# Patent

## Title
Methods for surface attachment of flipped active components

## Abstract

A method for selectively transferring active components from a source substrate to a destination substrate includes providing a source substrate having a process side including active components and a back side opposite the process side, the active components having respective primary surfaces including electrical connections thereon adjacent the process side and respective secondary surfaces opposite the primary surfaces and facing the back side; pressing a first stamp having first pillars protruding therefrom against the active components on the process side of the source substrate to adhere the respective primary surfaces of the active components including the electrical connections thereon to respective transfer surfaces of the first pillars; pressing a second stamp having second pillars protruding therefrom against the active components on the first pillars of the first stamp to adhere the respective secondary surfaces of the active components to respective transfer surfaces of the second pillars, wherein the respective transfer surfaces of the second pillars have greater adhesive strength than those of the first pillars; and pressing the second stamp including the active components on the second pillars thereof against a destination substrate to adhere the respective primary surfaces of the active components including the electrical connections thereon to a receiving surface of the destination substrate.

## Background
CLAIM OF PRIORITY
The present application is a continuation of U.S. patent application Ser. No. 13/491,196 filed Jun. 7, 2012, which claims priority under 35 USC §119 to U.S. Provisional Patent Application Ser. No. 61/494,514 entitled “Methods for Surface Attachment of Flipped Active Components,” filed on Jun. 8, 2011, the disclosure of which is incorporated by reference herein in its entirety.
CROSS REFERENCE TO RELATED APPLICATIONS
The present application is related to U.S. Provisional Patent Application Ser. No. 61/494,507 entitled “Substrates with Transferable Chiplets,” filed on Jun. 8, 2011, the disclosure of which is incorporated by reference herein in its entirety.


