeegee, is translated from the working zone in a synchronized movement, such that in course of the translation movement the filling head remains in full contact with the substrate. 
     Lossen et al. (2015), Pattern Transfer Printing (PTP™) for c-Si solar cell metallization, 5 th  Workshop on Metallization for Crystalline Silicon Solar Cells, Energy Procedia 67:156-162, which is incorporated herein by reference in its entirety, teaches pattern transfer printing (PTP™) as a non-contact printing technology for advanced front side metallization of c-Si PV solar cells, which is based on laser-induced deposition from a polymer substrate. 
     SUMMARY OF THE INVENTION 
     The following is a simplified summary providing an initial understanding of the invention. The summary does not necessarily identify key elements nor limit the scope of the invention, but merely serves as an introduction to the following description. 
     One aspect of the present invention provides a pattern transfer printing (PTP) system comprising: a tape handling unit configured to handle a tape comprising, as sections thereof, a plurality of pattern transfer sheets having respective patterns of trenches, and to controllably deliver the pattern transfer sheets for paste filling and consecutively for pattern transfer, a paste filling unit configured to fill the trenches on the delivered pattern transfer sheets with conductive printing paste, a wafer handling unit configured to controllably deliver a plurality of wafers for the pattern transfer at a close proximity to the pattern transfer sheet, a paste transfer unit configured to transfer the conductive printing paste from a respective one of the pattern transfer sheets onto a respective one of the delivered wafers, by releasing the printing paste from the trenches upon illumination by a laser beam, wherein the tape handling unit is configured to move the tape in a step-and-repeat manner from an unwinder roll to a re-winding roll and optionally to clean and dry the tape after printing during such movement. 
     Another aspect of the present invention provides a pattern transfer printing (PTP) system comprising a wafer handling system in which each of two x,z-stages working in parallel comprise two chucks for holding wafers, each chuck ensuring wafer movement in y, θ - axis thus enabling faster wafer handling and continuous wafers movement during pattern transfer. Multiple cameras imaging incoming wafers enable more accurate wafer alignment within the printing system thus more accurate alignment of printed conductive lines onto wafer pattern. 
     These, additional, and/or other aspects and/or advantages of the present invention are set forth in the detailed description which follows; possibly inferable from the detailed description; and/or learnable by practice of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of embodiments of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections throughout. 
       In the accompanying drawings: 
         FIG.  1 A  is a high-level schematic illustration of a pattern transfer printing (PTP) system, according to some embodiments of the invention. 
         FIGS.  1 B and  1 C  are high-level schematic illustrations of maintenance options and arrangements of PTP systems in a dual-lane production line, according to some embodiments of the invention. 
         FIGS.  2 A and  2 B  are high-level schematic side view and front view illustrations, respectively, of units and elements in PTP systems, according to some embodiments of the invention. 
         FIG.  3 A  is a high-level schematic side view illustration of a tape handling unit, according to some embodiments of the invention. 
         FIGS.  3 B- 3 E  are high-level schematic illustrations of tapes with pattern transfer sheets and of the pattern transfer stage, according to some embodiments of the invention. 
         FIG.  4    is a high-level schematic illustration of a tape re-use unit, according to some embodiments of the invention. 
         FIGS.  5 A and  5 B  are high-level schematic illustrations of paste filling units, according to some embodiments of the invention. 
         FIGS.  5 C- 5 E  are high-level schematic illustrations of printing heads, according to some embodiments of the invention 
         FIGS.  6 A- 6 C  are high-level schematic illustrations of wafer handling units and of their operation, according to some embodiments of the invention. 
         FIGS.  6 D and  6 E  are high-level schematic illustrations of wafer measurement units, according to some embodiments of the invention. 
         FIGS.  7 A- 7 C  are high-level schematic illustrations of tape stretching units and trench alignment monitoring units, according to some embodiments of the invention. 
         FIG.  8    is a high-level schematic illustration of a print quality control unit, according to some embodiments of the invention. 
         FIG.  9 A  is a high-level flowchart illustrating PTP methods, according to some embodiments of the invention. 
         FIG.  9 B  is a high-level flowchart illustrating the parallel processes in the pattern transfer printing (PTP) method, according to some embodiments of the invention. 
         FIG.  10    is a high-level block diagram of exemplary computing device, which may be used with embodiments of the present invention. 
     
    
    
     It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, various aspects of the present invention are described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the present invention. However, it will also be apparent to one skilled in the art that the present invention may be practiced without the specific details presented herein. Furthermore, well known features may have been omitted or simplified in order not to obscure the present invention. With specific reference to the drawings, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
     Before at least one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments that may be practiced or carried out in various ways as well as to combinations of the disclosed embodiments. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. 
     Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as “processing”, “computing”, “calculating”, “determining”, “enhancing”, “deriving” or the like, refer to the action and/or processes of a computer or computing system, or similar electronic computing device, that manipulates and/or transforms data represented as physical, such as electronic, quantities within the computing system’s registers and/or memories into other data similarly represented as physical quantities within the computing system’s memories, registers or other such information storage, transmission or display devices. 
     Embodiments of the present invention provide efficient and economical methods and mechanisms for pattern transfer printing and thereby provide improvements to the technological field of producing electrical contacts, and specifically of producing photovoltaic cells. Pattern transfer printing (PTP) systems and methods are provided to improve the quality, accuracy and throughput of pattern transfer printing. PTP systems comprise a tape handling unit for handling a tape with pattern transfer sheets and for controllably delivering the pattern transfer sheets one-by-one for paste filling and consecutively for pattern transfer, with the tape moving from an unwinder roll to a re-winding roll. PTP systems further comprise a paste filling unit which enables continuous paste filling using a supporting counter roll opposite to the paste filling head, a wafer handling unit controllably delivering wafers for the pattern transfer in a parallelized manner that increases throughput, a paste transfer unit with enhanced accuracy and efficiency due to exact monitoring and wafer alignment, as well as a print quality control. The PTP system may be configured to be used in a dual lane configuration with two parallel wafer flows so that the tape and the paste replacement and maintenance in each system are accessible from their front sides. 
       FIG.  1 A  is a high-level schematic illustration of a pattern transfer printing (PTP) system  100 , according to some embodiments of the invention.  FIGS.  1 B and  1 C  are high-level schematic illustrations of maintenance options and arrangements of PTP systems  100  in a dual-lane production line  101 , according to some embodiments of the invention.  FIG.  1 A  is a schematic perspective front view of PTP system  100 ,  FIG.  1 B  is a schematic perspective view of a front side  102  of PTP system  100 , made accessible for easy maintenance, and  FIG.  1 C  is a schematic top view of PTP systems  100  arranged in a dual-lane production line  101 .  FIGS.  2 A and  2 B  are high-level schematic side view and front view illustrations, respectively, of units and elements in PTP systems  100 , according to some embodiments of the invention. The highly schematic side view illustration of  FIG.  2 A  provides a non-limiting example for the arrangement of elements in tape handling unit  200  with respect to paste filling unit  120  (carrying out a paste filling stage  203 ) and paste transfer unit  350  (carrying out a pattern transfer stage  353 , see, e.g., in  FIGS.  2 A and  2 B ), while the highly schematic front view illustration of  FIG.  2 B  provides a non-limiting example for the arrangement of elements in wafer handling unit  400  and with respect to tape handling unit  200 . Units and elements illustrated in  FIGS.  2 A and  2 B  are described briefly below, with more details of non-limiting embodiments providing in the consecutive figures. One or more control units  105  (see, e.g., in  FIGS.  2 A and  2 B ), may be configured to monitor and/or control the units of PTP system  100 , possibly via various processors, and coordinate the operation of PTP system  100 . 
     PTP system  100  is configured to apply patterns of conductive material onto wafers by non-contact printing. PTP system  100  comprises a tape handling unit  200  configured to handle a tape  205  (see, e.g.,  FIG.  2 A ) comprising, as sections thereof, a plurality of pattern transfer sheets  205 A,  205 B (see, e.g.,  FIG.  2 A ) having respective patterns of trenches, and to controllably deliver pattern transfer sheets for paste filling  205 A and consecutively for pattern transfer  205 B, respectively. Tape handling unit  200  is configured to move tape  205  in a step-and-repeat manner (sheet by sheet) from an unwinder roll  222  to a re-winding roll  242 . PTP system  100  further comprises a paste filling unit  120  configured to fill the trenches on delivered pattern transfer sheets  205 A with conductive printing paste. Tape handling unit  200  may be further configured to deliver the pattern transfer sheets one-by-one for the paste filling (denoted sheets  205 A) and/or for the pattern transfer (denoted sheets  205 B) with continuous monitoring of the tension and the Machine Direction (MD, along the tape movement) and Cross Machine Direction (CMD, perpendicularly to MD) positions of tape  205 . PTP system  100  further comprises a wafer handling unit  400  configured to controllably deliver a plurality of wafers  90  (see, e.g.,  FIG.  2 B ) for the pattern transfer, at a close proximity (e.g., in a range of between 0.1 mm and 0.5 mm) to the pattern transfer sheet. The PTP system  100  further comprises a paste transfer unit  350  configured to transfer the conductive printing paste from respective pattern transfer sheet  205 B onto respective delivered wafer  90 B, by releasing the printing paste from the trenches upon illumination by a laser beam  80  (illustrated, e.g., in  FIG.  3 A ). 
