Packaged UV-LED device with anodic bonded silica lens and no UV-degradable adhesive

A packaged UV-LED device comprises a die carrier member having a cup-shaped recess, a fused silica lens member that is anodic bonded to the die carrier member, and a UV-LED die that is flip-chip mounted within a sealed cavity formed by the carrier member and the lens member. The carrier member involves a unitary cup member fashioned in an economical way from monocrystalline silicon wafer material. A dielectric/aluminum reflector that is effective for UV radiation and that does not degrade and overheat is disposed on the sidewalls of the recess. The lens member is anodic bonded to a silicon surface of the rim of this unitary cup member at a time when the UV-LED die is disposed in the recess. The anodic bonding is done in such way that the die is not damaged and such that the entire packaged UV-LED device includes no UV-degradable adhesive.

TECHNICAL FIELD

The described embodiments relate to Ultraviolet Light Emitting Diode (UV-LED) assemblies.

BACKGROUND INFORMATION

High power UV-LEDs are increasingly popular and are finding new applications. For example, high output power UV-LED devices are now being used in UV-curing and UV-printing applications.FIG. 1(Prior Art) is a top-down diagram of one commercially available UV-LED product50. A packaged UV-LED51is mounted on a star board Metal Core Printed Circuit Board (MCPCB) substrate52.FIG. 2(Prior Art) is a diagram of the packaged UV-LED51ofFIG. 1. The UV-LED die53is mounted on a ceramic substrate54. A glass lens55that is inexpensive and tolerant of the high temperatures involved is employed. The overall assembly50ofFIG. 1is available as part number A00X-UV4 from LEDSupply, P.O. Box 326, 44 Hull Street, Randolph, Vt. 05060. The packaged UV-LED device51is a C3535U-UNx1 series LED available from SemiLEDs Corporation of 3F, No. 11, KeJung Road, Chu-Nan Site, Hsinchu Science Park, Chu-Nan 350, Miao-Li County, Taiwan, ROC. To increase directionality of the emitted UV radiation, a heat tolerant 20 mm polycarbonate lens made by Carclo Optics of Aylesbury, England, is sold along with the UV-LED device ofFIG. 1. This secondary optic is mounted on top of the MCPCB substrate52such that legs of the secondary optic sit into corresponding holes on the MCPCB. UV-LED products such as these are adequately inexpensive and are believed to work well in their intended environments.

SUMMARY

Rather than using a desirably inexpensive glass lens or borosilicate glass lens, which absorbs UV radiation and is typically about half as expensive as a lens made of fused silica, a less common fused silica lens is employed in a novel packaged UV-LED device. Also, rather than using an inexpensive ceramic submount such as in the prior art device ofFIG. 1andFIG. 2, a novel die carrier member is employed. This novel die carrier member includes a unitary silicon cup member. A cup-shaped recess is formed into the upper surface of this unitary silicon cup member. In one example, the unitary silicon cup member is fashioned from homogenous monocrystalline silicon of a silicon wafer. The crystal lattice orientation of monocrystalline silicon atoms in the silicon of the unitary silicon cup member is employed during an anisotropic wet etching step to form the cup-shaped recess in an economical way. An aluminum reflector is then formed on the inside sidewalls of the cup-shaped recess. A thin layer of dielectric material is then formed over the aluminum in order to improve light extraction efficiency and to increase the overall total reflectivity of the reflector. In one example, this thin dielectric layer has a thickness of approximately one quarter wavelength of the wavelength of the UV radiation to be emitted from the UV-LED die. In another example, this thin dielectric layer has a thickness of approximately five quarters of the UV radiation wavelength. Regardless of the exact thickness of the thin dielectric layer, the dielectric has a thickness over the aluminum that is less than twice the UV radiation wavelength. The dielectric material has an index of refraction in a range of from about 1.4 to 2.5 and may, for example, be silicon nitride or silicon dioxide. The dielectric is preferably deposited using sputtering, but it may also be deposited by ebeam deposition or chemical vapor deposition.

There are no wirebonds on any top surface of the die carrier member, but rather metal through-silicon vias (TSVs) that extend vertically through the unitary silicon cup member are filled with metal. The resulting vertically-extending metal vias provide electrical contact between surface mount pads on the inside bottom of the cup-shaped recess and corresponding metal terminals on the bottom side of the overall die carrier member.