FIELD
The present invention is directed to methods for providing substrates having separate electrically active components distributed thereon and related devices.
BACKGROUND
Substrates with electronically active components distributed over the extent of the substrate may be used in a variety of electronic systems, for example, flat-panel imaging devices such as flat-panel liquid crystal or organic light emitting diode (OLED) display devices. Substrates with electrically active components are also found in flat-panel solar cells. A variety of methods may be used to distribute electronically active circuits over substrates, including forming the electronically active circuits on a substrate and forming the components on separate substrates and placing them on a substrate. In the latter case, a variety of assembly technologies for device packaging may be used.
The electronically active components are typically formed by sputtering a layer of inorganic semiconductor material or by spin-coating organic material over the entire substrate. Inorganic semiconductor materials can be processed to improve their electronic characteristics, for example amorphous silicon can be treated to form low-temperature or high-temperature poly-crystalline silicon. In other process methods, microcrystalline semiconductor layers can be formed by using an underlying seeding layer. These methods typically improve the electron mobility of the semiconductor layer. The substrate and layer of semiconductor material can be photo-lithographically processed to define electronically active components, such as transistors. Such transistors are known as thin-film transistors (TFTs) since they are formed in a thin layer of semiconductor material, typically silicon. Transistors may also be formed in thin layers of organic materials. In these devices, the substrate is often made of glass, for example Corning Eagle® or Jade® glass designed for display applications.
The above techniques may have some limitations. Despite processing methods used to improve the performance of thin-film transistors, such transistors may provide performance that is lower than the performance of other integrated circuits formed in mono-crystalline semiconductor material. Semiconductor material and active components can be provided only on portions of the substrate, leading to wasted material and increased material and processing costs. The choice of substrate materials may also be limited by the processing steps necessary to process the semiconductor material and the photo-lithographic steps used to pattern the active components. For example, plastic substrates have a limited chemical and heat tolerance and do not readily survive photo-lithographic processing. Furthermore, the manufacturing equipment used to process large substrates with thin-film circuitry is relatively expensive. Other substrate materials that may be used include quartz, for example, for integrated circuits using silicon-on-insulator structures as described in U.S. Patent Application 2010/0289115 and U.S. Patent Application 2010/0123134. However, such substrate materials can be more expensive and/or difficult to process.
Other methods used for distributing electronically functional components over a substrate in the circuit board assembly industry include, for example, pick-and-place technologies for integrated circuits provided in a variety of packages, for example, pin-grid arrays, ball-grid arrays, and flip-chips. However, these techniques may be limited in the size of the integrated circuits that can be placed.
In further manufacturing techniques, a mono-crystalline semiconductor wafer is employed as the substrate. While this approach can provide substrates with the same performance as integrated circuits, the size of such substrates may be limited, for example, to a 12-inch diameter circle, and the wafers are relatively expensive compared to other substrate materials such as glass, polymer, or quartz.
In yet another approach, thin layers of semiconductor are bonded to a substrate and then processed. Such a method is known as semiconductor-on-glass or silicon-on-glass (SOG) and is described, for example, in U.S. Pat. No. 7,605,053, issued Oct. 20, 2009. If the semiconductor material is crystalline, high-performance thin-film circuits can be obtained. However, the bonding technique and the processing equipment for the substrates to form the thin-film active components on large substrates can be relatively expensive.
Publication No. 11-142878 of the Patent Abstracts of Japan entitled “Formation of Display Transistor Array Panel” describes etching a substrate to remove it from a thin-film transistor array on which the TFT array was formed. TFT circuits formed on a first substrate can be transferred to a second substrate by adhering the first substrate and the TFTs to the surface of the second substrate and then etching away the first substrate, leaving the TFTs bonded to the second substrate. This method may require etching a significant quantity of material, and may risk damaging the exposed TFT array.
Other methods of locating material on a substrate are described in U.S. Pat. No. 7,127,810. In this approach, a first substrate carries a thin-film object to be transferred to a second substrate. An adhesive is applied to the object to be transferred or to the second substrate in the desired location of the object. The substrates are aligned and brought into contact. A laser beam irradiates the object to abrade the transferring thin film so that the transferring thin film adheres to the second substrate. The first and second substrates are separated, peeling the film in the abraded areas from the first substrate and transferring it to the second substrate. In one embodiment, a plurality of objects is selectively transferred by employing a plurality of laser beams to abrade selected area. Objects to be transferred can include thin-film circuits.
U.S. Pat. No. 6,969,624 describes a method of transferring a device from a first substrate onto a holding substrate by selectively irradiating an interface with an energy beam. The interface is located between a device for transfer and the first substrate and includes a material that generates ablation upon irradiation, thereby releasing the device from the substrate. For example, a light-emitting device (LED) is made of a nitride semiconductor on a sapphire substrate. The energy beam is directed to the interface between the sapphire substrate and the nitride semiconductor releasing the LED and allowing the LED to adhere to a holding substrate coated with an adhesive. The adhesive is then cured. These methods, however, may require the patterned deposition of adhesive on the object(s) or on the second substrate. Moreover, the laser beam that irradiates the object may need to be shaped to match the shape of the object, and the laser abrasion can damage the object to be transferred. Furthermore, the adhesive cure takes time, which may reduce the throughput of the manufacturing system.
Another method for transferring active components from one substrate to another is described in “AMOLED Displays using Transfer-Printed Integrated Circuits” published in the Proceedings of the 2009 Society for Information Display International Symposium Jun. 2-5, 2009, in San Antonio Tex., US, vol. 40, Book 2, ISSN 0009-0966X, paper 63.2 p. 947. In this approach, small integrated circuits are formed over a buried oxide layer on the process side of a crystalline wafer. The small integrated circuits, or chiplets, are released from the wafer by etching the buried oxide layer formed beneath the circuits. A PDMS stamp is pressed against the wafer and the process side of the chiplets is adhered to the stamp. The chiplets are pressed against a destination substrate coated with an adhesive and thereby adhered to the destination substrate. The adhesive is subsequently cured. This method, however, may not locate the process side of the chiplets against the destination substrate. Thus, additional process steps may be needed to form patterned conductors that electrically connect the chiplets to destination-substrate circuitry. Moreover, chiplets with, a variable topography may not be readily transferred.
SUMMARY
It should be appreciated that this Summary is provided to introduce a selection of concepts in a simplified form, the concepts being further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of this disclosure, nor is it intended to limit the scope of the disclosure.
According to some aspects of the present invention, in a method for transferring active components from a source substrate to a destination substrate, a source substrate having a process side including active components therein or thereon is provided. The active components have respective primary surfaces including electrical connections thereon adjacent the process side and respective secondary surfaces opposite the primary surfaces. A first stamp having first pillars protruding therefrom is pressed against the active components on the process side of the source substrate to adhere the respective primary surfaces of the active components including the electrical connections thereon to respective transfer surfaces of the first pillars. A second stamp having second pillars protruding therefrom is pressed against the active components on the first pillars of the first stamp to adhere the respective secondary surfaces of the active components to respective transfer surfaces of the second pillars. The respective transfer surfaces of the second pillars have greater adhesive strength than those of the first pillars. The second stamp including the active components on the second pillars thereof is pressed against a destination substrate to adhere the respective primary surfaces of the active components including the electrical connections thereon to a receiving surface of the destination substrate.