     The units of PTP system  100  are mounted on a rigid frame in a compact manner, to minimize the system’s footprint. As a general design feature, tape handling is carried out along a vertical direction (denoted “z”) and along one horizontal direction (denoted “y”, termed machine direction, MD), while wafer handling is carried out along a perpendicular direction thereto, e.g., in another horizontal direction (denoted “x”, termed cross machine direction, CMD). 
     Tape handling unit  200  may be configured to deliver pattern transfer sheets one-by-one for the paste filling (e.g., pattern transfer sheet  205 A) - at a paste filling process stage  203 , by help of a moving paste filling head  122  - and/or for the pattern transfer (e.g., pattern transfer sheet  205 B) - in a paste pattern transfer unit carrying out paste transfer process stage  353 , by help of a movable scanner  355  (e.g., moveable along the x and y axes and optionally tiltable at an angle θ, or possibly an optical head that scans along the y axis, is moveable along the x axis and optionally tiltable at an angle θ), see, e.g.,  FIG.  2 A . Details concerning tape  205  and pattern transfer sheets thereupon are provided below. It is noted that in  FIG.  2 B  pattern transfer sheet  205 A is illustrated schematically at the plane of the drawing as it is located in the paste filling unit  120  almost vertically. For example, paste filling unit  120  and pattern transfer sheet  205 B plane may be set at an angle deviating 0-30° from the vertical x-z plane. 
     In embodiments, one or more top dancer  225  and bottom dancer  245  (see, e.g.,  FIG.  2 A ) may be configured to buffer the step-and-repeat movement of tape  205  from unwinder roll  222  and to re-winding roll  242 , respectively, as pattern transfer sheet  205 A is being filled with paste and/or as paste from pattern transfer sheet  205 B is being transferred, so as to ensure that these are carried out with the respective pattern transfer sheet in static positions. Top dancer(s)  225  and/or bottom dancer(s)  245  may be configured to maintain the tension in tape  205  moving through at least a part of PTP system  100 . 
     It is noted that in PTP system  100 , paste filling unit  120  is positioned almost vertically (along the z axis) to ensure a short travelling distance of pattern transfer sheet  205 A from paste filling to pattern transfer sheet  205 B at paste transfer unit  350  thus enabling to minimize changes of the filled paste condition (e.g., due to drying before printing) . For example, the near-vertical position may be configured to enable a smaller movement distance for the pattern transfer sheets from state  205 A to state  205 B, and thereby optionally to locate the laser scanner just behind the vertical filling unit, closer to roll  227 A positioned between  205 A and  205 B, illustrated schematically in  FIG.  2 A . The near-vertical position of paste filling unit  120  is advantageous with respect to prior art horizontal position of paste filling units, as it reduces the distance between the paste filling position (205A) and the paste transfer position (205B) of the pattern transfer sheet  205 . 
     As illustrated schematically in  FIG.  1 B , front side  102  of PTP system  100  may be configured to have unwinder roll  222  and re-winding roll  242  easily accessible for replacement and maintenance requirements, as well as have paste filling unit  120  easily accessible for paste filling and maintenance requirements - from same front side  102 . Moreover, as illustrated schematically in  FIG.  1 C , PTP systems  100  may be arranged back-to-back, in dual-lane production line  101 , with each of front sides  102  of PTP systems  100 A,  100 B easily accessible for maintenance. Dual-lane production line  101  may be configured to include two lanes  101 A,  101 B, each with one or more (serially arranged) PTP systems  100 A,  100 B, which operate on two (or more) paths of wafers  90  (e.g., independent paths for higher throughput), using a relatively small footprint. 
     PTP system  100  may further comprise a tape re-use unit  250  (see, e.g.,  FIG.  2 A ) configured to clean pattern transfer sheets after the pattern transfer to provide reusable pattern transfer sheets. For example, tape re-use unit  250  may comprise a tape cleaning unit  252  in which tape  205  may be cleaned mechanically, e.g., using scraper(s), ultrasound, and/or other means, and/or chemically using cleaning solutions; and a tape drying unit  255 , with idle rolls  244 ,  246  positioned as needed to maintain safe tape movements. Tape  205  may be moved by one or more tape drive motor(s)  230  (illustrated schematically), and further supported by one or more rolls  227  along the way of tape  205  through PTP system  100 . A non-limiting example for tape handling unit  200  is illustrated with more details in  FIGS.  3 A- 3 E . A non-limiting example for tape re-use unit  250  is illustrated with more details in  FIG.  4   . 
     Paste filling unit  120  (see, e.g.,  FIG.  2 A ) may comprise moveable paste filling head  122  and a countering moveable roll  125  configured to support a back side of pattern transfer sheet  205 A during the paste filling. A non-limiting example for paste filling unit  120  is illustrated with more details in  FIGS.  5 A- 5 E . 
     In certain embodiments, wafer handling unit  400  may comprise at least one stage  410  enabling movement in x and z axes (termed in the following - the x,z-stage), with each stage  410  comprising at least one holder (e.g., chuck)  415 , and with each holder supporting wafer  90  and enabling wafer movement in y and θ axes (θ axis relates to rotation of a wafer with respect to the x-y plane). In certain embodiments, two x,z-stages  410  of wafer handling unit  400  may be configured to operate in parallel with respect to each other. Each stage  410  may comprise two holders  415  for holding wafers  90 , each holder  415  ensuring wafer movement in y, θ - axis thus enabling faster wafer handling and continuous wafers movement during pattern transfer. Multiple cameras imaging incoming wafers enable more accurate wafer alignment within the printing system thus more accurate alignment of printed conductive lines onto wafer pattern. 
     Wafer handling unit  400  (see, e.g.,  FIG.  2 B ) may comprise two stages  410 A,  410 B, with each stage  410  supporting two wafers  90 , e.g., via wafer holders  415 A,  415 B (e.g., vacuum chucks). Each stage  410  may support at least one wafer holder (chuck)  415 , e.g., two or three holders (chucks)  415 . As a non-limiting example, two holders (chucks)  415 A,  415 B for each of two stages  410 A,  410 B are illustrated. Stages  410  may be configured to enable movement along the x and z axes, while each holder (chuck)  415  may be configured to support wafer  90  and enable additional wafer movement at least along the y and θ axes, as explained below. 
     Wafer handling unit  400  may be configured to alternate two stages  410 A,  410 B during operation to enable parallel operation on wafers  90  by different units of PTP system  100 . For example, the position of wafer  90 A may be measured by a wafer alignment unit  420  while wafer  90 B received transferred paste by paste transfer unit  350  and wafer  90 C is inspected by a print quality control unit  450 , as disclosed herein. During the alteration of stages  410 , two wafers  90  may be processed by respective units, increasing the overall throughput of PTP system  100 . For example, wafer handling unit  400  may be configured to provide simultaneous (i) wafer measurement of two wafers  90 A (first and second wafers, mounted on holders (chucks)  415 A and  415 B on stage  410 A, respectively), (ii) pattern transfer to a third wafer  90 B (mounted on holder (chuck)  415 A on stage  410 B) and (iii) print quality control of a fourth wafer  90 C (mounted on holder (chuck)  415 B on stage  410 B). Then, wafer handling unit  400  may be configured to move stages  410 A,  410 B according to arrows  411 A, so that the wafers are move to the consecutive operations (e.g., wafer  90 A is moved from wafer measurement to pattern transfer, wafer  90 B is moved from pattern transfer to print quality control and wafer  90 C is moved out of the system, while a new wafer is moved to wafer measurement). Following the linear stage movements along CMD, stages  410 A,  410 B may be switched (arrow  411 B), so that wafer  90  are further processed, and the movements repeat cyclically, as illustrated schematically in  FIG.  6 C ). 
     Wafer handling unit  400  may further comprise mechanical elements such as input wafer conveyor  412  for supplying wafers  90  and mounting them on wafer holders  415  of respective stage  410  and output wafer conveyor  419  for receiving printed wafers  90  from wafer holders  415  of respective stage  410 . A non-limiting example for wafer handling unit  400  and a schematic description of its operation is illustrated with more details in  FIGS.  6 A- 6 C . 
     Wafer alignment unit(s)  420  (e.g.,  420 A and  420 B, see  FIG.  2 B ) may be configured to detect and measure features on wafers and to adjust the wafer position to the pattern transfer accordingly - is illustrated with more details in a non-limiting example in  FIGS.  6 D and  6 E . 