After the UV-LED die has been flip-chip surface mounted onto the pads on the bottom of the cup-shaped recess, the fused silica lens is bonded in a low temperature anodic bonding process down onto a silicon surface of the unitary silicon cup member of the die carrier member, thereby sealing the UV-LED die in an air-filled cavity. The thin dielectric has two purposes. First, it provides electrical and physical isolation for the aluminum reflector. Second, it increases the total reflectivity of the aluminum reflector. By virtue of the novel use of low temperature anodic bonding, there is no epoxy and no silicone and no other potentially UV-degradable adhesive material disposed between the lens and the unitary silicon cup member of the die carrier member. There are other expensive bonding techniques that might be usable for fixing the lens to the die carrier, but advantageously due to the use of anodic bonding the expensive materials involved in these other techniques need not be, and are not, provided. For example, gold/tin eutectic bonding materials are not employed in bonding the lens, and there is neither any gold or any tin or any other metals in contact with any part of the fused silica lens.

The anodic bonding is carried out at a temperature that is higher than 300 degrees Celsius so that the temperature is high enough to facilitate anodic bonding when a DC voltage of about 300-500 volts is present between the lens and die carrier member. The temperature employed during anodic bonding is, however, kept lower than 350 degrees Celsius. By keeping the temperature below 350 degrees Celsius, and by keeping the high temperature step to a duration of not more than ten minutes, the UV-LED die is not damaged in the anodic bonding step.

The resulting packaged UV-LED device is robust in that it employs no UV-degradable adhesives. For a device of this robustness, it is relatively inexpensive to manufacture. After the packaged UV-LED device has been manufactured, it is then typically mounted onto a star board MCPCB substrate. The packaged UV-LED device along with its star board MCPCB substrate are referred to together here as an UV-LED assembly.

Further details and embodiments and methods and techniques are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the invention, examples of which are illustrated in the accompanying drawings. In the description and claims below, when a first object is referred to as being disposed “over” or “on” a second object, it is to be understood that the first object can be directly on the second object, or an intervening object may be present between the first and second objects. Similarly, terms such as “upper”, “top”, “up”, “down”, “across”, “horizontally”, “laterally” and “bottom” are used herein to describe relative orientations between different parts of the structure being described, and it is to be understood that the overall structure being described can actually be oriented in any way in three-dimensional space.

FIG. 3is a top-down diagram of an Ultraviolet Light Emitting Diode (UV-LED) assembly1in accordance with one novel aspect. UV-LED assembly1includes a packaged UV-LED device2and a “Star Board” Metal Core Printed Circuit Board (MCPCB) substrate3. There are two “+” positive supply voltage metal terminals4and5and two “−” negative supply voltage metal terminals6and7. A positive supply DC voltage with respect to the potential on the terminals6and7is driven onto one or both of the positive supply voltage terminals4and5. A DC current flows onto the assembly1via one or both of the positive supply voltage metal terminals4and5, through the packaged UV-LED device2, and out of the assembly1via one or both of the negative supply voltage metal terminals6and7. This current causes a UV-LED die within the UV-LED device2is emit ultraviolet radiation. In the present example, the UV-LED die emits radiation having a center wavelength in a range from 250 nanometers to 300 nanometers, and in particular in one embodiment the center wavelength of emissions is about 280 nanometers.

FIG. 4is a top-down diagram of the star board MCPCB substrate3with the packaged UV-LED device2not shown so that the P positive supply voltage pad8, the thermal pad9, and the N negative supply voltage pad10are in view. The top surface of the star board MCPCB substrate3that is not one of the terminals4-7or one of the pads8-10is a top surface of a thin insulative solder mask material11. The star board MCPCB substrate3is also called a “submount”. The star board MCPCB substrate3may be made using MOCVD (Metal-Organic Chemical Vapor Deposition) techniques whereby a layer of metal can be deposited, followed by a layer of a dielectric, followed by another layer of metal. A metal layer is deposited by depositing thin layers of atoms in a chemical vapor deposition process. The star board MCPCB substrate3may also be made by photolithographically etching and laminating layers together in a process similar to that used to make FR4 printed circuit boards.

FIG. 5is a cross-sectional side view of the star board MCPCB substrate3taken along sectional line A-A′ ofFIG. 4. The entire bottom portion of the substrate3is a copper structure and layer12. This copper structure12extends upward to the upper surface13of the substrate3in the area of the thermal pad9. In addition to copper structure12, the substrate3includes two other copper structures14and15. Part of the upper surface of the copper structure14is left exposed by a hole in the solder mask11. This part of the copper structure14is the P metal pad8. Part of the upper surface of the copper structure15is left exposed by a hole in the solder mask11. This part of the copper structure15is the N metal pad10. The copper structures14and15are separated from the copper structure and layer12by an intervening layer16of dielectric material. The copper structures may be plated with thin layers of another metal such as gold.