In some embodiments, the first pillars may be dimensioned such that the first pillars contact the active components between the electrical connections on the respective primary surfaces thereof.
In some embodiments, the respective surfaces of the second pillars may have a greater surface area than the respective surfaces of the first pillars.
In some embodiments, the second pillars may include a material having a greater adhesive strength per unit area than that of the first pillars.
In some embodiments, the second pillars may include vacuum collets configured to provide a vacuum strength greater than the adhesive strength of the first pillars to adhere the respective secondary surfaces of the active components thereto.
In some embodiments, the electrical connections may include conductive pads on the respective surfaces of the active components.
In some embodiments, the electrical connections may include conductive posts protruding from the respective surfaces of the active components.
In some embodiments, the electrical connections on the respective primary surfaces of the active components may be adhered to respective electrical contacts on the receiving surface of the destination substrate.
In some embodiments, the electrical connections on the respective primary surfaces of the active components may be adhered to the respective electrical contacts on the receiving surface of the destination substrate by an adhesive layer thereon.
In some embodiments, the electrical contacts on the destination substrate may include a conductive material softer than that of the electrical connections of the active components, and pressing the second stamp against the destination substrate may drive the electrical connections of the active components through the electrical contacts on the destination substrate.
According to one aspect of the present invention, a method for selectively transferring active components from a source substrate to a destination substrate comprises: providing a source substrate having a process side and a plurality of active components formed on or in the process side of the source substrate; providing a first stamp having a plurality of first pillars formed thereon and spatially aligned to the active components, each first pillar having a first area; pressing the first pillars against corresponding active components to adhere the active components to the first pillars; providing a second stamp having a plurality of second pillars spatially aligned to the first pillars, each second pillar having a second area larger than the first area; pressing the second pillars against corresponding active components to adhere the active components to the second pillars with van der Waal's forces; and pressing the active components against the destination substrate to adhere the active components to the destination substrate.
According to another aspect of the present invention, an active component comprises: a substrate having a process side; a circuit formed on the process side; connection posts formed on the process side, the connection posts electrically connected to the circuit; and wherein the connection posts have a base width, a height, a base area, and a peak area, and wherein the height is greater than the base width and the peak area is less than the base area.
According to yet another aspect of the present invention, an active substrate comprises: a destination substrate having a plurality of destination substrate contacts, the destination substrate contacts having a surface; a plurality of active components distributed over the destination substrate, each active component including a component substrate different from the destination substrate, each active component having a circuit and connection posts formed on a process side of the component substrate, wherein the connection posts have a base width and a height that is greater than the base width, wherein the connection posts are in electrical contact with the circuit and the destination substrate contacts; and wherein the connection posts are driven through the surface of the destination substrate contacts into the destination substrate connection pads to electrically connect the connection posts to the destination substrate contacts.
Embodiments of the present invention provide high-performance active components having variable topography over substrates with the process side of the chiplets in contact with a destination substrate.
Other methods and/or devices according to some embodiments will become apparent to one with skill in the art upon review of the following drawings and detailed description. It is intended that all such additional embodiments, in addition to any and all combinations of the above embodiments, be included within this description, be within the scope of the invention, and be protected by the accompanying claims.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a schematic cross-section of a source substrate having a plurality of active components having post connections formed on a process side of the source substrate in accordance with embodiments of the present invention;
FIG. 1B is a schematic top-view of a source substrate having a plurality of active components having post connections formed on a process side of the source substrate in accordance with embodiments of the present invention;
FIG. 1C is a schematic cross-section of an active component having post connections in accordance with embodiments of the present invention;
FIG. 1D is a schematic cross-section of an active component having pad connections in accordance with embodiments of the present invention;
FIG. 1E is a schematic top-view of an example connection post in accordance with embodiments of the present invention;
FIG. 2A is a schematic of a stamp and source substrate in accordance with embodiments of the present invention;
FIG. 2B is a schematic of a stamp in alignment with a source substrate in accordance with embodiments of the present invention;
FIG. 2C is a schematic of a stamp and source substrate in accordance with embodiments of the present invention;
FIG. 2D is a schematic of a stamp with adhered active components in accordance with embodiments of the present invention;
FIG. 3A is a schematic of a first stamp with adhered active components and a second stamp in accordance with embodiments of the present invention;
FIG. 3B is a schematic of a first stamp with adhered active components in alignment with a second stamp in accordance with embodiments of the present invention;
FIG. 3C is a schematic of a first stamp with adhered active components and a second stamp in accordance with embodiments of the present invention;
FIG. 3D is a schematic of a second stamp with adhered active components in accordance with embodiments of the present invention;
FIG. 3E is a schematic of a second stamp employing a vacuum collet to adhere an active component in accordance with embodiments of the present invention;
FIG. 3F is a schematic of a second stamp with a vacuum-adhered active component in accordance with embodiments of the present invention;
FIG. 3G is a schematic of a second stamp employing a vacuum collet to adhere active components in accordance with embodiments of the present invention;
FIG. 3H is a schematic of a second stamp with vacuum-adhered active components in accordance with embodiments of the present invention;
FIG. 4A is a schematic of a second stamp with adhered active components and a destination substrate in accordance with embodiments of the present invention;
FIG. 4B is a schematic of a second stamp with adhered active components in alignment with a destination substrate in accordance with embodiments of the present invention;
FIG. 4C is a schematic of a second stamp with adhered active components and a destination substrate in accordance with embodiments of the present invention;
FIG. 4D is a schematic of a destination substrate with adhered active components in accordance with embodiments of the present invention;
FIG. 5 is a schematic cross-section of an active component and electrical contacts on a destination substrate in accordance with embodiments of the present invention;
FIG. 6 is a schematic cross-section of an active component and electrical contacts with an adhesive layer on a destination substrate in accordance with embodiments of the present invention;
FIG. 7 is a schematic top-view of an active component and electrical contacts on a destination substrate in accordance with embodiments of the present invention; and
FIG. 8 is a flow diagram illustrating methods in accordance with embodiments of the present invention.