     In certain embodiments, wafer handling unit  400  may comprise more than two stages  410 , accompanied with multiplication of any of wafer alignment unit  420 , paste transfer unit  350  and/or print quality control unit  450  - to further increase the throughput of PTP system  100 . 
     PTP system  100  may further comprise a tape stretching unit  270  (see, e.g.,  FIGS.  2 A and  7 A ) configured to affix and flatten respective pattern transfer sheet  205 B during the paste pattern transfer from pattern transfer sheet  205 B to wafer  90 B. PTP system  100  may further comprise a trench alignment monitoring unit  300  configured to monitor a position (e.g., in x, y and theta (tilt) directions) and a distortion of the trenches prior to the pattern transfer. A non-limiting example for tape stretching unit  270  and trench alignment monitoring unit  300  is illustrated with more details in  FIGS.  7 A- 7 C . 
     Paste transfer unit  350  (e.g., a laser scanner) may comprise a laser scanner (scanning head)  355  (e.g., movable along CMD, e.g., by a linear stage, a ball-screw stage, etc.) configured to control the illumination of pattern transfer sheets  205 B by the laser beam for depositing the paste from the patterned trenches of pattern transfer sheets  205 B. 
     PTP system  100  may further comprise a print quality control unit(s)  450  (e.g.,  450 A and  450 B, see  FIG.  2 B ) configured to control a print quality of the pattern transfer, in particular to detect tiny defects such as openings or gaps within the printed fingers or other defects in the pattern that was transferred onto the wafer. For example, print quality control unit(s)  450  may be based on imaging cameras, which transfer the acquired images of inspected wafers to processor(s)  452  for image processing. A non-limiting example for a print quality control unit  450  is illustrated with more details in  FIG.  8   . 
       FIG.  3 A  is a high-level schematic side view illustration of tape handling unit  200 , according to some embodiments of the invention. Tape handling unit  200  may be configured to move tape  205 , while delivering pattern transfer sheets  205 A one-by-one for the paste filling (at paste filling unit  120 , see, e.g.,  FIG.  2 A ) and/or for the pattern transfer (at pattern transfer unit  350 ) by continuously controlling the tape tensions and accurate position of the sheets in both MD and CMD coordinates.  FIGS.  3 B- 3 E  are high-level schematic illustrations of tape  205  with pattern transfer sheets  205 B (see, e.g.,  FIG.  2 A ) and of paste pattern transfer unit  350 , according to some embodiments of the invention. 
     The CMD positions of unwinder roll  222  and re-winding roll  242  may be continuously controlled and if needed corrected by help of one or more control unit(s)  105 , e.g., by controlling driving motor(s) thereof. Top dancer(s)  225  and bottom dancer(s)  245  may be configured to support fast stepwise movements of pattern transfer sheets  205 A,  205 B (as segments of tape  205 ) to their respective positions for paste filling and pattern transfer. Idle rolls  227  (only some of which indicated) may be configured to direct tape movement through tape handling unit  200 . 
     Tape handling unit  200  may be configured to enable fast and accurate provision and changing of the tape segments (pattern transfer sheets) used to print the wafers. Tape handling unit  200  may be further configured to have a compact design with a minimal footprint, and be set within a stable and rigid frame or chassis for supporting its operation and also for enabling easy maintenance. Tape re-use unit  250  may be set within the frame and in the path of tape  205  and enable reusing tape  205  - making the overall process more efficient and economical. 
       FIGS.  3 B- 3 E  are high-level schematic illustrations of tape  205  with pattern transfer sheets  205 B and of pattern transfer unit  350 , according to some embodiments of the invention, which are disclosed in more details in Chinese Patent Applications Nos. 202111034191X and 2021221306455, incorporated herein by reference in their entirety. 
     Highly schematic  FIG.  3 B  illustrates transfer of patterned paste from the pattern transfer sheet  205 B to substrate (e.g., wafer)  90 B using laser illumination by laser scanner(s)  355 . Pattern transfer sheets  205 B comprises a plurality of trenches  210  arranged in a specified pattern and configured to receive printing paste and release the printing paste from trenches  210  upon illumination by laser beam  80  onto a receiving substrate such as wafer  90 B.  FIGS.  3 B and  3 E  schematically illustrate the filling of trenches  110  on an empty pattern transfer sheet on tape  205  with paste to yield filled pattern transfer sheet  205 A, which is then moved further in PTP system  100  to have the paste released from trenches  210  of pattern transfer sheet  205 B onto wafer  90 B. It is noted that while the tape is denoted generally by the numeral  205 , sections of tape  205  that are designed as used as pattern transfer sheets are denoted by numeral  205 A when they are in paste filling stage  203  and are denoted by numeral  205 B when they are in paste pattern transfer stage  353  (illustrated schematically in  FIG.  3 B  by an arrow). 
     Pattern transfer sheets may further comprise at least one trace mark  220  that is located outside the specified pattern of trenches  210  and is configured to receive the printing paste. Trace mark(s)  220  is aligned with respect to respective trench(es)  210  and is wider than a width of laser beam  80 . Upon illumination by laser beam  80 , only a part of the paste in trace mark(s)  220  is released (off pattern transfer sheet  205 B), because the width of trace mark(s)  220  is larger than the width of laser beam  80  - yielding a gap that may be used to detect the actual position of the laser beam relative to the position of the corresponding trench. 
     Pattern transfer sheet may further comprise a plurality of working window marks  223  that are located outside the specified pattern of trenches  210  and are configured to receive the printing paste. Working window marks  223  are set at specified offsets with respect to specified trenches  210  of the specified pattern, with different working window marks  223  being set at different offsets. Working window marks  223  may be used to monitor the power of laser beam  80  needed for releasing paste from all the trenches. 
     In certain embodiments, pattern transfer sheet may comprise both trace mark(s)  220  and working window marks  223 , which may be configured to enable unambiguous detection by image processing, e.g., by a trench alignment monitoring unit  300 . 
     Pattern transfer sheet may further comprise a plurality of alignment marks (not shown) that are located outside the specified pattern of trenches  210 , aligned with respective trenches  210 , configured to receive the printing paste and used to provide initial laser scanner alignment with respect to the specified pattern of trenches  210 . 
     A trench alignment monitoring unit  300  may be configured to monitor the pattern transfer process optically, e.g., monitoring the transfer of the printing paste by emptying of trenches  210  and of marks  220 ,  223  onto the substrate, as explained herein. One or more processor(s)  356  or controller(s), in communication with control unit(s)  105 , may be in communication with laser scanner(s)  355  (in paste transfer unit  350 ) and imaging unit(s)  300  and be configured to adjust optical parameters of laser illumination by modifying the settings of power and position of laser scanner(s)  355  according to image analysis of images taken by imaging unit(s)  300 . These adjustments and modifications improve the quality and accuracy of pattern transfer stage  353 . For example, processor(s)  356  or controller(s) may be configured to calculate an alignment of laser beam  80  according to traces on pattern transfer sheet (after the paste is released therefrom), e.g., detect misalignment of laser scanner  355  upon detection of asymmetric trace(s) as disclosed in Chinese patent application Nos. 202111034191X and 2021221306455, incorporated herein by reference in their entirety. Processor(s)  356  or controller(s) may be further configured to calculate an effective working window of laser illumination  80  using remaining working window marks  223  on pattern transfer sheet (after the paste is released therefrom), and adjust laser power of the laser scanner  355  accordingly. Additional non-limiting details for PTP systems  100  are provided, e.g., in U.S. Pat. No. 9,616,524. 
     Disclosed PTP systems  100  and tape  205  may be used to print fine lines  92  of thick metallic paste to produce electronic circuits, e.g., creating conductive lines or pads or other features on laminates for PCBs or other printed electronic boards, or on silicon wafers, e.g., for photovoltaic (PV) cells. Other applications may comprise creating conductive features in the manufacturing processes of mobile phones antennas, decorative and functional automotive glasses, semiconductor integrated circuits (IC), semiconductor IC packaging connections, printed circuit boards (PCB), PCB components assembly, optical biological, chemical and environment sensors and detectors, radio frequency identification (RFID) antennas, organic light-emitting diode (OLED) displays (passive or active matrix), OLED illuminations sheets, printed batteries and other applications. For example, in non-limiting solar applications, the metallic paste may comprise metal powder(s), optional glass frits and modifier(s), volatile solvent(s) and non-volatile polymer(s) and/or or resin(s). A non-limiting example for the paste includes SOL9651B™ from Heraeus™. 
       FIG.  3 C  is a high-level schematic cross section illustration of tape (pattern transfer sheet)  205 , according to some embodiments of the invention. In certain embodiments, tape  205  may be transparent to laser illumination and comprise at least a top polymer layer  214  comprising trenches  210  and marks  220 ,  223  (illustrated schematically in  FIG.  3 B ) which formed by press molding , pneumatic molding or laser molding thereon. In the illustrated non-limiting example, trenches  210  are illustrated as being trapezoid in cross section. 
