Flat panel displays (FPD), which include liquid crystal displays (LCD), plasma display panels (PDP), electrophoretic display (EPD), organic light emitting diode (OLED) displays, and light emitting diode (LED) displays, can be used in many electronic devices such as mobile phones, tablets, laptop computers, monitors and televisions.
Pixels of the flat panel displays are arranged in a matrix form and controlled by an array of driver circuits, for example, to provide selections of on or off pixels. The driver circuits, which can be implemented with thin film-transistors (TFTs), can be fabricated on a substrate together with interconnect lines for linking the driver circuits and the pixels, called a display backplane, on which the array of pixels can be formed.
The backplane can function as a series of switches, through the driver circuits to the interconnect lines to control a current or voltage applied to each individual pixel. The thin film-transistors can use amorphous silicon (a-Si) channels, low temperature polycrystalline silicon channels, or other channels such as IGZO, all of which typically have a lower mobility than single crystal silicon channels.
In some embodiments, methods to form a display can include assembling micro components on a substrate using solder bumps and bond pads connectivity. LED displays can also be formed with micro LED chips, assembling on a backplane.
The backplane can also be fabricated by the assembling process with micro driver chips.
In some embodiments, the present invention discloses self assembling of micro components on a substrate using solder bumps. The micro components can be smaller than 100 microns, 50, 20, 10, or 5 microns in a dimension.
In some embodiments, the present invention discloses methods, and backplanes and flat panel displays formed from the methods, to form backplanes and flat panel displays by assembling driver circuit components and/or pixel circuit components on a substrate.
In some embodiments, the substrate can include interconnect lines, e.g., periodic interconnect lines for forming connections to the individual driver circuits and pixel circuits. The substrate can also include bond pads, which are coupled to the interconnect lines.
The driver circuits and the pixel circuits can be fabricated on separate substrates, and diced to form individual driver and pixel components. The driver and pixel components can include bond pads for interconnection. Solder bumps can be formed on the bond pads, to be bonded to bond pads of the interconnect lines in the substrate, such as in a flip chip process.
The assembling of the driver and pixel components on the substrate can be performed by a solder bump process, e.g., placing solder bumps on corresponding bond pads, and heating the assembly to a high temperature to melt the solder bumps, which can form secure bonds with the bond pads. The molten solder bumps can pull the components into perfect alignment with the bond pads, e.g., with the substrate, providing a flat panel self assembling process.
In some embodiments, the present invention discloses an assembly process of a large array of similar devices or circuits, e.g., the driver circuits and the pixel circuits. These arrays can be small micro components, e.g., if micron size such as 5 microns to 100 microns, such as small silicon chips consisting of a few transistors and capacitors making up switching circuits, in the case of an LCD display, or making up switching and driver circuits, in the case of an AMOLED or LED display. The micro component may consist of several such adjacent driver and switching circuits of the display, which can be called driver chips, e.g., driver circuit components fabricated and packaged into semiconductor chips. The micro component may consist of several pixel circuits of the display, which can be called pixel chips, e.g., pixel circuit components fabricated and packaged into semiconductor chips. The display backplane can include a very large number such repeating units, or driver chips, interconnected to form a display backplane. The display backplane can include pixel chips interconnected with the driver chips.
In some embodiments, the present invention discloses driver circuits on single crystal silicon substrate, which can possesses high performance, such as high mobility, as compared to thin film transistors such as a-Si, LTPS, and IGZO. The thin film transistors have performance limitations when compared to devices made in single crystal silicon wafers.
In some embodiments, the driver circuits and the LED circuits (e.g., LED pixel) can be fabricated on single crystal silicon substrate in semiconductor wafer fabs, diced them into individual driver and pixel chips, and gang assembled, in some embodiments, on a temporary carrier and join them on to a display substrate such as glass or other material, in a self aligned attachment to form a large pixel array for a display backplane or display.
Even given the advances in automated assembly tools and methods, the task of assembling large number circuits and would be huge, both in terms of defects that could come about in alignment and assembly, and defects in making the electrical contacts needed, and speed of the overall process and reliability of the overall process are affected.
In some embodiments, a number of driver chips and/or pixel chips can be provided with solder terminals. The chips can be placed in approximate alignment on the corresponding bond pads on the display substrate, and reflowing the solder, which when molten pulls these individual chips into perfect alignment to the later, and to each other, due to surface tension forces. A large number of chips, such as chips placed in 1-5 cm square areas, can be assembled at one time.
In some embodiments, the present invention discloses employing a self-alignment characteristic of flip-chip interconnections for assembling precision circuits.
In some embodiments, driver transistors can be formed on a substrate for a backplane. These transistors can be first fabricated on a wafer using very optimal CMOS process in a fab. In the fab, these will necessarily be processed as a regular array next each other for the economic, good process control and for the process flow, such as for economy, and uniformity.
A substrate, such as a glass substrate for an LCD display, or another material substrate for an LED display, can have interconnect lines fabricated thereon. The substrate then can be populated with the driver transistors, to form a backplane. Pixel elements can then be placed on the backplane to form a display.
Each pixel of such a backplane will need two or three of these transistors, in the given pixel area, which is not necessarily next to each other, but in a distributed arrangement.
