TWI524555B - Shunting layer arrangement for leds - Google Patents

Description

Diverter configuration of light-emitting diode

The present invention relates to light emitting diodes (LEDs) and, in particular, to a patterned metal layer on a light emitting surface of an LED die that improves current distribution without increasing light blocking.

Figure 1 of the prior art is a top view of one of the LED dies 10, and Figure 2 is a simplified cross-sectional view of one of the LED dies 10 along line 2-2 of Figure 1. In an example, the LED die 10 is based on GaN and has its growth substrate removed. The structure is well known. Typically, a bottom metal anode 12 is bonded directly to a sub-substrate pad or a circuit board. One of the metal reflectors 14 on the electrode 12 reflects the light upward. The epitaxial growth semiconductor layer of the LED die comprises a first p-type layer 16, a p-type cladding layer 18, an active layer 20, an n-type cladding layer 22, a first n-type layer 24 and a second N-type layer 26. The various p-type and n-type layers between the cladding layer and the metal contacts can have different doping amounts and different compositions to achieve different functions, such as lattice matching and current spreading. There can be more layers. The semiconductor layer is transparent.

A transparent current spreading layer 28 is formed on the second n-type layer 26, and a metal cathode 30 is electrically connected to one of the edges of the current spreading layer 28. A lead (not shown) is bonded to the anode 30. The current spreading layer material is selected to have low light loss, low electrical resistivity, and good electrical contact. Materials suitable for the current spreading layer 28 comprise indium tin oxide, zinc oxide or other transparent conductive oxide. Current spreading layer 28 is only a few microns thick and therefore has a low vertical resistance and a very high lateral resistance. What is important is the p-type cladding layer 18 and the n-type cladding layer 22 The current distribution thereon is fairly uniform to achieve uniform light generation across the active layer 20.

To compensate for the higher lateral resistance of the current spreading layer 28, a low resistance metal shunt layer 32 is patterned to extend through the current spreading layer 28 but only blocks a small amount of light. There is a trade-off between minimizing current crowding and minimizing light blocking. The shunt pattern shown in Figure 1 is typical in that the metal bus bars and the vertical metal bus bars along the periphery of the LED die 10 are connected to the layers. These shunt bars are formed to be very narrow to minimize light blocking.

2 shows the current through the LED die 10 as indicated by the thick arrow 36 and some of the photon tracks indicated by the thin arrow 38. A simplified emission light pattern 39 is also shown.

The top surface of the LED die 10 is roughened to increase light extraction.

One problem with conventional shunt designs is the contact resistance at one of the interfaces of the subdivided stream bar and the current spreading layer 28, wherein the contact resistance is directly related to the width of the strip.

For the particular case of patterning the shunt layer as one of the features of the bus bar as shown in Figure 1, the contact resistance of one of the three inner cross-bundle bars can be expressed as:

Wherein R _s is the sheet resistance of the current spreading layer 28 (in units of square ohms (Ω / □)), the length of the L-series bus bar section, w is the width of the bus bar, and the L _t transmission length is expressed as unit length. The transfer length is defined as:

The contact resistivity of the ρ _c- based metal semiconductor interface is in units of ohm square meters (Ω·m ² ).

As is well known, the lateral current between a conductive layer and a metal contact does not evenly traverse the metal contacts. The voltage is highest near the edge of the metal contact and exponentially decreases with distance. The 1/e distance of the voltage curve is another way of determining the transfer length.

3 when L = L _t represents normalized to the amount of w / L _t is the contact resistance R _s of expressing a function of normalized above. The curve indicates that when the contact width is less than 2L _t , the contact resistance increases inversely proportional to w, such as R _C(ric) tends to . On the other hand, when the contact width is higher than 2L _t , the contact resistance follows Tend to 1 and close to the amount .

As can be seen, although the narrow width can be expected to block less light, the width of the bus bar in Figure 1 should not be too small, otherwise the contact resistance would be very high.

Therefore, it is desirable to reduce the contact resistance between a metal shunt layer and a current spreading layer without adversely affecting light extraction of the LED dies. Conversely, it is desirable to increase the light extraction of the LED dies without reducing the contact resistance between a metal shunt layer and the current spreading layer. It is also desirable to improve the uniformity of current distribution across the surface of the LED die.

Various metal shunt patterns that reduce contact resistance and improve current distribution uniformity without reducing light extraction are disclosed herein.