The figures are not drawn to scale since the individual elements of the drawings have too great a size variation to permit depiction to scale.
DETAILED DESCRIPTION OF EMBODIMENTS
The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. However, this invention should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout.
It will be understood that when an element such as a layer, region or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “in contact with” or “connected to” or “coupled to” another element, it can be directly contacting or connected to or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “in direct contact with” or “directly connected to” or “directly coupled to” another element, there are no intervening elements present.
It will also be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present invention.
Furthermore, relative terms, such as “under” or “lower” or “bottom,” and “over” or “upper” or “top,” may be used herein to describe one element's relationship to another element as illustrated in the Figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in one of the figures is turned over, elements described as being on the “lower” side of other elements would then be oriented on “upper” sides of the other elements. The exemplary term “lower”, can therefore, encompasses both an orientation of “lower” and “upper,” depending of the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as “below” or “beneath” other elements would then be oriented “above” the other elements. The exemplary terms “below” or “beneath” can, therefore, encompass both an orientation of above and below.
The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used in the description of the invention and the appended claims, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
Embodiments of the invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the invention. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. In other words, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of the invention.
Unless otherwise defined, all terms used in disclosing embodiments of the invention, including technical and scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs, and are not necessarily limited to the specific definitions known at the time of the present invention being described. Accordingly, these terms can include equivalent terms that are created after such time. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the present specification and in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entireties.
Embodiments of the present invention provide methods and devices for the transfer of active components, also referred to herein as ‘chiplets,’ from a source substrate to a destination substrate. Chiplets are small integrated circuits, each integrated circuit having a separate and distinct substrate so as to define a separate transferable or printable electronic component. FIGS. 1-7 are schematic diagrams illustrating various fabrication operations according to embodiments the present invention, while FIG. 8 is a flow diagram describing fabrication operations in various methods according to embodiments of the present invention. Transferring integrated circuits from the source substrate to the destination substrate is also referred to herein as ‘printing’ the integrated circuits onto the destination substrate.
Referring to the flow diagram of FIG. 8 and to the illustration of FIG. 1A, some embodiments of the present invention provide a source substrate 20 having a process side 24 of the source substrate 20 in step 100. The source substrate 20 can be a wafer having a process side 24 opposite the back side used to handle and transport the wafer. Active components 22 are formed using lithographic processes in an active layer 25 on or in the process side 24 of the source substrate 20. An empty release layer 50 space is formed beneath the active components 22 with tethers 60 connecting the active components 22 to the source substrate 20 in such a way that pressure applied against the active components 22 breaks the tethers 60 to release the active components 22 from the source substrate 20. FIG. 1B is a schematic top-view of the source substrate 20 including the release layer 50, tethers 60, connection posts 26 and active components 22 arranged in a regular, rectangular array. Methods of forming such structures are described, for example, in the paper “AMOLED Displays using Transfer-Printed Integrated Circuits” referenced above. Lithographic processes for forming active components 22 in a source substrate 20, for example transistors, wires, and capacitors, are used in the integrated circuit art. Active components 22 are small integrated circuits, or chiplets, as described further below. The active components 22 can have a thin substrate, for example less than or equal to 25 microns, less than or equal to 15 microns, or less than or equal to 10 microns.
According to various embodiments of the present invention, the source substrate 20 can be provided with the active components 22, release layer 50, tethers 60, and connection posts 26 already formed, or they can be constructed as part of the process of the present invention in step 100.
Referring to FIGS. 1C and 1D, the active components 22 include electronic circuits 30 formed using lithographic processes and can include electrical connections such as wires 32 to the connection posts 26. Connection posts 26 are illustrated in FIG. 1C and connection pads 28 in FIG. 1D. Connection pads 28 are planar electrical connections formed on the surface of the active component 22 and process side 24 of the source substrate 20. Such connection pads 28 are typically formed from metals such as aluminum or polysilicon using masking and deposition processes used in the art. The connection pads 28 are electrically connected to the circuit 30 with wires 32. In some embodiments, connection pads 28 and the circuit 30, together with other functional structures formed in the active layer 25 on the source substrate 20 make up the active component 22, or chiplet.
Connection posts 26 are electrical connections formed on the surface of the active component 22 and process side 24 of the source substrate 20 that extend perpendicularly to the process side 24 surface. Such connection posts 26 are also formed from metals such as aluminum, titanium, tungsten, copper, silver, gold, or other conductive metals and can be formed by repeated masking and deposition processes that build up three-dimensional structures. In some embodiments, the connection posts 26 are made of one or more high elastic modulus metals, for example tungsten. As used herein, a high elastic modulus is an elastic modulus sufficient to maintain the function and structure of the connection post 26 when pressed into a destination substrate contact 16, as described further below. Such structures can also be made by forming a layer above the active component 22 surface, etching a well into the surface, filling it with a conductive material such as metal, and then removing the layer. Processes for making three-dimensional structures are used in the art. In some embodiments, the post connections 26 are electrically connected to the circuit 30; post connections 26 and the circuit 30, together with other functional structures formed in the active layer 25 on the source substrate 20 make up the active component 22.
Referring to FIGS. 1C and 1E, the connection posts 26 can have a base width W representing a planar dimension of the connection post 26 on the process side 24 and a height H representing the extent of the connection post 26 from the process side 24 to the peak of the connection post 26. The base of the connection post 26 can have a base area, for example W2 for a square base, of the area of the connection post 26 in contact with the process side 24 and a peak area, for example a peak area equal to T2 for a square peak, of the area of the connection post 26 farthest from the process side 24. According to some embodiments of the present invention, the connection post 26 has a base area greater than the peak area. The connection post 26 can also have a height greater than a base dimension. In some embodiments of the present invention, the connection posts have a sharp point.