     Tape  205  may comprise at least one polymer layer, which may be selected from at least one of: polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, fully aromatic polyester, other copolymer polyester, polymethyl methacrylate, other copolymer acrylate, polycarbonate, polyamide, polysulfone, polyether sulfone, polyether ketone, polyamideimide, polyether imide, aromatic polyimide, alicyclic polyimide, fluorinated polyimide, cellulose acetate, cellulose nitrate, aromatic polyamide, polyvinyl chloride, polyphenol, polyarylate, polyphenylene sulfide, polyphenylene oxide, or polystyrene. 
     It is noted that while schematic  FIG.  3 C  shows periodical trenches  210 , marks  220  and/or  223  (illustrated schematically in  FIG.  3 B ) may comprise trenches, recesses and/or indentations that are embossed in a similar manner into top polymer layer  214 , and may have similar or different profiles. For example, trenches  210 , trace marks  220  and/or working window marks  223 , and alignment marks may have various profiles (cross section shapes), such as trapezoid, rounded, square, rectangular and triangular profiles. In various embodiments, the pattern of trenches  210  on tape  205  may comprise continuous trenches  210  and/or arrays of separated dents. It is noted that the term “trenches” is not to be construed as limiting the shape of trenches  210  to linear elements, but is understood in a broad sense to include any shape of trenches  210 . 
     Tape  205  may comprise a top polymer layer  214  and a bottom polymer layer  212 , the bottom polymer layer  212  having a melting temperature that is higher than an embossing temperature of the top polymer layer  214 . In some embodiments, top polymer layer  214  may be made of semi-crystalline polymer and have a melting temperature, e.g., below 150° C., below 130° C., below 110° C. or have intermediate values. In some embodiments, top polymer layer  214  may be made of amorphous polymer and have a glass temperature below 160° C., e.g., below 140° C., below 120° C., below 100° C. or have intermediate values. Bottom polymer layer  212  may have a higher melting temperature than the melting temperature or the glass temperature of top polymer layer  214 . For example, bottom polymer layer  212  may have a melting temperature above 150° C., above 160° C. (e.g., bi-axially-oriented polypropylene), above 170° C., and up to 400° C. (e.g., certain polyimides), or have intermediate values. 
     In certain embodiments, top and bottom polymer layers  214 ,  212  (respectively) may be between 10 µm and 100 µm thick, e.g., between 15 µm and 80 µm thick, between 20 µm and 60 µm thick, between 25 µm and 45 µm thick, or have other intermediate values - with bottom polymer layer  212  being preferably at least as thick as top polymer layer  214 . The polymer layers may be attached by an adhesive layer  213  that is thinner than 10 µm (e.g., thinner than 8 µm, thinner than 6 µm, thinner than 4 µm, thinner than 2 µm or have intermediate values) and is likewise transparent to the laser illumination. For example, in certain embodiments, top polymer layer  214  may be thicker than the depth of trenches  210  by several µm, e.g., by 5 µm, by 3-7 µm, by 1-9 µm, or by up to 10 µm. For example, trenches  210  may be 20 µm deep, top polymer layer  214  may be 20-30 µm thick and bottom polymer layer  212  may range in thickness between 25 µm and 45 µm (it is noted that thicker bottom polymer layer provide better mechanical performances). It is noted that the term “trenches” is not to be construed as limiting the shape of trenches  210  to linear elements, but is understood in a broad sense to include any shape of trenches  210 . 
     The temperature and thickness of top and bottom polymer layers ( 214 ,  212  respectively) may be designed so that top polymer layer  214  has good molding, ductility and certain mechanical strength, while bottom polymer layer  212  has good mechanical strength. Both top and bottom polymer layers ( 214 ,  212  respectively) may be designed to have good bonding properties. 
       FIGS.  3 D and  3 E  are high-level schematic illustrations of dynamic PTP system  100 , according to some embodiments of the invention. Dynamic PTP system  100  comprises at least one laser scanner optical head(s)  355  configured to illuminate with laser beam(s)  80  pattern transfer sheet  205 B with trenches  210  arranged in a first pattern  206  and holding printing paste in filled trenches  92 , which is then released onto wafers  90  upon the illumination by laser beam  80  from laser scanner optical head(s)  355  configured to have a fast-scanning axis along machine direction  87  (y axis, MD) and may be moved along cross machine direction  85  (x axis, CMD). The releasing of the paste from the trenches implements pattern transfer stage  353 , which is indicated schematically in  FIGS.  3 D and  3 E  by arrows. 
     Dynamic PTP system  100  may comprises moveable stages  410  with wafer holders  415  affixing wafers  90  (e.g., by help of vacuum clamping) to moveable stage  410  during the releasing of printing paste  92  from pattern transfer sheet  205 B. Moveable stage  410  may comprise any type of stage or wafer holder that can affix and move wafers  90 . Moveable stage  410  may be moved by any type of actuator, e.g., by linear or step motors. 
     Dynamic PTP system  100  may further comprises controller(s) and/or processor(s)  357 , possibly associated with control unit(s)  105 , and configured to control laser scanner optical head  355  to direct laser beam  80  along trenches  210  (along machine direction  87  -MD), and at a cross machine direction  85  (CMD, scanning direction) across trenches  210 . Processor(s)  357  may further be configured to move moveable stage  410  (the movements are denoted schematically by numeral  417 ) to yield a second pattern  96  of deposited paste on wafer  90 , which is different from first pattern  206  of trenches  210  on pattern transfer sheet  205 B. Advantageously, in contrast to current practice which is limited to transferring the same pattern (e.g., of lines) from pattern transfer sheet  205 B to wafer  90 , various embodiments of dynamic PTP system  100  enable to deposit the transferred metal paste onto wafer  90  at patterns (second pattern  96 ) which are different from first pattern  206  of trenches  210  on pattern transfer sheet  205 B. 
     As illustrated schematically in  FIG.  3 E , wafer  90 B may comprise a pattern of substantially parallel linear locations arranged with a certain receiving pitch p 2 , for receiving the paste release from trenches  210  of pattern transfer sheet  205 B, in a close proximity (e.g., in a range of between 0.1 mm and 0.5 mm) to pattern transfer sheet  205 B in such a way that the first trench on pattern transfer sheet  205 B is located exactly opposite to the first linear location on wafer  90 B. Scanning the paste-filled trench pattern  206  on pattern transfer sheet  205 B by laser beam  80  sequentially from the first trench to the last trench results in the deposition of the paste onto the specified locations on wafer  90 B to yield deposited paste in a specified pattern  96 . As processor(s)  357  and/or control unit  105  move wafer  90 B during the scanning (movements indicated schematically by numeral  417 ), the paste is deposited at a different pitch (p 2 ≠p 1 ), depending on the direction and speed of motion. 
     It is noted that scanning along x-axis may be carried out in forward and/or backward directions, and respective movements  417  of wafer  90  may be adjusted accordingly. In the present disclosure cross machine direction  85  is illustrated in one direction, as a non-limiting example. 
     For example, first pattern  206  of trenches  210  on pattern transfer sheet  205 B may have a first pitch (“p 1 ”) and second pattern  96  of deposited paste on wafer  90  may have a second pitch (“p 2 ”), that may be smaller or larger than first pitch (“p 1 ”), e.g., p 1 &gt;p 2  or p 1 &lt;p 2 . It is noted that second pattern  96  may differ from first pattern  206  over the whole extent of wafer  90  or over a part of the extent of wafer  90 . In some examples, the difference of pattern may comprise p 1 &gt;p 2  in some area(s) of wafer  90  while p 1 &lt;p 2  in other area(s) of wafer  90 . 
     In certain embodiments, with first pitch p 1  being larger than second pitch p 2  (p 1 &gt;p 2 ), processor  357  may be configured to move moveable stage  410  along scanning direction  85  (CMD, denoted  417 A) at a forward speed set to convert first pitch p 1  to second pitch p 2 . For example, with forward speed denoted as v F  and the time between deposition of consecutive lines denoted as t, p 2 =p 1 -v F -t. Alternatively or complementarily, denoting the scanner speed across trenches  210  as v S =p 1 /t, the approximate relation between the pitches is p 2 =p 1 -(v S -v F )/v S . 
     In certain embodiments, with first pitch p 1  being smaller than second pitch p 2  (p 1 &lt;p 2 ), processor  357  may be configured to move moveable stage  410  against (in a contrary direction to) scanning direction  85  (CMD, denoted  417 B) at a backward speed set to convert first pitch p 1  to second pitch p 2 . For example, with backward speed denoted as v B  and the time between deposition of consecutive lines denoted as t, p 2 =p 1 +v B -t. Alternatively or complementarily, denoting the scanner speed across trenches  210  as v S =p 1 /t, the approximate relation between the pitches is p 2 =p 1• (v S +v B )/v S . 