In one embodiment, the shunt pattern comprises metal circular dots having a diameter that is wider than the width of the conventional bus bar and the crossbar but that is in the order of 2 L _t to 10 L _t so as not to block an array of a large amount of light. In one embodiment, the radius of each point is greater than 2 L _t and less than 10 L _t , and preferably less than 5 L _t . The total area of the dots is smaller than the total area of the prior art bus bars and cross bars, so the light blocking is less. Shapes other than circular points, such as polygons (such as squares and rectangles), can be used. All such shapes are referred to herein as points.

In one embodiment, a typical metal used for the current spreading layer and used, the width of point (between 2L _t to 10L _t) of about 15 microns to ensure low contact resistance. Each point represents a current injection zone. Typically, there will be a density of one of 50 to 60 discrete implant zones per square millimeter for a good current distribution. For a minimum 2 L _t width and 50 square dots per square millimeter, about 1% of the top surface area of an LED die will be covered by dots. For an LED die of one size of 1 square millimeter, the total area of the dots will be approximately 0.01 square millimeters. In one embodiment, the top surface area of one of the LED dies covered by the dots is preferably less than 5%.

In order to evenly distribute the current on the top surface of the LED die, the point system is meshed with one of the very fine metal connectors, wherein the contact resistance between the metal connector and the current spreading layer is much smaller than 2L _t due to the width of the connector. Higher, but because the current is injected by the point, it does not affect the current injection.

Due to the dot array, there is a lower total contact resistance and less light blocking, thus improving the efficiency of the LED die.

In one embodiment, a wire bond electrode is formed adjacent the middle of the top surface of the LED die to create a more uniform current distribution.

In one embodiment, the point array is interconnected by a grid of fine metal connectors, some of which are also connected to the wire bond electrodes via radially extending thin metal connectors to cause point and wire bond electrodes The connection resistance between them is more uniform.

In one embodiment, the dots become larger as the points are further away from the wire bond electrodes to produce a more uniform current distribution across the entire surface of the LED die.

In an embodiment, the dots extend closer to the wire bonding electrode as the dots extend closer to produce a more uniform current distribution.

In one embodiment, a dielectric is present between the wire bond electrode and the current spreading layer to reduce current crowding below and around the perimeter of the wire bond electrode.

In an alternative embodiment and to avoid the use of a dielectric layer between the wire bond electrode and the current spreading layer, a concentric shunt ring system surrounding the wire bond electrode at a specific distance is used to reduce the perimeter and surroundings of the wire bond electrode perimeter The current is crowded.

In one embodiment in which there is one of the shunt bars extending around the perimeter of the top surface of the LED die, the width of the strip is reduced near the corner to reduce or eliminate current crowding near the corner.

In one embodiment, a cornered mirror structure is formed at each point and below the connection grid. The mirrors underneath the dots and connectors not only reflect light away from the bottom side of the dots/connectors, but also avoid the dots directly below (and below the connectors) by causing the active layer below each point to not produce light. To a small extent)) any current crowded. In one embodiment, each mirror is formed in a groove extending through the points and the active layer below the connector.

Other embodiments of the invention are described.

Identical or equivalent elements are labeled with the same element symbols.

4 illustrates a top surface of an LED die according to an embodiment of the present invention. One embodiment of one of the metal shunt patterns 40. The LED die can have the same layer as the prior art LED die of FIG.

According to Equation 1 above, the position of the current injection into the semiconductor along the bus bar is controlled by appropriately adjusting the geometric parameter w. The round metal contacts 42 (dots) are preferred due to their substantially uniform current pattern. The contact resistance of a circular metal contact with a radius of r _c can be expressed as follows:

In Equation 3, I0 and I1 are the modified Bessel functions of the first and second classes, respectively. As in the case of the bus bar, the contact resistance of a circular metal contact sharply increases at r _c < 2L _t . Thus, in a preferred embodiment, the radius of each of the circular metal contacts is between about 2 L _t and 10 L _t .

Thus, a shunt pattern can be composed of a number of geometries that allow for selective control of the location of current injection but are limited in size to not adversely affect light output. This can be used, for example, to improve current uniformity across the active layer of the device and to minimize metal semiconductor contact regions.

The narrow metal connectors 44 are configured as a grid to connect the metal contacts 42 together. The connector 44 has a width preferably less than the 2L _t, because it will be like without the current injected into the LED die, and the wider the connector block of light increases.