In step 105 of FIG. 8, a first stamp 90 is provided having a plurality of first pillars 92 formed thereon and spatially aligned to the active components 22, each first pillar 92 having a first area. The first stamp 90 can be made of an elastomeric material, such as PDMS. The first pillars 92 protrude from the first stamp 90 surface. The first pillars 92 are spatially arranged on the surface of the first stamp 90 so that each first pillar 92 can be aligned with an active component 22 on the source wafer 20. In some embodiments of the present invention, each active component 22 in an array of active components 22 spatially corresponds to a first pillar 92. In another embodiment, a subset of the active components 22 spatially corresponds to a first pillar 92.
In step 110 of FIG. 8 and as shown in FIG. 2A, the first pillars 92 are pressed against corresponding active components 22 into the release layer 50 to adhere the active components 22 to the first pillars 92 to transfer the pressed active components 22 from the source substrate 20 to first stamp pillars 92 (FIG. 2B). By pressing the first stamp 90 against the active components 22, the tethers 60 are broken and the active components 22 are adhered to the first pillars 92, for example by van der Waal's forces. The first stamp 90 is removed from the source substrate 20 (FIG. 2C), leaving the active components 22 adhered to the first pillars 92 (FIG. 2D).
In some embodiments of the present invention, the first pillars 92 have a planar dimension L1, for example a width, smaller than the distance P between the connection posts 26 on the active components 22 (FIG. 2A). Thus, the first pillars 92 fit between the connection posts 26 to make intimate contact with the surface of the active components 22 to enhance the adhesive effect of the van der Waal's forces and improve adhesion between the active components 22 and the first pillars 92. If the first pillars 92 were located over the connection posts 26, the connection posts 26 would form a standoff between the active components 22 and the first pillars 92, greatly decreasing the attractive force of the van der Waal's force between the active components 22 and the first pillars 92.
Referring to step 115 of FIG. 8 and as shown in FIG. 3A, a second or intermediate stamp 94 having second pillars 96 is provided. In some embodiments of the present invention, the second pillars 96 are made of the same material as the first pillars 92. In other embodiments of the present invention, the second pillars 96 are made of a different material than the first pillars 92. In some embodiments of the present invention, the second pillars 96 form vacuum collets 98.
If the first and second pillars 92, 96 are made of the same material, the second pillars 96 can have a larger surface area than the first pillars 92. As shown in FIG. 3A, the second pillars 96 have at least one planar dimension width L2 that is larger than one of the planar dimensions L1 of the first pillars 92. As shown in FIG. 3B, the active components 22 adhered to the first pillars 92 are brought into contact with the second pillars 96. Because the area of the second pillars 96 is larger than the area of the first pillars 92, the van der Waal's forces between the active components 22 and the second pillars 96 is greater than the van der Waal's forces between the active components 22 and the first pillars 92. Therefore, the active components 22 will transfer (step 120) to the second pillars 96 when the first stamp 90 is removed (FIG. 3C) leaving the active components 22 adhered to the second pillars 96 of the second stamp 94 (FIG. 3D). If the first and second pillars 92, 96 are made of different material, the second pillars 96 should have a surface area sufficient to transfer the active components 22 to the second pillars 96 from first pillars 92. If the second pillars 96 form a vacuum collet, the vacuum collet 98 must be small enough to contact single active components 22 and the vacuum must be strong enough to remove the contacted single active component 22 from the first pillar 92 and transfer it to the second pillar 96.
As illustrated in FIGS. 3A-3D, the first stamp 90 has more first pillars 92 than the second stamp 94 has second pillars 96. Thus, not all of the active components 22 on the first pillars 92 will transfer to the second pillars 96. The second stamp 94 can be laterally translated with respect to the first stamp 90 to sequentially transfer subsets of the active components 22 from the first pillars 92 to the second pillars 96. Since the first pillars 92 are spatially aligned to the active components 22 on the source substrate 20, to enable a sparser distribution of active components 22 on the second stamp 94, the second stamp 94 can have fewer second pillars 96 than first pillars 92 so as to spatially distribute the active components 22 farther apart.
As illustrated in FIGS. 3E and 3F, a second stamp 94 can include second pillars 96 that form vacuum collets 98. By applying a vacuum (or partial vacuum) to the vacuum collets 98, the active components 22 can be transferred to the second stamp 94. As shown in FIG. 3E, the second stamp 94 is aligned with the first stamp 90, vacuum is applied to the vacuum collet 98, and the second stamp 94 is removed from the first stamp 90, leaving the active component 22 associated with the second pillar 96 adhered to the second pillar 96 (FIG. 3F). The illustrations of FIGS. 3E and 3F employ a single second pillar 96 vacuum collet 98. In another embodiment of the present invention, and as illustrated in FIGS. 3G and 3H, a plurality of second pillars 96 can have vacuum collets 98. As shown in FIG. 3G, the second stamp 94 is aligned with the first stamp 90, vacuum is applied to the plurality of vacuum collets 98, and the second stamp 94 is removed from the first stamp 90, leaving the active components 22 associated with the second pillars 96 adhered to the second pillars 96 (FIG. 3F).
The spatial distribution of the active components 22 is a matter of design choice for the end product desired. In one embodiment of the present invention, all of the active components 22 in a source substrate 20 array are transferred to the first stamp 90. In another embodiment, a subset of the active components 22 in the source substrate 20 array is transferred. Similarly, in some embodiments of the present invention, all of the active components 22 on the first pillars 92 of the first stamp 90 array are transferred to the second pillars 96 of the second stamp 94. In another embodiment, a subset of the active components 22 on the first pillars 92 of the first stamp 90 are transferred to the second pillars 96 of the second stamp 90. By varying the number and arrangement of first and second pillars 92, 96 on the first and second stamps 90, 94, the distribution of active components 22 on the second pillars 96 of the second stamp 94 can be likewise varied.
In a further embodiment of the present invention, referring to step 125 of FIG. 8, a destination substrate 10 is provided. An optional adhesive layer 18 (FIG. 6) can be coated over the destination substrate 10 (optional step 130). As shown in FIG. 4A, the active components 22 on the second pillars 96 of the second stamp 94 are brought into alignment with the destination substrate 10 and pressed onto the destination substrate 10 (FIG. 4B). The second stamp 94 is then removed (FIG. 4C) leaving the active components 22 adhered to the destination substrate 10 (FIG. 4D) (step 135).