FIG. 6is a top-down diagram of the packaged UV-LED device2. The top of the packaged UV-LED device2is the top semispherical surface of a fused silica lens member17. Fused silica is a noncrystalline (glass) form of silicon dioxide. The fused silica may be doped with sodium in order to promote future anodic bonding with silicon.

FIG. 7is a top-down diagram of the packaged UV-LED device2with its fused silica lens member17removed so that the top of the underlying die carrier member18is in view. The top of the die carrier member18forms a cup-shaped recess. The UV-LED die19is mounted in this recess in the center of the die carrier member18.

FIG. 8is a top-down diagram of the die carrier member18with the UV-LED die19removed so that the underlying first metal pad20and second metal pad21are in view.

FIG. 9is a cross-sectional side view taken along sectional line B-B′ ofFIG. 6. The die carrier member18includes a unitary cup member22, a thin insulative silicon dioxide layer23, an aluminum reflector24, a dielectric layer25, the first metal pad20, the second metal pad21, a first bottom side terminal26, a second bottom side terminal27, a third bottom side thermal terminal conduction46, a first metal via28that couples the first bottom side terminal26to the first metal pad20, and a second metal via29that couples the second bottom side terminal27to the second metal pad21. The unitary cup member22has four flaring sidewall portions39-42and a central base portion45. The unitary cup member22is “unitary” in the sense that all these “portions” are portions of a single piece of monocrystalline silicon. The first bottom metal terminal26, the first metal via28, and the first metal pad20are three different parts of the same metal structure. The first metal via28extends from the first metal pad20, through the base portion45of the unitary cup member18, and to the first bottom side terminal26. Likewise, the second bottom metal terminal27, the second metal via29, and the second metal pad21are three different parts of the same metal structure. The second metal via29extends from the second metal pad21, through the base portion45of the unitary cup member18, and to the second bottom side terminal27. The UV-LED die19is flip-chip surface mounted to the first and second metal pads20and21by amounts of solder30and31. More specifically, a first terminal and pad32of the die19is soldered to by amount of solder30to the top of the first metal pad20, and a second terminal and pad33of the die19is soldered by amount of solder31to the top of the second metal pad21. Horizontal dashed line34represents the active semiconductor layers of the flip-chip mounted die19. The upward-facing top surface47of the die is a surface of the sapphire substrate die portion35upon which the active layers were deposited. This sapphire is substantially transparent to the UV radiation emitted from the active layers. The unitary cup member22has a rim36. This rim36extends peripherally around the cup recess. The fused silica lens member17is anodic bonded directly to the rim36so that the UV-LED die19is disposed inside a sealed cavity37formed by the die carrier member18and the fused silica lens member17.

FIG. 10is a flowchart of a method100of manufacturing the packaged UV-LED device2. In a wet etching step, a two-dimensional array of cup-shaped recesses is formed into the top of a single monocrystalline silicon wafer. The 54.7 degree angle (the angle is illustrated inFIG. 9) of the four flaring inside sidewalls of the recess is due to the wet etching of a properly oriented monocrystalline silicon. The silicon atoms on each sidewall of the recess have the [1,1,1] orientation, whereas the silicon atoms on the surface of the bottom of the recess have the [0,0,1] orientation. The hole in the photomask through which this wet etching takes place is a square hole. Techniques and methods for etching recesses of the shape pictured inFIG. 7, having 54.7 degree sloped sidewalls, into silicon are known and employed in the MEMS industry (MicroElectroMechanical Systems industry). A heated potassium hydroxide (KOH) solution may be employed as an etchant in this wet etching step.

After the wet etching step, two downwardly-extending cylindrical via holes are formed (for example, by etching or laser drilling or ion beam milling) through the base portion45at the bottom of each of the cup-shaped recesses. The via holes can be formed from the wafer backside such as in another etching step different from the etching of the topside etching of the wafer. Regardless of whether these via holes are formed from the wafer topside or wafer backside or a combination of the two, each via hole extends all the way through the silicon wafer. Next, a thin layer of silicon dioxide is formed over the entire silicon surface of the wafer structure so as to form silicon dioxide layer23. This thin silicon dioxide layer23may be a thermal oxide so that the inside surfaces of the via holes are covered with silicon dioxide.

The metal structures26,28,20and27,29,21and46ofFIG. 9are then formed. Copper may, for example, be plated over the entire bottom surface and over the entire top surface, and then may be selectively removed to leave the metal structures illustrated inFIG. 9. Copper is employed for its thermal conduction properties for these metal structures.