The chips that are going to be made in the wafer fab are tested on the wafer, diced into individual chips, and assembled onto the corresponding receiving pads in the driver areas (for the driver chips), or in the pixel areas (for the pixel chips). A typical display would have millions of these chips, so such an assembly of individual chips onto a substrate would be prohibitively difficult and expensive.
In some embodiments, hundreds or thousands of these chips can be assembled on the substrate at a same time, in an easy and self-aligned way. In some embodiments, all the chips can be assembled in one furnace operation.
The driver and pixel arrays can be regular repeating units. Thus a manageable size of these driver and pixel chips can be selected. The manageable size can be large enough to minimize the numbers that we need to assemble. The manageable size should not be too large or too heavy to ease the assembling process, such as not to frustrate self-alignment during the solder reflow. The number of such repeats units in one chip could be, say, a thousand pixels in a reasonable sized chip, such as a manageable size of one cm by one cm chip.
The chips can also be provided with suitable tin-based solder bumps on the terminal metallurgy such as Cr—Cu—Ni—Au, commonly used in flip chip assembly. The solder bumps can be performed in the wafer fabrication process.
Corresponding locations with Cr—Cu—Ni—Au solder bumps for the terminals in the receiving circuit on the substrate preserves the lithography accuracy of the locations on the backplane.
In some embodiments, the present invention gathers a number of micro circuits on temporary plates, and then joins the temporary plates with the substrate to populate the entire substrate by traditional pick and place method. Pick and place and alignment method only requires approximate alignment. Then when the solder is reflowed under proper flux and temperature ramping conditions, followed by the removal of the temporary plates holding these micro circuits.
In some embodiments, a large array of micro circuits can be fabricated on a suitable substrate in a lithographically defined total array. The substrate can be diced, e.g., the micro circuits can be singulated into individual circuits. Hundreds or thousands of these individual circuits can be temporary assembled on a donor plate.
The donor plates, which contain the micro circuits, can be assembled on a substrate onto the corresponding receiving pads on the backplane circuitry of the substrate in approximate alignment. The donor plates can be assembled by available pick and place flip assembly tooling.
After multiple donor plates are assembled on the substrate, the entire assembly can be reflowed, e.g., heated, in one furnace operation so that the solder bumps will melt and realign and join very robustly to the corresponding pads on the backplane structure and board. The donor plates can be removed, leaving millions of micro circuits on the substrate, which are assembled in perfect alignment and making very good electrical contact with tin solder. The assembly of any population of micro circuits can be a good robust manufacturing process.
The bonds these transistors make to the corresponding pads will be strong and reliable because they are self-aligned solder attachments which have very good electrical and thermal properties and will make a very strong interconnection between the pad on the board and the transistor terminals.
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In some embodiments, the present invention discloses forming an LED display by assembling micro LED components on a backplane. The backplane can be formed by assembling micro driver components on a substrate having interconnect lines.
The micro LED devices can be made on a sapphire wafer in a regular array on a single wafer. Thousands of these can be made on a single wafer. The individual LED chips on the wafer are tested and marked for goodness, diced, and sorted with only the good ones picked. These then need to be assembled onto to the backplane substrate in the pixel areas, in a distributed way, not necessarily next to each other.
After the dicing, the individual LED chips can be assembled onto a temporary carrier or plate which can be made of display glass or other material, as if to form a virtual backplane array, and stuck to it with flux dots deposited by screen or nozzle printing.
The LED chips are also assembled in an array to replicate the array of the pads on the backplane circuit in the glass. So the glass is the ultimate substrate on which these have to be assembled to form a Smartphone backplane.
The flux-assembled LED array is then ‘diced’ into convenient sized pixel chips. Let us say you have assembled say 1000 of these backplane chips each containing 100 pixels onto this backplane glass, and held there by solder flux.
When we melt the solder by heating to 400 degrees C. in the presence of a suitable flux to melt the solder. When the solder melts it's very nicely wets, which means it attaches onto the terminal in a self-aligned way, correcting any misalignments in placement and cooling, one achieves a solder interconnection that is strong, robust and electrical interconnections between the chip terminals against the glass backplane.
After the reflow we will remove the temporary carrier and the other temporary scaffolding material that was holding these chips together by easy means because we have deliberately chosen materials that make these processes easy.
Once the temporary substrate is removed, we are left with an array of millions of these transistors joined very robustly to this backplane in a very robust electrically and physically robust and metallurgical sound interconnection in a self aligned way as if they were all assembled in one batch, far better than if they are assembled one at a time.
Two separate heating processes can be performed, one after the assembly of the micro driver chips, and one after the assembly micro LED chips. Alternatively, a heating process can be performed after the assembly of the micro driver chips and micro LED chips.
Variations of the process can be used, such as without the assembling of micro LED chips, or the fabrication of the LED display without the assembling of the micro driver chips.
The present invention claims priority from U.S. Provisional Patent Application No. 62/367,092, entitle “Self-Aligned Assembly Of Large Arrays Of Micro-components, such as Silicon based pixel circuits, or Micro-LEDs Onto A System Board”, filed on Jul. 26, 2017, which is incorporated herein by reference.
Number | Date | Country | |
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62367092 | Jul 2016 | US |