       FIG.  4    is a high-level schematic illustration of tape re-use unit  250 , according to some embodiments of the invention. Tape cleaning unit  252  of tape re-use unit  250  may comprise a pre-cleaning compartment  252 A and a cleaning compartment  252 B configured to remove paste remains having different characteristics (e.g., pre-cleaning may remove rougher clumps of paste while cleaning may remove finer paste remains), and possibly may be preceded by a scraper device for removing paste smears. Tape cleaning may be carried out physically, e.g., by turbulent liquid flow, agitation, application of ultrasound, etc., and/or chemically, e.g., applying corresponding solvents. Fluid introduction to and removal from pre-cleaning compartment  252 A and/or cleaning compartment  252 B may be managed by a recirculation unit  254  (shown schematically), comprising, e.g., pump(s) and filter(s) for reusing respective cleaning solution(s). Tape drying unit  255  may follow tape cleaning unit  252  and be configured to dry tape  205  and prepare it for future use, prior to rolling tape  205  onto re-winding roll  242 . Idle rolls  244 ,  246  and optionally additional rolls may be set to direct tape movement through and after tape re-use unit  250 . During the tape segment advance movement, one or more idles rolls  248  shown in the bottom part of unit  250  may move upward enabling smooth tape movement and continuous tension control performing as dancers, similar to dancers  225 ,  245 . After finishing the segment advance, rolls  248  may go downwards under their own weight. Alternatively or complementarily, tape re-use unit  250  may comprise one or more dancers configured to maintain the tension in tape  205  moving through tape re-use unit  250 . 
       FIGS.  5 A and  5 B  are high-level schematic illustrations of paste filling unit  120 , according to some embodiments of the invention.  FIG.  5 A  is a perspective view and  FIG.  5 B  is a side view.  FIGS.  5 C- 5 E  are high-level schematic illustrations of filling head  122 , according to some embodiments of the invention.  FIGS.  5 C and  5 E  are schematic side view in cross section and  FIG.  5 D  is a perspective view from below filling head  122 . 
     As illustrated schematically in  FIGS.  5 A and  5 B , paste filling unit  120  may comprise a frame on which paste filling head  122  and bottom roll  125  are mounted, and with respect to which they are moved simultaneously. Movements of the paste filling head assembly may be controlled by one or more control unit(s)  105  e.g., via controlling respective flexible rack  126  attached to paste filling head  122 , drive motors  124  and/or gantry motion system  128 . 
     Bottom roll  125  may be configured to counter paste filling head  122  and support pattern transfer sheet  205 A of tape  205  during the filling of pattern transfer sheet  205 A with the paste by paste filling head  122 . Bottom roll  125  may be configured to roll during operation, possibly controllably. 
     Paste filling unit  120  may be configured to enable fast, uniform and accurate filling of the high viscosity paste into the trenches having a high aspect ratio. Paste filling unit  120  may be further configured to clean the surface of tape  205  after filling, e.g., disclosed in WIPO Publication No. 2015128857, which is incorporated herein by reference in its entirety. 
     As illustrated schematically in  FIGS.  5 C- 5 E , and disclosed in more details in Chinese Patent Applications Nos. 2021106730065 and 2021213505781, incorporated herein by reference in their entirety, filling head  122  of paste filling unit  120  may comprise at least two feeding openings  161 ,  169 , an internal cavity  165  and at least one dispensing opening  160  that are in fluid communication (see, e.g.,  FIGS.  5 D and  5 E ) and a pressurized paste supply unit  155  configured to circulate paste  190  through printing head  150 . The pressure in the pressurized paste supply unit may be adjusted to maintain continuous circulation of the paste through feeding openings  161 ,  169  and internal cavity  165  and to control dispensing of the paste through dispensing opening(s)  160 . For example, pressurized paste supply unit  155  may comprise a pressurized paste reservoir  154  and a paste pump  152  in fluid communication with internal cavity  165  of printing head  150 , which are configurated to circulate the paste therethrough. In non-limiting examples, paste pump  152  may comprise rotating pressure-tight displacement systems with self-sealing, rotor/stator designs for dispensing precise volumes such as eco-PEN450™ from Dymax™. 
     In various embodiments, paste filling unit  120  comprises at least one pressure sensor  140  configured to measure the pressure of the circulating paste, e.g., pressure sensor  140  illustrated schematically in  FIGS.  5 C- 5 E  and associated with a paste mixer  130  or pressure sensors  140 A,  140 B illustrated schematically in  FIG.  5 C , at either ends of printing head  150 , as non-limiting examples. Alternatively or complementarily, pressure measurement may be implemented within elements of pressurized paste supply unit  155 , such as paste pump  152  and/or paste reservoir  154 . Paste filling unit  120  may further comprise at least one processor  167  and/or controller (shown schematically in  FIG.  5 E ), in communication with control unit(s)  105  and configured to adjust the pressure in pressurized paste supply unit  155  (or its components) with respect to the measured pressure of the circulating paste. Paste filling unit  120  may further comprise one or more paste mixer(s)  130  configured to mix the circulating paste. For example, paste mixer(s)  130  may be a static mixer, mixing the paste by utilizing its pressurization. In non-limiting examples, paste mixer(s)  130  may comprise plastic disposable static mixers such as GXF-10-2-ME™ from Stamixco™ made of large diameter plastic housing that includes multiple mixing elements. 
     Pressurized paste supply unit  155  may be further configured to introduce the paste into internal cavity  165  via at least one entry opening  161  of the at least two feeding openings and to receive the circulated paste via at least one exit opening  169  of the feeding openings in printing head  150 . Typically, entry opening(s)  161  and exit opening(s)  169  are at the top of printing head  150 , opposite to dispensing opening(s)  160  which faces the pattern transfer sheet with trenches which are to be filled by the paste. Alternatively or complementarily, entry opening(s)  161  and/or exit opening(s)  169  may be positioned on sides and/or extension(s) of printing head  150 . 
     Pressurized paste supply unit  155  may comprise pressure-controlled paste reservoir  154 , paste pump  152  and mixer  130  that are in fluid communication. Pressure-controlled paste reservoir  154  may be configured to deliver paste to paste pump  152 , which may be configured to deliver the paste through mixer  130  to entry opening(s)  161 . Pressurized paste supply unit  155  may be further configured to mix paste from exit opening(s)  169  with the paste delivered from pressure-controlled paste reservoir  154  to paste pump  152 . For example, as illustrated schematically in  FIGS.  5 C and  5 E , paste  190  in paste reservoir  154  may be delivered ( 191 ) to paste pump  152 , mixing ( 192 ) with paste  197  exiting from exit opening(s)  169  of printing head  150 , to be pumped by paste pump  152  into mixer  130 . Paste  193  from mixer  130  may be delivered ( 194 ) to entry opening(s)  161  of printing head  150 , wherein paste  196  moves along internal cavity  165  and some paste  195  may be dispensed through dispensing opening(s)  160  to form patterns on the transfer sheet  205 A such as lines to be then printed (after the tape movement, from transfer sheet  205 B) on the receiving substrate such as wafer  90  (e.g., silver lines of about 20 µm width on silicon wafers for PV cells, see, for non-limiting examples, Lossen et al. 2015). Remaining paste  197  is then mixed with paste  191  from paste reservoir  154  (e.g., delivered through nozzle  163  at junction  151 ) to compensate for the dispensed amount, and the paste is circulated through Paste filling unit  120  to maintain its mechanical characteristics and support continued mixing of the paste to maintain its uniform composition. In certain embodiments, paste filling unit  120  may be further configured to modify paste composition, e.g., by adding additives such as solvents to keep the paste homogenized, possibly in relation to the monitored pressures throughout paste filling unit  120 . For example, additives such as solvents may be added to the paste entering mixer  130  if needed. 
     In various embodiments, printing head  150 , internal cavity  165  and dispensing slit as opening  160  limited by slit edges  162  (e.g., metallic slit lips) may be elongated (see, e.g.,  FIG.  5 D ) and configured with respect to paste properties (e.g., viscosity values), specified throughput and specified features (e.g., length, width and optionally cross section) of the lines or other elements that are to be dispensed by printing head  150 . In certain embodiments, dispensing opening  160  may comprise one or more slits, one or more opening, a plurality of linearly-arranged openings, e.g., one or more lines of circular or elliptical openings, and so forth. 
     In various embodiments, paste material may comprise conductive silver based metallic paste, and may typically be of high viscosity (e.g., in the range of several tens to several hundreds of Pa·s). For example, in non-limiting solar applications, the metallic paste may comprise metal powder(s), optional glass frits and modifier(s), volatile solvent(s) and non-volatile polymer(s) and/or or resin(s). A non-limiting example for the paste includes SOL9651B™ from Heraeus™. 