As shown in FIGS. 4A-4D, the destination substrate 10 can include destination substrate contacts 16. The destination substrate contacts 16 can be electrically conductive and connected through wires or conductive traces to other components or structures on the destination substrate 10. If the active components 22 include protruding connection posts 26, and the active components 22 are aligned with the destination substrate 10 so that the connection posts 26 are aligned with the destination substrate contacts 16, the connection posts 26 can electrically contact the destination substrate pads and electrically connect the active component circuit 30 to electronic components on the destination substrate 10. By pressing the connection posts 26 of the second stamp 94 with sufficient mechanical pressure against the destination substrate contacts 16, the connection posts 26 can be driven through a surface of the destination substrate contacts 16 into the destination substrate contacts 16 to form a robust electrical contact with the destination substrate contacts 16. A sufficient mechanical pressure can be an amount of force needed to cause the destination substrate contact 16 to plastically deform as the connection post 26 is pressed into it. Thus, in this embodiment, the connection posts 26 on the active components 22 may have sharp points and/or a high elastic modulus, for example, by incorporating tungsten. A connection post 26 can have a sharp point, for example, if the top of the post has an area less than 10 microns square, less than 5 microns square, or less than 1 micron square. The destination substrate contacts 16 can also provide adhesion to help adhere the active components 22 to the destination substrate 10.
Referring to FIG. 5, the destination substrate contacts 16 can be made of a relatively soft metal, such as tin, solder, or tin-based solder, to assist in forming good electrical contact with the connection posts 26 and adhesion with the active components 22. As used herein, a soft metal may refer to a metal into which a connection post 26 can be pressed to form an electrical connection between the connection post 26 and the destination substrate contact 16. In this arrangement, the destination substrate contact 16 can plastically deform and flow under mechanical pressure to provide a good electrical connection between the connection post 26 and the destination substrate contact 16. In a further method of the present invention, the destination substrate contacts 16 (or connection posts 26) are heated, causing the destination substrate pad metal to reflow and improve adhesion between the active components 22 and the destination substrate 10 and improve the electrical connection to the connection posts 26.
In another embodiment of the present invention, the connection posts 26 can include a soft metal and the destination substrate contacts 16 include a high elastic modulus metal. In this arrangement, the connection posts 26 can plastically deform and flow under mechanical pressure to provide a good electrical connection between the connection post 26 and the destination substrate contact 16.
Referring to FIG. 6, if the optional adhesive layer 18 is formed on the destination substrate 10, the connection posts 26 can be driven through the adhesive layer 18 to form an electrical connection with the destination substrate contacts 16 beneath the adhesive layer 18. The adhesive layer 18 can be cured to more firmly adhere the active components 22 to the destination substrate 10 and maintain a robust electrical connection between the connection posts 26 and destination substrate contacts 16 in the presence of mechanical stress. The adhesive layer 18 can undergo some shrinkage during the curing process that can further strengthen the electrical connectivity and adhesion between the connection post 26 and the destination substrate contacts 16.
FIG. 7 provides a top view of the destination substrate 10 with destination substrate contacts 16 after the active components 22 are adhered to the destination substrate 10.
Thus, methods of the present invention include selectively transferring active components from a source substrate to a destination substrate by providing a source substrate 20 having a process side 24 and a plurality of active components 22 formed on or in the process side 24 of the source substrate 20. A first stamp 90 having a plurality of first pillars 92 formed thereon is spatially aligned to the active components 22. Each first pillar 92 has a first area. The first pillars 92 are pressed against corresponding active components 22 to adhere the active components 22 to the first pillars 92. A second stamp 94 having a plurality of second pillars 96 is spatially aligned to the first pillars 92. Each second pillar 94 has a second area greater than the first area. The second pillars 96 are pressed against corresponding active components 22 to adhere the active components 22 to the second pillars 96. The active components 22 are pressed against the destination substrate 10 to adhere the active components 22 to the destination substrate 10.
According to another embodiment of the present invention, an active component 22 includes a substrate having a process side 24 and a circuit 30 formed on the process side 24. Connection posts 26 are formed on the process side 24 and electrically connected to the circuit 30. The connection posts 26 have a base width, a height, a base area, and a peak area. The height is greater than the base width and the peak area is less than the base area.
In yet another embodiment of the present invention, an electronically active substrate includes a destination substrate 10 having a plurality of destination substrate contacts 16. The destination substrate contacts 16 have a surface. A plurality of active components 22 are distributed over the destination substrate 10. Each active component 22 includes a component substrate different from the destination substrate 10. Each active component 22 has a circuit 30 and connection posts 26 formed on a process side 24 of the component substrate. The connection posts 26 have a base width and a height that is greater than the base width. The connection posts 26 are in electrical contact with the circuit 30 and the destination substrate contacts 16. The connection posts 26 are driven through the surface of the destination substrate contacts 16 into the destination substrate contacts 16 to electrically connect the connection posts 26 to the destination substrate contacts 16.
In another embodiment, an adhesive layer 18 is formed over the destination substrate 10 between the active components 22 and the destination substrate 10, so that the connection posts 26 pass through the adhesive layer 18 into the destination substrate contacts 16. The adhesive layer 18 can be a curable adhesive layer and the adhesive layer can be cured to adhere the active components 22 to the destination substrate 10.
According to one embodiment of the present invention, the source substrate 20 can be provided with active components 22 and connection pads 28 or connection posts 26 already formed on the process side 24 of the source substrate 20. Alternatively, an unprocessed source substrate 20 can be provided and the active components 22 formed on the process side 24 of the source substrate 20. An unprocessed source substrate 20 is a substrate that does not yet include active components 22. The unprocessed source substrate 20 can have other processing steps completed, for example, cleaning, deposition of material layers, or heat or chemical treatments, as are used in the photo-lithographic arts. Active components 22 are formed, for example using photo-lithographic processes including forming masks over the source substrate 20, etching materials, removing masks, and depositing materials. Such processes are used in the photo-lithographic arts. Using such processes, active components 22 are formed on or in the process side 24 of the source substrate 20.
