LED package and manufacturing method

An LED package (40) and manufacturing method in which the package has LED substrate (50) and a circuit substrate (54) bonded together, with the LED over the integrated circuit, and with electrical connection between the LED and corresponding integrated circuit. The package has package terminals (56a, 56b) on one face only with through vias (58a, 58b) providing connection between the LED substrate and the circuit substrate.

FIELD OF THE INVENTION

This invention relates to LED packages and methods of manufacturing such packages.

BACKGROUND OF THE INVENTION

Various LED packages are known. For example, wafer level chip scale LED packages are known which are directly solderable on a suitable substrate. Such a package typically has two contacts to the p-n junctions of the diode. The LED package can for example be mounted to a substrate carrying control circuitry for the LED, for example ESD diodes or control transistors.

By way of example, an LED die can be mounted on a silicon substrate, in which the substrate contains an embedded ESD protection diode. Contacts on the top of the substrate makes electrical connection with the LED die terminals, and the substrate has further external contacts on the same top face, outside the area where the LED die is mounted. This requires individual placement of each LED die over the substrate, for example using a ball grid array.

It is also known that in addition to ESD protection, by associating LEDs with control transistors, it becomes possible to drive and control strings of LEDs. For example, multiple LEDs in series can be controlled individually by connecting individual FET transistors parallel to each LED. By closing a transistor switch, a corresponding LED is shorted and will be switched off.

There remains a need for a cost effective and compact packaging solution for LEDs and associated control devices (such as a transistor or ESD protection diode or more complicated control circuit).

SUMMARY OF THE INVENTION

According to the invention, there is provided a method and apparatus as defined in the independent claims.

According to one aspect, there is provided a method of forming an LED package, comprising:forming an array of integrated circuit LEDs as part of a first semiconductor substrate;forming an array of integrated circuit components as part of a second semiconductor substrate;bonding the first and second semiconductor substrates together, with each LED of the first semiconductor substrate positioned over a corresponding integrated circuit component or components of the second semiconductor substrate, thereby making electrical connection between the LED and corresponding integrated circuit component or components;dicing the bonded first and second semiconductor substrates to form individual LED packages or groups of LED packages.

This method provides a circuit as part of the LED package, and directly under the LED. This allows a compact design. Wafer level bonding takes place between the LED substrate and the circuit substrate, so that only one dicing step is required to form the packages, and only one alignment process is needed between the two substrates.

Forming the array of integrated circuit components can comprise forming through vias extending between opposite faces of the second semiconductor substrate. In this way, one side of the second substrate has all the required package terminals to connect to the LED (using the vias) and to the integrated circuit component or components (directly).

Forming the array of integrated circuit components can comprise forming an array of transistors. These can be used as control devices, to control the individual LEDs. This can for example be used for dimming control or control of the pattern of illumination of an array of LEDs.

Forming the array of integrated circuit components can comprise forming an array of diodes, which can then be used for ESD protection.

According to another aspect, there is provided an LED package, comprising:an integrated circuit LED formed as a first semiconductor substrate and having LED connection terminals on one face;an integrated circuit formed as a second semiconductor substrate, wherein the one face of the first semiconductor substrate and a first face of the second semiconductor substrate are bonded together, with the LED of the first semiconductor substrate positioned over the integrated circuit, and with electrical connection between the LED and corresponding integrated circuit,
wherein the package has package terminals on the opposite face of the second semiconductor substrate to first face, and the second substrate has at least one through via providing connection between a package terminal and an LED connection terminal.

This package has a single connection face enabling connection to an LED and associated circuit. The arrangement is space efficient by stacking the LED over its associated circuit.

Connection to at least one LED terminal is enabled. For example, if the LED is in series with the integrated circuit, then the package terminals can make connection to one LED terminal and one integrated circuit terminal (i.e. the ends of the series connection). The second substrate can however have at least two through vias providing connection between respective package terminals and the two LED connection terminals. Thus, connection to both LED terminals is enabled.