     Paste filling unit  120  may comprise one or more pressure sensor(s)  140 ,  140 A,  140 B configured to measure the pressure of the circulating paste at one or more respective locations along the paste circulation path. For example, pressure sensor(s)  140 ,  140 A, 140 B may be set adjacent to entry opening(s)  161 , exit opening(s)  169 , in fluid communication with internal cavity  165  of printing head  150  and/or in association with any of mixer  130 , paste reservoir  154  and/or paste pump  152 . Pressure-related indications from pressurized paste reservoir  154  and/or paste pump  152  may also be used to monitor paste circulation through Paste filling unit  120  and/or to monitor and possibly modify the paste properties such as its viscosity, e.g., by adding solvent. Paste filling unit  120  may further comprise at least one controller (e.g., as part of or in communication with control unit  105  and/or as at least one computer processor  173  as illustrated in  FIG.  10   ), in communication with any of the components of paste filling unit  120 , e.g., via communication link(s)) configured to adjust the pressure in pressure-controlled paste reservoir  154  and/or paste pump  152  with respect to the measured pressure of the circulating paste, e.g., as received from one or more pressure sensor(s)  140 ,  140 A, 140 B. In non-limiting examples, any of pressure sensor(s)  140  may comprise, e.g., small profile, media compatible, piezoresistive silicon pressure sensors packaged in a stainless-steel housing (e.g., MEAS 86A™ from T.E. connectivity™ or equivalent sensors). 
     In various embodiments, pressure-controlled paste reservoir  154  and paste pump  152  may open adjacently to exit opening(s)  169  of printing head  150  and paste filling unit  120  may comprise a conduit  135  connecting the exit of mixer  130  to entry opening(s)  161  of printing head  150 . In some embodiments, pressure-controlled paste reservoir  154  and paste pump  152  may open adjacently to exit opening(s)  169  of printing head  150 , mixer  130  may be adjacent to entry opening(s)  161  of printing head  150 , and conduit  135  may connect paste pump  152  to mixer  130 . Pressure sensor  140  may be associated with mixer  130 . The dimensions and orientations of paste reservoir  154  and paste pump  152  may vary, e.g., both paste reservoir  154  and paste pump  152  may be set perpendicularly to printing head  150  (see, e.g.,  FIG.  5 C ), or one or both of paste reservoir  154  and paste pump  154  may be set at an angle to printing head  150 . For example, paste pump  152  may be set obliquely to spread its weight more evenly over printing head  150 , as illustrated schematically in  FIGS.  5 D and  5 E . 
     In various embodiments, conduit  135  may be adjusted to conform to any arrangement of paste reservoir  154 , paste pump  152  and mixer  130 , so as to make paste filling unit  120  more compact or adjust it to a given space and weight distribution requirements within the printing machine. Holder  145  (see, e.g.,  FIG.  5 C ) is illustrated schematically as an attachment element for attaching paste filling unit  120  to the printing machine (see, e.g., U.S. Pat. No. 9,616,524 for a non-limiting example). In non-limiting examples, conduit  135  may be connected between an opening  131  in mixer  130  and an opening  138  of connector  137  at entry opening  161  in printing head  150  (see, e.g.,  FIG.  5 C ) or between opening  131  in paste pump  152  and opening  138  in mixer  130  (see, e.g.,  FIG.  5 E ). 
       FIGS.  6 A- 6 C  are high-level schematic illustrations of wafer handling unit  400  and of its operation, according to some embodiments of the invention.  FIG.  6 A  is a side view,  FIG.  6 B  is a partial perspective view and  FIG.  6 C  is a schematic illustration of the wafer handling. 
     Wafer handling unit  400  is configured to increase the throughput of PTP system  100  by enabling parallel processing of different wafers  90 . Wafer holders  415 A,  415 B (see  FIG.  6 B , e.g., vacuum chucks) may be configured to move in parallel (e.g., along the horizontal x axis and also along vertical z axis) and apply wafer position corrections during movements along the horizontal y axis and with respect the wafer’s tilting angle (denoted θ). For example, each stage  410 A,  410 B may be configured to adjust both wafer holders  415 A,  415 B (that may adjust the wafer positions) along the x-axis and the z axis. Correspondingly, wafer handling unit  400  may comprise one or more motors  413 , e.g., a linear motor  413 A (illustrated schematically) for moving stage  410 A, along the x axis and a motor  418 A (illustrated schematically) for adjusting the position of wafer stage  410 A (with holders  415 A and  415 B) along the z axis. The positions along the y and θ axes are adjusted by each holder  415  separately. Accordingly, a linear motor  413 B (not shown), which is mounted in parallel to motor  413 A, moves stage  410 B along x axis and motor  418 B adjusts wafer stage  410 B (holders  415 A and  415 B) along the z axis. Both stages  410 A and  410 B are operated in parallel along the x axis by motors  413 A and  413 B and are separated when moving in opposite directions by changing their z-position by motors  418 A and  418 B, accordingly. 
     In various embodiments, wafer handling unit  400  may be configured to have two stages working in parallel which are each movable along x and z directions. Each stage  410  may comprise two holders  415  for holding wafers  90 , with each holder  415  ensuring wafer movement along the y and θ axes (θ denoting tilting of the wafer) thus enabling faster wafer handling and continuous wafers movements during the pattern transfer process. Multiple cameras may be configured to capture images of the incoming wafers to enable more accurate wafer alignment within the printing system thus more accurate alignment of printed conductive lines onto wafer pattern(s). 
     As illustrated schematically in  FIG.  6 C , wafer handling unit  400  may be configured to move wafers  90  from input conveyor  412  through pre-alignment measurement stage (receiving the wafer at position  90 A), pattern transfer printing stage (receiving the wafer at position  90 B) and print quality control stage (receiving the wafer at position  90 C) to output conveyor  419 , while parallelizing wafer treatment so that wafers supported by either stage  410 A,  410 B are processed in parallel. (e.g., during pattern transfer  90 B to Wafer 1 (held by holder  415 A) of Stage 1 ( 410 A), Wafer 2 (held by holder  415 B) of Stage 2 ( 410 B) may already be pre-aligned (wafer  90 A), while Wafers 1 and 2 (held by holders  415 A,  415 B, respectively) of Stage 1 ( 410 A) are pre-aligned ( 90 A) and printed ( 90 B), respectively, Wafer 1 (held by holder  415 A) of Stage 2 ( 410 B) undergoes quality control (wafer  90 C), etc. Wafer handling unit  400  may be mounted on a granite base  405  (see, e.g., in  FIG.  6 B ) to stabilize all the modules and reduce inaccuracies that may result from frequent and fast movements of wafer stages and other moving parts. 
     Wafers  90  may be a silicon wafer, as used, e.g., for manufacturing PV cells of different types as described in detail, e.g., in Luque and Hegedus (eds.) 2011, Handbook of photovoltaic science and engineering, pages 276-277, incorporated herein by reference in its entirety. 
       FIGS.  6 D and  6 E  are high-level schematic illustrations of wafer alignment unit  420 , according to some embodiments of the invention. Each wafer  90 A may be identified by specified features thereof and its placement may be adjusted relative to the paste transfer printing unit according to the exact locations of the specified features. For example, selective emitter (SE) solar cells comprise localized lines (SE lines) of heavy doping in Si substrate onto which the metal contacts are printed by paste transfer. Wafer alignment unit  420  may be configured to measure the locations of the SE lines on wafer  90 A and the position of the wafer may be adjusted so that the paste transfer for each printed finger is done by paste transfer unit  350  with respect to the positions of the SE lines, as determined by wafer alignment unit  420  in order to increase the overall printing accuracy. 
     Wafer alignment unit  420  may comprise camera array(s)  430  with associated illumination, configured to measure the locations of specific features on wafer  90 A, e.g., of the SE lines. For example, wafer alignment unit  420  may comprise multiple imaging cameras configured to capture at least a part of a perimeter of wafer  90 , possibly most or all of the wafer perimeter. The cameras of array(s)  430  may be configured to image the wafer corners (using e.g., four cameras for the areas near the wafer corners) as well as features at a middle of the wafer (using, e.g., two or more cameras to image areas including two opposite ends of the specific features, such as several trenches located in the middle of the wafer). 
     In case of two wafer holders  415 A,  415 B with respective wafer per stage  410 , camera arrays  430  may comprise two respective sub-arrays  430 A,  430 B, each comprising, e.g., two rows of cameras, configured to measure wafer  90 A at the respective position (e.g., as Wafer 1 or Wafer 2, illustrated schematically in  FIG.  6 C ). In non-limiting examples, each camera sub-array  430 A,  430 B may comprise six cameras  435  with respective illumination sources (e.g., four LEDs boards  422  per sub-array configured to provide uniform illumination of the camera’s Field of View (FoV) with high contrast of the SE lines images) - which may be configured to provide accurate x and θ coordinates of the SE lines and the SE pitch even if the SE pitch is not unform in the x direction (CMD). 