The metal contacts 42 and the connectors 44 are preferably constructed of a multilayer metal of low resistance, but do not migrate into the semiconductor layer.

Figure 5 shows the shunt pattern of Figure 4 with a larger wire bond electrode 46 positioned near the middle of the LED die surface to provide a substantially uniform current distribution. The size of the electrode 46 is preferably a minimum size to achieve a good wire bond.

6 shows the shunt pattern of FIG. 5 with additional radial connectors 48 interposed between wire bond electrodes 46 and various metal contacts 42. These radial connectors 48 provide a parallel connector path to the outer metal contacts 42 to make the current distribution more uniform because the combined resistance of the mesh connector 44 paths increases from the wire bond electrodes 46.

7 is a top plan view of a top surface of an LED die showing a metal shunt pattern having a size (diameter) of metal contacts 50 that increases as the metal contacts 50 are moved further away from the wire bond electrodes 46. A larger area of metal contacts necessarily reduces the space between the metal contacts near the perimeter, which increases current injection near the perimeter to counteract the increased resistance of the connectors 44 and 48 that are routed to the outer metal contacts 50.

Figure 8 is a top plan view of a top surface of an LED die showing a metal shunt of a metal contact 54 having a density that increases as the metal contact 54 moves further away from the wire bond electrode 46 to achieve a more uniform current density. pattern.

9 is a top plan view of a top surface of an LED die 55 showing a metal shunt pattern having a square dot 56, an enlarged center wire bond 57, and a narrow connector 58 connecting the dots. The configuration and width of the square dots can be similar to the above-mentioned circular dots.

In one embodiment, the width of the dots (between 2 L _t and 10 L _t ) is about 15 microns for the typical metal used and the current spreading layer used to ensure low contact resistance (based on the particular material used) Similar to one of the graphs in Figure 3.) Each point represents a current injection zone. Typically, there will be a density of one of 50 to 60 discrete implant zones per square millimeter for a good current distribution. For a minimum 2 L _t width and 50 square dots per square millimeter, about 1% of the top surface area of an LED die will be covered by dots. For an LED die of one size of 1 square millimeter, the total area of the dots will be approximately 0.01 square millimeters. A circular point with the same width as the square point covers less area, so it will block less light. In one embodiment, the top surface area of one of the LED dies covered by the dots is preferably less than 5%, such as 2%. The width of the dots is preferably less than 5 L _t but slightly greater than 2 L _t because a width greater than 2 L _t does not provide a significantly reduced contact resistance and minimizes light blocking.

Figure 10 is a cross-sectional view of one of the wire bond electrodes 46 regions having a lower dielectric layer 64 to avoid current crowding below and around the periphery of the electrode 46. The metal contacts the current spreading layer 28 and a ring has a width Wx. Wx is preferably 0.5 L _t < Wx < L _t . A lead 60 and bonding metal 62 are also shown.

Figure 11 shows a concentric shunt ring 65 surrounding one of the wire bonding electrodes 46 at a particular distance. The concentric shunt ring 65 reduces current crowding below and around the perimeter of the wire bond electrode 46. The width (Wr) of the concentric shunt ring 65 is proportional to L _t (preferably above 0.1 L _t and below L _t ) to provide a sufficiently low current resistance. The diameter (D) of the concentric shunt ring 65 is preferably at least 20% larger than the diameter of the wire bonding electrode 46.

Figure 12 is a top plan view of a top surface of a prior art LED die (similar to Figure 1) showing a metal shunt 66 surrounding the perimeter of the top surface of the LED die with a wire bond electrode 68 near a corner . Since the arms of the metal shunt 66 are close to each other at the corners, there will be current crowding near the corners, resulting in uneven light output and one of these zones may be overcurrent. The metal shunt configuration of Figure 13 can be used to substantially prevent the turn This current is crowded at the corner.

Figure 13 is a close-up view of one of the corners of an LED die showing the metal shunt 70 having a reduced width Wc at one of the corners to reduce current injection from each leg near the corner. Wc preferably less than L _t (e.g. 0.1L _t) desired to increase the contact resistance is generated near the corners of one of the substantially uniform distribution of current. The width of the remainder of the metal shunt is greater than L _t . Unlike the inner metal contacts, in the edge metal contacts, the current flows only from one side of the contact area, so the minimum width of 2L _t is not applicable here. Preferably, a wire bond electrode is positioned along a halfway of a metal shunt branch to avoid current crowding near a corner. Each corner will be similar to Figure 13.