Next, an aluminum reflector is formed in each cup-shaped recess. Aluminum reflector24is one of these reflectors. Sputtering, ebeam deposition, or chemical vapor deposition may be employed to deposit the aluminum. Once deposited, the aluminum can be patterned and etched, or a lift off process can be employed. In one example, the aluminum reflectors are 50 nanometers thick. As long as the aluminum thickness is greater than about 40 nanometers thick, it can fully reflect UV radiation. Accordingly, the metal structures26,28,20,27,29,21and46are made from a metal that is different from the metal from which the reflector24is made.

After the formation of the aluminum reflectors, a thin dielectric layer is formed directly on and over the aluminum reflector in each cup-shaped recess. The thin dielectric is formed so that it does not cover the metal at the bottoms of the recesses. In one example, the thin dielectric is silicon nitride that is approximately one quarter wavelength thick where the dielectric is disposed on aluminum. The wavelength is the wavelength of the UV radiation emitted by the UV-LED die, which is about 280 nanometers. In particular, the silicon nitride layer over the top of the aluminum is about 50 nanometers thick. After deposition of this thin silicon nitride layer, the top of the wafer can be resurfaced and cleaned, such as by chemical mechanical polishing, so that the rim36is a clean and planar silicon surface.

At this point, the wafer has the structure of many instances of the die carrier member18ofFIG. 9adjoining each other in the wafer structure. This overall wafer structure104is called a “die carrier array structure”. UV-LED dice are then flip-chip mounted (step101) to this die carrier array structure104such that there is one UV-LED die disposed in each of the cup recesses of the die carrier array structure104. The bottom surface of a wafer-shaped fused silica lens array structure105is cleaned and resurfaced so that its bottom surface is clean and is as planar as possible. The wafer-shaped fused silica lens array structure105is then anodic bonded (step102) to the carrier array structure104such that each UV-LED die is disposed in a separate sealed cavity formed by the die carrier array structure (from below) and the fused silica lens array structure (from above).

FIG. 11is an illustration showing how the fused silica lens array structure105is brought down and into contact with the carrier array structure104in this low temperature anodic bonding step. Arrows106represent the pressing of the two structures104and105together. More specifically, the structure104is disposed on a chuck (not shown) and a top tool (not shown) is pressed downward on the structure105from the top, thereby forcing the two structures104and105together. The top tool has a downward-facing surface that is conformal to the shape of the upward facing surface of the fused silica lens array structure105. Each of the two joining surfaces (the bottom surface of structure105and the upward facing planar rim surface of structure14) has a surface roughness that is less than 100 nmRa (Ra<100 nm). The structures104and105are heated to a temperature of at least 300 degrees Celsius and less than 350 degrees Celsius, and are held at this temperature for a time of ten minutes while a DC voltage of about 400 volts is disposed between the two structures. The high temperature assists the dissociation of alkali oxides present in the structure105, thereby creating alkali ions and oxygen ions. The silicon of the die carrier array structure has a positive 400 volt potential with respect to the fused silica lens array structure. As a result of these temperature, pressure and voltage conditions, there is a migration of oxygen ions to the boundary between the two structures104and105. This causes an irreversible bond to form between the two structures104and105without the use of any epoxy or silicone or other adhesive. There is no epoxy or silicone or other potentially UV-degradable adhesive disposed anywhere between the rims of the die carrier members of the die carrier array structure104and the fused silica lens array structure105. The result of this anodic bonding is a die carrier/lens array structure107.

Although the die carrier member18ofFIG. 9in the example described above involves a unitary cup member22and is fashioned from a homogenous monocrystalline silicon wafer, in another example it is not fashioned from a silicon wafer. Rather, the die carrier member involves a ceramic (for example, AlN) substrate over which a molded EMC (Epoxy Molding Compound) structure is disposed, where the molded EMC structure is molded to form the cup-shaped recesses. The bottom surface of each of the cup-shaped recesses in this case is a top surface of the ceramic substrate. The dielectric/aluminum reflector is formed on the flaring EMC sidewalls, and metal is deposited and patterned to form the pads, vias, and bottom side terminals. In yet another example, the die carrier member involves an FR4 substrate board portion. In yet another example, the die carrier member is fashioned from a unitary piece of glass. The glass has been formed and/or is etched and/or is machined to have the desired shape with the cup-shaped recesses. Although not illustrated in the example ofFIG. 9described above, one or more of the metal structures may include surface plating layer and/or may include a barrier metal layer.