     Camera array(s)  430  may be mounted to the system chassis to ensure their stability and accuracy of the measurements. Processors  425  may receive the images from camera arrays  430 , apply high resolution image processing algorithms to yield an accuracy of the SE line measurements of one to few microns, and provide the data to control unit(s)  105  to adjust the wafer position relative to paste transfer unit  350 . Cameras  435  may be, for example, of 5 Mpix CMOS type, with an imaging lens, for example, of 25 mm focal length. It should be noted that number of cameras  435  in each array  430  affects the accuracy of wafer alignment to the transfer sheet pattern thus of the position of printed finger lines  92  onto SE lines on wafer  90 B (see, e.g.,  FIG.  3 E ). As a non-limiting example, the number of cameras may be six thus enabling to determine accurate x, θ - positions of the first and the last SE lines by four corner cameras as well as to estimate the SE pitch along the CMD by help of two intermediate cameras. In a non-limiting example, four LED boards  422  may be used to enable uniform illumination of FOVs of all the cameras by near-normal illumination, which enables high contrast imaging of the SE lines. 
       FIGS.  7 A- 7 C  are high-level schematic illustrations of tape stretching unit  270  and trench alignment monitoring unit  300 , according to some embodiments of the invention.  FIG.  7 A  is a front view (without tape  205 ) with an inset side view and  FIGS.  7 B and  7 C  are perspective views from below (facing the front of tape stretching unit  270 ) and from above (facing the back of tape stretching unit  270  and alignment monitoring unit  300 ). Tape stretching unit  270  and trench alignment monitoring unit  300  may be set with respect to plate  271  and secured by supports  273  to the frame of PTP system  100 . 
     Tape stretching unit  270  may be configured to stretch tape  205  at the pattern transfer stage (e.g., pattern transfer sheet  205 B) to keep pattern transfer sheet (tape segment)  205 B straight and flat, avoiding deformations to the shape of paste-filled trenches thereupon and to prevent direct contact between pattern transfer sheet  205 B and wafer  90 B onto which the paste is transferred (e.g., keeping a gap of, e.g., in the range of 100 µm to 500 µm between pattern transfer sheet  205 B and wafer  90 B). Moreover, tape stretching unit  270  may be configured to avoid interference to wafer movements by wafer handling unit  400 . For example,  FIGS.  7 A and  7 B  illustrate schematically the use of vacuum bars  280  configured to affix and flatten tape  205  using vacuum application and a stretching mechanism  275  and may comprise recesses  272  for applying vacuum to the transfer sheet by keeping its planarity. 
     Trench alignment monitoring unit  300  may be configured to monitor the trenches&#39; x, θ - positions and distortions, e.g., using multiple imaging cameras configured and/or located to capture ends of the trenches and to capture at least middle-sections of the trenches. For example, trench alignment monitoring unit  300  may be configured to measure the ends of the trenches using cameras  285  (e.g., four pairs of alignment cameras, one camera of each pair at each end of the trenches) as well as tilted cameras  290  (see, e.g., one of tilted cameras  290  illustrated  FIG.  7 C  and in a larger scale on the left part side view in  FIG.  7 A ) - set to measure trench distortions at central portions of the trenches. Tilted imaging cameras  290  may be tilted with respect to the vertical z direction so as to capture the middle-sections of the trenches without obstructing the illumination of the trenches (e.g., not positioned above the pattern transfer sheet). 
     Corresponding image processing algorithms may be applied to the images of cameras  285 ,  290  in one or more processor(s)  310  associated with control unit(s)  105  - to measure trench positions (e.g., x and θ-position(s)) and distortions, and optimize the positioning accuracy of laser beam  80  with respect to some or every paste-filled trench of pattern transfer sheet  205 B during scanning. The measurements may be used to increase accuracy and/or to reduce the required beam width (wider beams  80  were previously used to compensate for inaccuracies). Trench alignment monitoring unit  300  may further comprise illumination unit(s), e.g., LED boards  287  (see, e.g.,  FIGS.  7 B and  7 C ) located below the camera assemblies, configured to provide the required illumination of the trenches for the fields of view of respective cameras  285 ,  290 . In certain embodiments, imaging cameras  285  and  290  may be assembled of the same CMOS camera and imaging lens as are used in the wafer alignment module. 
     Paste transfer unit  350  may comprise a high-power laser and optical head  355 , which forms laser beam  80  that releases the paste from the trenches in pattern transfer sheet  205 B onto wafer  90 B. 
     Optical head  355  may be movable and configured to be moved along CMD (x axis), e.g., with a velocity of about 0.5 m/s or more, with optical head  355  configured to focus laser beam  80  to specified spot shape(s) (that are effective in releasing the paste from the trenches of pattern transfer sheet  205 B) and to move this spot in along MD (y axis) with very high velocity, e.g. 500 m/s. Optical head  355  may be movable along CMD (x axis), e.g., by a precise linear motor to adjust the exact location of laser beam  80  with respect to the actual locations of the trenches on the pattern transfer sheet  205 B. Optical head  355  may be controllably tiltable (e.g., by the same or by an additional motor) to adjust for tilts of pattern transfer sheet  205 B that may remain with disclosed tape stretching, and as measured by trench alignment monitoring unit  300 . The laser used in pattern transfer unit  350  may be any of one of the following groups: a) CW, QCW, pulse; b) IR, NIR, Visible; c) solid state, fiber, gaseous, laser diode. The scanner for MD axis may be any commercially available linear scanner enabling the scanning velocity of several hundreds of m/sec. The motor assembly for CMD axis movement of optical head  355  may be based on a linear motor or a ball screw motor. 
     Paste transfer unit  350  may be controllable by control unit(s)  105  with respect to the illumination and the various movement parameters, possibly adjusted and monitored by associated processor(s). 
       FIG.  8    is a high-level schematic illustration of print quality control (QC) unit  450 , according to some embodiments of the invention. Print quality control unit  450  may be configured to detect defects in the pattern transfer using one or more cameras  455  configured to capture high resolution images (e.g., having 20 megapixels or more) of printed wafers  90 C, with corresponding illumination  457  (e.g., four dark field LEDs boards along the x direction, CMD, configured to provide uniform illumination of whole wafer  90 C). Print quality control unit  450  may be configured to provide high contrast and avoid or reduce optical noise. 
     Print QC unit  450  may comprise two respective cameras  455 A,  455 B (the latter indicated schematically by an arrow, cameras  455 B are opposite to camera  455 A but are not visible on  FIG.  8   , except for the schematic illustration of its FoV). Cameras  455  are configured to measure wafer  90 C at the respective position (e.g., as Wafer 1 or Wafer 2, illustrated schematically in  FIG.  6 C ). Processors  452  may receive the images from cameras  455 , apply high resolution image processing algorithms to detect tiny printing defects like small cuts or local misprints, and provide the data to control unit(s)  105  to correct parameters of the process. Cameras  455 A and  455 B may be, in a non-limiting example, of 20 Mpix CMOS type equipped with an imaging lens enabling FoV of about 230 mm by 230 mm. The LED boards may be installed on two sides and on two height levels in order to ensure uniform illumination of the whole wafer, as is shown schematically in  FIG.  8   . 
     Elements from  FIGS.  1 - 8    may be combined in any operable combination, and the illustration of certain elements in certain figures and not in others merely serves an explanatory purpose and is non-limiting. 
       FIG.  9 A  is a high-level flowchart illustrating a pattern transfer printing (PTP) method  500 , according to some embodiments of the invention.  FIG.  9 B  is a high-level flowchart illustrating in general the parallel processes in pattern transfer printing (PTP) method  500 , according to some embodiments of the invention. The method stages may be carried out with respect to PTP system  100  described above, which may optionally be configured to implement method  500 . Method  500  may be at least partially implemented by at least one computer processor or by at least one control unit  105  (e.g., one or more personal computers, PCs and/or one or more programmable logic controllers, PLCs) or by their combinations). Certain embodiments comprise computer program products comprising a computer readable storage medium having computer readable program embodied therewith and configured to carry out the relevant stages of PTP method  500 . PTP method  500  may comprise the following stages, irrespective of their order. 
     As illustrated in  FIG.  9 A , PTP method  500  may comprise handling a tape through a PTP system, the tape comprising, as sections thereof, a plurality of pattern transfer sheets having respective patterns of trenches - to controllably deliver the pattern transfer sheets for paste filling (stage  510 ), filling the trenches on the delivered pattern transfer sheets with conductive printing paste (stage  520 ), controllably delivering a plurality of wafers (one-by-one) for the pattern transfer (stage  530 ), and transferring the conductive printing paste from a plurality of filled trenches on the pattern transfer sheets onto a respective one of the delivered wafers, by releasing the printing paste from the trenches upon illumination by a laser beam, e.g., by help of two dimensional x, y-scanning (stage  540 ). 
     PTP method  500  may further comprise cleaning the pattern transfer sheets after the pattern transfer to provide reusable pattern transfer sheets, and optionally re-using the cleaned pattern transfer sheets (stage  560 ). 
     PTP method  500  may further comprise supporting a back side of the pattern transfer sheet by a countering moveable roll during the paste filling (stage  522 ). PTP method  500  may further comprise carrying out the trench filling at a nearly vertical position (stage  524 ), e.g., at a nearly vertical angle (in the range of 0-30° from the vertical x-z plane). For example, the paste filling unit and the pattern transfer sheet plane may be set at an angle deviating 0-30° from the vertical x-z plane. 