Active components 22 are small electronic integrated circuits, for example, having a size of about 5 microns to about 5000 microns in a dimension. The electronic circuits can include semiconductor materials (for example inorganic materials such as silicon or gallium arsenide, or inorganic materials) having various structures, including crystalline, microcrystalline, polycrystalline, or amorphous structures. The active components 22 can also include insulating layers and structures such as silicon dioxide, nitride, and passivation layers and conductive layers or structures including wires made of aluminum, titanium, silver, or gold that form an electronic circuit. Connection posts 26 or connection pads 28 can be formed of metals such as aluminum or polysilicon semiconductors and can be located on the top surface of the active components 22. Methods and materials for making active component 22 electronic circuits are used in the integrated circuit arts. Large numbers of such small integrated circuits are formed on a single source substrate 20. The active components 22 are typically packed as closely as possible to use the surface area of the source substrate 20 as efficiently as possible.
Embodiments of the present invention provides advantages over other printing methods described in the prior art. By employing connection posts 26 on active components 22 and a printing method that provides active components 22 onto a destination substrate 10 with the process side 24 and connection posts 26 adjacent to the destination substrate 10, a low-cost method for printing chiplets in large quantities over a destination substrate 10 is provided. Furthermore, additional process steps for electrically connecting the active components 22 to the destination substrate 10 are obviated.
The source substrate 20, first stamp 90, second stamp 94, and destination substrate 10 can be made separately and at different times or in different temporal orders or locations and provided in various process states.
In some embodiments, the active components 22 are small integrated circuits formed in a semiconductor wafer source substrate 20, for example gallium arsenide or silicon, which can have a crystalline structure. Processing technologies for these materials typically employ high heat and reactive chemicals. However, by employing transfer technologies that do not stress the active component 22 or substrate materials, more benign environmental conditions can be used compared to thin-film manufacturing processes. Thus, the present invention has an advantage in that flexible substrates that are intolerant of extreme processing conditions (e.g. heat, chemical, or mechanical processes) can be employed for the destination substrates 10. Furthermore, it has been demonstrated that crystalline silicon substrates have strong mechanical properties and, in small sizes, can be relatively flexible and tolerant of mechanical stress. This is particularly true for substrates of 5 micron, 10 micron, 20 micron, 50 micron, or even 100-micron thicknesses. Alternatively, the active components 22 can be formed in a microcrystalline, polycrystalline, or amorphous semiconductor layer.
The active components 22 can be constructed using foundry fabrication processes used in the art. Layers of materials can be used, including materials such as metals, oxides, nitrides and other materials used in the integrated-circuit art. Each active component 22 can be a complete semiconductor integrated circuit and can include, for example, transistors. The active components 22 can have different sizes, for example, 1000 square microns or 10,000 square microns, 100,000 square microns, or 1 square mm, or larger, and can have variable aspect ratios, for example 1:1, 2:1, 5:1, or 10:1. The active components 22 can be rectangular or can have other shapes.
The first and second stamps 90, 94 can be structured, for example with pillars 92 matched to the shapes, sizes, and locations of the active components 22 to enhance adhesion. The first and second stamps 90, 94 can be elastomeric, for example made of polydimethylsiloxane (PDMS), rubber, or a reinforced composite.
The adhesion between the active components 22 and the receiving side 12 of the destination substrate 10 should be greater than the adhesion between the active components 22 and the second stamp 94. As such, when the second stamp 94 is removed from the receiving side 12 of the destination substrate 10, the active components 22 adhere more strongly to the receiving side 12 than to the second stamp 94, thereby transferring the active components 22 from the second stamp 94 to the receiving side 12 of the destination substrate 10.
Referring to the flow diagrams of FIG. 8, the method of the present invention can be iteratively applied to a single or multiple destination substrates 10. By repeatedly transferring sub-arrays of active components 22 from a second stamp 94 to a destination substrate 10 and relatively moving the second stamp 94 and destination substrates 10 between stamping operations by a distance equal to the spacing of the selected active components 22 in the transferred sub-array between each transfer of active components 22, an array of active components 22 formed at a high density on a source substrate 20 can be transferred to a destination substrate 10 at a much lower density. In practice, the source substrate 20 is likely to be expensive, and forming active components 22 with a high density on the source substrate 20 will reduce the cost of the active components 22, especially as compared to forming active components on the destination substrate 10. Transferring the active components 22 to a lower-density destination substrate 10 can be used, for example, if the active components 22 manage elements distributed over the destination substrate 10, for example in a display, digital radiographic plate, or photovoltaic system.
In particular, in the case wherein the active component 22 is an integrated circuit formed in a crystalline semiconductor material, the integrated circuit substrate provides sufficient cohesion, strength, and flexibility that it can adhere to the destination substrate 10 without breaking as the second stamp 94 is removed.
In comparison to thin-film manufacturing methods, using densely populated source substrates 20 and transferring active components 22 to a destination substrate 10 that requires only a sparse array of active components 22 located thereon does not waste or require active layer material on a destination substrate 10. The present invention can also be used in transferring active components 22 made with crystalline semiconductor materials that have higher performance than thin-film active components. Furthermore, the flatness, smoothness, chemical stability, and heat stability requirements for a destination substrate 10 used in embodiments of the present invention may be reduced because the adhesion and transfer process is not substantially limited by the material properties of the destination substrate. Manufacturing and material costs may be reduced because of high utilization rates of more expensive materials (e.g., the source substrate) and reduced material and processing requirements for the destination substrate.
Many different embodiments have been disclosed herein, in connection with the above description and the drawings. It will be understood that it would be unduly repetitious and obfuscating to literally describe and illustrate every combination and subcombination of these embodiments. Accordingly, the present specification, including the drawings, shall be construed to constitute a complete written description of all combinations and subcombinations of the embodiments described herein, and of the manner and process of making and using them, and shall support claims to any such combination or subcombination.
In the specification, there have been disclosed embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation.