The same technique will be used for the corners of any cross bus bars.

The metal contacts 42, 50, 56 in the intermediate region of the LED die are connected to the metal shunt 70 using the aforementioned narrow connectors 44, 48.

14 is a cross-sectional view of one of the LED dies in accordance with an embodiment of the present invention, wherein a corner mirror 76 is formed in each of the circular metal contacts 80 and the trenches 78 below the mesh connectors. Mirror surface 76 reduces light blocking by overlying circular metal contacts 80 and connectors and prevents high current density regions from being created under and below the respective circular metal contacts 80. The details of the construction of such mirrors 76 are found in U.S. Patent Application Serial No. 12/770,550, the entire disclosure of which is incorporated herein by reference.

The geometry of the mirror 76 can be tailored to increase light extraction efficiency. It is assumed that the top metal contact 80 (similar or identical to any of the foregoing metal contacts) can be used with a mirrored wall in a semiconductor positioned below the metal contact 80, such as It is depicted in Figure 14. The illustration shows the case where mirror 76 (usually a metal) penetrates into the semiconductor and spans one of the active layers 20 in the region below the metal contacts 80. The mirror 76 is covered by a transparent dielectric 84 to prevent electrical shorting between the layers. The inclusion of such mirror walls in the wafer enhances light extraction, but is not conducive to reducing the area of action that produces photons. Due to this trade-off, the width Ws of each metal joint 80 is preferably minimized and thus the number of mirrors is minimized. This translates into a minimization of the distance between the mirrors that maximizes light extraction.

Preferably, the pattern of the shunt layer should be designed to optimize the following performance-related aspects: - homogenization of current injection into the semiconductor active layer (eg, control of the distribution of metal contacts); - voltage drop across the shunt layer Minimization (eg using a thin metal connector); - maximizing light extraction (eg optimizing the size of the metal contacts and forming a mirror); - maximizing the action area (eg optimizing the size of the mirror).

While the invention has been shown and described with respect to the specific embodiments of the embodiments of the invention All such changes and modifications within the precise spirit and scope of the invention are covered by the scope of the claims.

10‧‧‧Light Emitting Diode (LED) Grains

12‧‧‧Anode

14‧‧‧ reflector

16‧‧‧First p-type layer

18‧‧‧p-type cladding

20‧‧‧Working layer

22‧‧‧n type cladding

24‧‧‧First n-type layer

26‧‧‧Second n-type layer

28‧‧‧current distribution layer

30‧‧‧ cathode

32‧‧‧Diversion Layer

36‧‧‧ Current

38‧‧‧Photon orbit

39‧‧‧emitting light pattern

40‧‧‧Split pattern

42‧‧‧Metal joints

44‧‧‧Connector

46‧‧‧ wire bonding electrode

48‧‧‧radial connectors

50‧‧‧Metal joints

54‧‧‧Metal joints

55‧‧‧LED dies

56‧‧‧Metal joints

57‧‧‧Center wire bonding point

58‧‧‧Connector

60‧‧‧ lead

62‧‧‧Joint metal

64‧‧‧ underlying dielectric layer

65‧‧‧Concentric Shunt Ring

66‧‧‧Metal shunt

68‧‧‧ wire bonding electrode

70‧‧‧Metal shunt

76‧‧‧With a mirror

78‧‧‧ trench

80‧‧‧Metal joints

84‧‧‧ dielectric

1 is a top plan view of one of the top surfaces of a prior art LED die showing a metal shunt pattern.

Figure 2 is a cross-sectional view taken along line 2-2 of Figure 1.

Fig 3 a graph of lines of one of the ohmic contact metal contacts the normalized and the normalized width, which is smaller than the display width 2L _t of the normalized result in increasing the contact resistance.

4 is a top plan view of a top surface of an LED die showing a metal shunt pattern in accordance with an embodiment of the present invention.

Figure 5 shows the shunt pattern of Figure 4 with a larger wire bond electrode positioned near the middle of the LED die surface to provide a substantially uniform current distribution.

Figure 6 shows the shunt pattern of Figure 5 with an additional radial connector interposed between the wire bond electrodes and various points.

Figure 7 is a top plan view of a top surface of an LED die showing a metal shunt pattern having a point that increases in size as the point increases from the wire bond electrode.