The integrated circuit can comprise an ESD protection diode. However, in a preferred example, the integrated circuit comprises a transistor. This can be used for controlling the operation of the LED. For example, the transistor can be in parallel with the LED. In this way, it can provide a bypass function and can thus be used to interrupt the current through the LED. This enables use as part of a resistive driver scheme, to switch off one or multiple LEDs in series between a voltage source.

The package terminals can then comprise connections to the transistor source, drain and gate.

An LED circuit can comprise at least one package of the invention mounted on a printed circuit board, wherein the printed circuit board has tracks for connection to the three package terminals.

The three terminals enable control of LED strings. The gate terminal provides a switching function which is activated by the associated package terminal at the bottom of the device.

DETAILED DESCRIPTION

The invention provides an LED package and manufacturing method in which the package has an LED substrate and a circuit substrate bonded together, with the LED over the integrated circuit, and with electrical connection between the LED and corresponding integrated circuit. The package has package terminals on one face only with through vias providing connection between the package terminals on one substrate and the LED connections of the other substrate. Thus, even though the package has two substrates, it has a single connection face for mounting over a carrier, such as a PCB, in simple manner.

The single connection face enables electrical connection to the LED and to the associated circuit. The arrangement is space efficient by stacking the LED over its associated circuit.

FIG. 1shows a known LED package. The LED10is formed as a discrete package which is mounted on a silicon submount12for example by solder balls. Connections to the LED package are made through the solder balls, and external connections from the submount are made by wirebonds14. As shown schematically inFIG. 1, the submount12can implement a pair of ESD protection diodes. Thus, ESD protection is one reason for associating additional components with each LED.

Another reason for associating additional components with each LED is to provide switching functionality.FIG. 2shows a known circuit which provides a parallel switch M1to M4for each LED of a string of LEDs, LED1to LED4. By turning on a switch, a bypass path is provided so that the individual LED is turned off.

FIG. 3shows a known way to implement the circuit ofFIG. 2. A number of discrete LED packages30is mounted over tracks on a PCB32. Two tracks31connect to two power lines. Additional tracks between the individual LED packages30provide the series connections between the LED packages. Further tracks connect to the transistors34, also mounted on the PCB, and the transistors have PCB tracks which form control lines36connecting to their gates.FIG. 3also shows in exploded form one of the LED packages. This arrangement takes up a significant amount of space. It also requires mounting of the LED packages as well as the transistors.

FIG. 4shows a circuit of the invention. The circuit again has LED packages40mounted on a PCB42, with PCB tracks42connecting to power lines for the series arrangement of LEDs. The LED packages40integrate the LED and associated transistor into a single package, with the LED overlying the transistor. The package has package terminals on only one face, namely the face against the PCB42. There are three package terminals, two power line terminals and a control terminal for connecting to the transistor gate within the package. The control terminal connects to a PCB track46. This provides a more compact arrangement and with reduced mounting requirements.FIG. 4also shows in exploded form one of the LED packages as well as a plan view.

FIG. 5shows one LED package40used in the circuit ofFIG. 4in more detail. Each LED package comprises an integrated circuit LED formed as a first semiconductor substrate50and having LED connection terminals52a,52bon one face. These terminals connect to the n- and p-junctions of the LED. The LED can be a pn diode, or a pin diode or any other known diode configuration. It can be vertical or lateral.

An integrated circuit (in this example the switching transistor) is formed as a second semiconductor substrate54. The face of the first second semiconductor substrate50which carries the terminals52a,52band a first face of the second semiconductor substrate54are bonded together. The two substrates are shown separated inFIG. 5for clarity.

The bonding face of the second substrate54carries contact pads55a,55bfor connection to the terminals52a,52b.

When connected, the LED of the first semiconductor substrate50is positioned over the integrated circuit, and with electrical connection between the LED and corresponding integrated circuit.

The package has package terminals56a,56b,56con the opposite face of the second semiconductor substrate54. The second substrate54has through vias58a,58bproviding connection between two of the package terminals56a,56band the contact pads55a,55bfor connection to the LED connection terminals52a,52b. A third package terminal56cconnects to the gate of the transistor, which is formed within the semiconductor substrate in conventional manner.

In this way, the single set of package terminals enables connection to the LED as well as to the transistor. If the transistor of the package is not needed for the particular use of the LED package, no connection needs to be made to the transistor gate, and the package can be used as a normal two-terminal LED package.

FIG. 6shows a cross-sectional view of a string of LED packages and shows the two substrates in contact with each other after wafer level bonding before dicing.

FIG. 7ashows an exploded perspective view of one package. The same reference numbers are used as inFIG. 5. The transistor structure of the second substrate54is shown generally at70, connected between the contact pads55a,55b.FIG. 7ais a perspective shaded view andFIG. 7bshows the same view as a line drawing.

In the example shown, the gate is formed in the first face of the second substrate. A via is formed to enable connection to the gate from the package terminals which are on the opposite side of the second substrate.

Note that the transistor structure could be on the bottom surface of the second semiconductor substrate54, and in this case no via for the gate would be needed.

The arrangement can be manufactured more easily. In particular, wafer level bonding between the two substrates is possible, before dicing.

As shown inFIG. 8, the two substrates50,54can be formed as full arrays. Wafer bonding between the two substrates is then carried out. Possible interconnect methods include micro-bumps or a gold nanosponge.FIG. 8shows a view from above, and shows the dicing lines at the top of the top substrate50and the contact pads at the top of the bottom substrate54.FIG. 9shows a view from below, and shows the LED contact pads at the bottom of the top substrate50and the package terminals at the bottom of the bottom substrate54.

The wafer level bonding should avoid high temperature processes, in order to avoid stress mismatch due to thermal expansion. With suitable substrate designs, the ultra-sonic bonding typically used for single die attach methods can be used. Alternatively, cold interconnect methods can be applied, such as electrically conductive gluing.

Only after wafer level bonding, the individual LEDs, or else groups of LEDs to form the individual packages, are separated. Wafer level connections using through silicon via technology are known for 3D packaging solutions. These allow contacts to be brought to the bottom of a device, as shown for the second substrate54.

In the example above, a parallel switch is added to each LED. Of course, serial switches can also be added in the same way. In this case, external connection to both LED terminals may not be needed. Instead, connections to each end of the series circuit, as well as the transistor gate, are then needed. Thus, the three package terminals can then comprise the transistor source or drain (the one not connected to the LED), the transistor gate and only one of the LED connection terminals.

The circuit is shown as a single transistor, but it may instead be an ESD diode. Of course more complex circuits could be provided in the second substrate. For example, an ESD diode can be integrated into the package as well as the transistor. Furthermore, a more complex transistor control circuit can be implemented in each package, for example for local dimming control.

The invention can be used to form LED packages generally. Of particular interest are packages for LED arrays that require control of individual LEDs. In particular, in automotive applications a dynamic LED matrix array is known for providing dynamic control of the light direction.

The use of a transistor with each LED enables multiple LEDs in series to be controlled individually. The invention provides the integration of a switch into the LED package, directly under the LED. This allows a compact driver design and simplifies the control of LED strings.

Furthermore, the components could be suitable for multi-LED sources driven from mains voltage in which the forward voltages across the LEDs are controlled according to the phase of the mains voltage. In a 220 V mains voltage network, the actual voltage oscillates periodically between +325 and −325 V. If an LED string contains 100 LEDs with an average forward voltage of around 3V, the mains voltage can be applied directly to the string without risk of overdriving the LEDs (in practice, such a circuit would typically contain a rectifier and a protective resistor). If all LEDs remain in the string at all times, the voltage at the individual LEDs is at times of the mains cycle below the switch-on voltage for light generation. It is known that in synchronism with the reducing mains voltage, an increasing number of LEDs can be switched off to allow for the remaining LEDs to provide generation of light efficiently.

The processes used to form the integrated circuit LED and the integrated circuit transistor have not been described in detail, as they are routine. Indeed any diode technology and any transistor technology can be used. Furthermore, the different substrates can use different materials and manufacturing processes as they are formed independently.