## Claims

1. A method for transferring a plurality of active components from a source substrate to a destination substrate, the method comprising:
providing a source substrate having a process side including the plurality active components therein or thereon, the active components having respective primary surfaces including electrical connections thereon adjacent the process side and respective secondary surfaces opposite the primary surfaces;
pressing a first stamp having first pillars protruding therefrom against the active components on the process side of the source substrate to adhere the respective primary surfaces of the active components including the electrical connections thereon to respective transfer surfaces of the first pillars; and
after pressing the first stamp against the plurality of active components, transferring the plurality of active components to a receiving surface of a destination substrate utilizing a second stamp having second pillars protruding therefrom, wherein each active component includes a component substrate different from the destination substrate, wherein transferring the plurality of active components to the receiving surface comprises:
pressing the second stamp against a destination substrate to adhere the plurality of active components to a receiving surface of the destination substrate; and
removing the second stamp from the receiving surface of the destination, thereby transferring the active components to the destination substrate and forming a display.

2. The method of claim 1, wherein the second stamp transfers the active components from the first stamp to the destination substrate.

3. The method of claim 1, wherein the plurality of active components comprises a plurality of integrated circuits.

4. The method of claim 1, wherein the receiving surface comprises an adhesive layer.

5. The method of claim 4, wherein the adhesive layer is a curable adhesive layer.

6. The method of claim 1, wherein transferring the plurality of active components to the receiving surface comprises: pressing the second stamp having second pillars protruding therefrom against the active components on the first pillars of the first stamp to adhere the respective secondary surfaces of the active components to respective transfer surfaces of the second pillars.

7. The method of claim 6, wherein the respective transfer surfaces of the second pillars have greater adhesive strength than those of the first pillars.

8. The method of claim 1, wherein pressing the second stamp against the destination substrate adheres the respective primary surfaces of the plurality of active components to the receiving surface of the destination substrate.

9. A method for transferring a plurality of active components from a source substrate to a destination substrate, the method comprising:
providing a source substrate having a process side including the plurality active components therein or thereon, the active components having respective primary surfaces including electrical connections thereon adjacent the process side and respective secondary surfaces opposite the primary surfaces;
pressing a first stamp having first pillars protruding therefrom against the active components on the process side of the source substrate to adhere the respective primary surfaces of the active components including the electrical connections thereon to respective transfer surfaces of the first pillars; and
after pressing the first stamp against the plurality of active components, transferring the plurality of active components to a receiving surface of a destination substrate utilizing a second stamp having second pillars protruding therefrom, wherein transferring the plurality of active components to the receiving surface comprises:
pressing the second stamp against a destination substrate to adhere the plurality of active components to a receiving surface of the destination substrate.

10. The method of claim 9, wherein the second stamp transfers the active components from the first stamp to the destination substrate.

11. The method of claim 9, wherein the plurality of active components comprises a plurality of integrated circuits.

12. The method of claim 9, wherein transferring the plurality of active components to the receiving surface comprises: removing the second stamp from the receiving surface of the destination, thereby transferring the active components to the destination substrate and forming a display.

13. The method of claim 9, wherein the receiving surface comprises an adhesive layer.

14. The method of claim 13, wherein the adhesive layer is a curable adhesive layer.

15. The method of claim 9, wherein the first stamp has more first pillars than the second stamp has second pillars.

16. The method of claim 9, wherein each active component includes a component substrate different from the destination substrate.

17. The method of claim 9, wherein transferring the plurality of active components to the receiving surface comprises: pressing the second stamp having second pillars protruding therefrom against the active components on the first pillars of the first stamp to adhere the respective secondary surfaces of the active components to respective transfer surfaces of the second pillars.

18. The method of claim 9, wherein the respective transfer surfaces of the second pillars have greater adhesive strength than those of the first pillars.

19. The method of claim 9, wherein pressing the second stamp against the destination substrate adheres the respective primary surfaces of the plurality of active components to the receiving surface of the destination substrate.

20. The method of claim 9, wherein the respective surfaces of the second pillars have a greater surface area than the respective surfaces of the first pillars.