Figure 8 is a top plan view of a top surface of an LED die showing a metal shunt pattern having a point at which the density increases as the point moves further away from the wire bond electrode.

Figure 9 is a top plan view of a top surface of an LED die showing a metal shunt pattern having a square dot and an enlarged center wire bond.

Figure 10 is a cross-sectional view of one of the wire bond electrode regions having a lower dielectric layer to avoid current crowding under the electrodes.

Figure 11 is a top plan view of a top surface of an LED die showing a metal shunt ring pattern surrounding the wire bond electrodes to mitigate current crowding below and around the perimeter of the wire bond electrodes.

Figure 12 is a top plan view of a top surface of a prior art LED die (similar to Figure 1) shows a metal shunt around the perimeter of the top surface.

Figure 13 is an enlarged plan view of one of the corners of an LED die showing how current crowding near the corner can be avoided by reducing the width at the corners of the metal shunt. The same technique will be used for the corners of any cross bus bars.

Figure 14 is a cross-sectional view of one of the LED dies in accordance with one embodiment of the present invention, wherein a corner mirror is formed in the trench below each point.

40‧‧‧Split pattern

42‧‧‧Metal joints

44‧‧‧Connector

Claims (18)

A light emitting diode (LED) device comprising: an LED die having a top surface including a current spreading layer; and a metal electrode pattern on the top surface for conducting current through the LED The die is energized to the LED die, the electrode pattern comprising: a plurality of metal contacts on the top surface, such as having a transfer length L _t of the metal contacts of between about 2 and 10 times Width, where the transfer length is defined as Where R _s is the sheet resistance of the current spreading layer in units of ohms per square (Ω/□), and ρ _c is the contact resistivity of the interface between one of the metal contacts and the current spreading layer, in ohms The square meter (Ω.m ² ) is a unit; and a plurality of metal connectors that connect the plurality of metal contacts together, the metal connectors having a width of less than 2 L _t . The device of claim 1, wherein the plurality of metal contacts on the top surface have a width between about 2 and 5 times the transfer length L _t of one of the metal contacts. The device of claim 1, wherein a total area of one of the metal contacts is less than 2% of a light emitting surface of the LED die. The device of claim 1, wherein a total area of one of the metal contacts is less than 5% of a light emitting surface of the LED die. The device of claim 1, wherein the total area of one of the metal contacts is less than 10% of the light-emitting surface of one of the LED dies. The device of claim 1, wherein the metal contacts are substantially circular and the widths are the diameters of the metal contacts. The device of claim 1, wherein the metal contacts are polygonal. The device of claim 1, further comprising a wire bond electrode connected to one of the metal contacts by at least one of the metal connectors. The device of claim 8, wherein the metal connectors form a grid of one of parallel and vertical connectors. The device of claim 8 wherein the metal connectors extend radially from the wire bond electrode. The device of claim 8, wherein at least some of the metal contacts are increased in size as the metal contacts are further away from the wire bonding electrode. The device of claim 8 wherein the density of one of the metal contacts increases as the metal contacts are further away from the wire bond electrode. The device of claim 8, further comprising a dielectric layer between the wire bond electrode and the current spreading layer to reduce a current density between the wire bond electrode and the current spreading layer. The device of claim 13, wherein the wire bonding electrode extends around the dielectric layer over a distance Wx of one of the edges of the dielectric layer, wherein 0.5 L _t < Wx < L _t . The device of claim 8, further comprising surrounding a concentric shunt ring of the wire bond electrode at a specific distance to reduce current crowding below and around a perimeter of the wire bond electrode, wherein the concentric shunt ring and the wire bond electrode There are no such metal contacts between them. The device of claim 15, wherein one of the concentric shunt rings has a width between 0.1 L _t and L _t , and wherein one of the concentric shunt rings has a diameter that is at least 20% larger than a diameter of one of the wire bond electrodes. The device of claim 1 further comprising a metal shunt surrounding one of the perimeters of the top surface of the LED die, the metal shunt having a first width along an edge of the LED die and for reducing the LED The current density at the corners of the die is one of the narrower widths of the corners of the LED die. The device of claim 17, wherein the first width of the metal shunt along the edges of the LED die is greater than _Lt , and the metal shunt at the corners of the LED die is narrower width less than 0.1L _t to reduce the current density at the corner of the die such that the LED.