     PTP method  500  may further comprise delivering the wafers using two alternating stages, with each stage supporting two wafers (stage  532 ), controllably delivering the wafers for the pattern transfer at a close proximity (e.g., in a range of between 0.1 mm and 0.5 mm) to the pattern transfer sheet (stage  534 ) and carrying out the wafer measurement (before printing), pattern transfer onto a wafer (printing) and a print QC inspection (after printing) simultaneously for at least three of the wafers (stage  536 ), wherein at least two of the wafers are supported by the same stage. The wafers are then advanced by consecutively moving the stage along CMD. The stages may be alternated following the printed wafers release to the output conveyor, parallelizing the printing process to increase throughput. In certain embodiments, only two wafers (e.g., positioned on the same stage) are processed simultaneously, alternating between (i) wafer alignment (before printing) and pattern transfer onto a wafer (printing), and (ii) pattern transfer onto a wafer (printing) and the print QC inspection (after printing) for the respective pair of wafers. 
     PTP method  500  may further comprise affixing and flattening the respective pattern transfer sheet during the pattern transfer (stage  542 ) and/or monitoring x,θ-positions and/or distortions of the trenches prior to the pattern transfer (stage  544 ). 
     PTP method  500  may further comprise detecting and measuring features on the wafer and adjusting the pattern transfer accordingly (stage  546 ). 
     PTP method  500  may further comprise inspecting the printing quality of the transferred paste pattern (stage  550 ), e.g., by measuring an accuracy of the pattern transfer and/or detecting defects in the transferred pattern on the wafer 
     As illustrated in  FIG.  9 B , PTP method  500  may comprise method stages implemented by tape handling unit  200 , pattern transfer unit  350  and wafer handling unit  400 , as illustrated schematically and described in detail herein. 
     PTP method  500  may comprise moving pattern transfer sheets from the unwinder roll towards the paste filling unit (stage  510 A), the filling of the trenches with paste (stage  520 ), moving the filled sheets to the pattern transfer unit (stage  510 B), e.g., stretching and affixing the filled sheets in the transfer unit (stage  542 A), as disclsoed herein. Following paste removal from the sheets, PTP method  500  may comprise moving the used sheets towards the re-use unit (stage  514 ), cleaning and drying the sheets (stage  560 A) and moving the cleaned sheets toward the re-winding roll, possibly for future use (stage  560 B). 
     PTP method  500  may further comprise measuring the positions of the trenches by the trench alignment unit (stage  544 A), laser-scanning the trenches to transfer the paste to respective wafers  90  (stages  540 A,  540 B,  540 C,  540 D), as provided by the wafer handling unit, until the trenches from the same sheet are laser-scanned onto the last provided wafer (stage  544 B). 
     PTP method  500  may further comprise handling the wafers using two stages  410 A,  410 B, which carry out the following stages, respectively: putting wafers on the wafer holders from the input conveyor (stages  530 A,  530 B), moving the wafers to the wafer alignment units (stages  532 A,  532 B), determining the wafer positions of the wafer holders (stages  534 A,  534 B), moving the wafer sequentially to the transfer printing unit, optionally during the printing of previous wafers (stages  535 A,  535 B and stages  535 C,  535 D, respectively for the two stages), and then moving the wafers to the print quality units (stages  550 A,  550 B), followed by releasing the wafers to the output conveyor (stages  552 A,  552 B) and returning the stages to their initial positions (stages  553 A,  553 B) to repeat stages  530 - 553 . 
     Advantageously, disclosed PTP systems may be optimized to increase accuracy, efficiency and throughput by providing continuous handling of wafers during pattern transfer and using dual-chuck wafer stages, alignment of wafers by multiple cameras, more accurate alignment of transfer sheet by multiple cameras at the print position and locating the paste filling module at near vertical position thus reducing time between paste filling and pattern transfer. 
       FIG.  10    is a high-level block diagram of exemplary computing device  170 , which may be used with embodiments of the present invention. Computing device  170  may include a controller or processor  173  that may be or include, for example, one or more central processing unit processor(s) (CPU), one or more Graphics Processing Unit(s) (GPU or general-purpose GPU - GPGPU), a chip or any suitable computing or computational device, an operating system  171 , a memory  172 , a storage system  175 , input devices  176  and output devices  177 . PTP system  100 , control unit(s)  105 , any of processors  310 ,  425 ,  452  and/or parts thereof may be or include a computer system as shown for example in  FIG.  10   . The processors may comprise multiple cores configured to enable parallel processing of different tasks, for example processing of images of all the cameras of the wafer alignment unit or/and of the trench monitoring units or/and of the print quality units. 
     Operating system  171  may be or may include any code segment designed and/or configured to perform tasks involving coordination, scheduling, arbitration, supervising, controlling, or otherwise managing operation of computing device  170 , for example, scheduling execution of programs. Memory  172  may be or may include, for example, a Random-Access Memory (RAM), a read only memory (ROM), a Dynamic RAM (DRAM), a Synchronous DRAM (SD-RAM), a double data rate (DDR) memory chip, a Flash memory, a volatile memory, a non-volatile memory, a cache memory, a buffer, a short-term memory unit, a long-term memory unit, or other suitable memory units or storage units. Memory  172  may be or may include a plurality of possibly different memory units. Memory  172  may store for example, instructions to carry out a method (e.g., code  174 ), and/or data such as user responses, interruptions, etc. 
     Executable code  174  may be any executable code, e.g., an application, a program, a process, task or script. Executable code  174  may be executed by processor  173  possibly under control of operating system  171 . For example, executable code  174  may when executed cause the production or compilation of computer code, or application execution such as VR execution or inference, according to embodiments of the present invention. Executable code  174  may be code produced by methods described herein. For the various modules and functions described herein, one or more computing devices  170  or components of computing device  170  may be used. Devices that include components similar or different to those included in computing device  170  may be used and may be connected to a network and used as a system. One or more processor(s)  173  may be configured to carry out embodiments of the present invention by for example executing software or code. 
     Storage system  175  may be or may include, for example, a hard disk drive, a floppy disk drive, a Compact Disk (CD) drive, a CD-Recordable (CD-R) drive, a universal serial bus (USB) device or other suitable removable and/or fixed storage unit. Data such as instructions, code, VR model data, parameters, etc. may be stored in a storage system  175  and may be loaded from storage system  175  into a memory  172  where it may be processed by processor  173 . In some embodiments, some of the components shown in  FIG.  10    may be omitted. 
     Input devices  176  may be or may include for example a mouse, a keyboard, a touch screen or pad or any suitable input device. It will be recognized that any suitable number of input devices may be operatively connected to computing device  170  as shown by block  176 . Output devices  177  may include one or more displays, speakers and/or any other suitable output devices. It will be recognized that any suitable number of output devices may be operatively connected to computing device  170  as shown by block  177 . Any applicable input/output (I/O) devices may be connected to computing device  170 , for example, a wired or wireless network interface card (NIC), a modem, printer or facsimile machine, a universal serial bus (USB) device or external hard drive may be included in input devices  176  and/or output devices  177 . 
     Embodiments of the invention may include one or more article(s) (e.g., memory  172  or storage system  175 ) such as a computer or processor non-transitory readable medium, or a computer or processor non-transitory storage medium, such as for example a memory, a disk drive, or a USB flash memory, encoding, including or storing instructions, e.g., computer-executable instructions, which, when executed by a processor or controller, carry out methods disclosed herein. 
     Aspects of the present invention are described above with reference to flowchart illustrations and/or portion diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each portion of the flowchart illustrations and/or portion diagrams, and combinations of portions in the flowchart illustrations and/or portion diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general-purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or portion diagram or portions thereof. 
     These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or portion diagram or portions thereof. 
     The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or portion diagram or portions thereof. 
     The aforementioned flowchart and diagrams illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present invention. In this regard, each portion in the flowchart or portion diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the portion may occur out of the order noted in the figures. For example, two portions shown in succession may, in fact, be executed substantially concurrently, or the portions may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each portion of the portion diagrams and/or flowchart illustration, and combinations of portions in the portion diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
     In the above description, an embodiment is an example or implementation of the invention. The various appearances of “one embodiment”, “an embodiment”, “certain embodiments” or “some embodiments” do not necessarily all refer to the same embodiments. Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment. Certain embodiments of the invention may include features from different embodiments disclosed above, and certain embodiments may incorporate elements from other embodiments disclosed above. The disclosure of elements of the invention in the context of a specific embodiment is not to be taken as limiting their use in the specific embodiment alone. Furthermore, it is to be understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in certain embodiments other than the ones outlined in the description above. 
     The invention is not limited to those diagrams or to the corresponding descriptions. For example, flow need not move through each illustrated box or state, or in exactly the same order as illustrated and described. Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belongs, unless otherwise defined. While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents.