Patent Application
20110089347
1. Field of the Invention
In one of its aspects, the present invention relates to an ultraviolet radiation light emitting diode (LED) device. In another of its aspects, the present invention relates to a fluid treatment system comprising an ultraviolet radiation LED device.
2. Description of the Prior Art
Fluid treatment systems are known generally in the art.
For example, U.S. Pat. Nos. 4,482,809, 4,872,980, 5,006,244, 5,418,370, 5,539,210 and Re:36,896 (all in the name of Maarschalkerweerd and all assigned to the assignee of the present invention) all describe gravity fed fluid treatment systems which employ ultraviolet (UV) radiation.
Generally, such prior fluid treatment systems employ an ultraviolet radiation lamp to emit radiation of a particular wavelength or range of wavelengths (usually between 185 and 400 nm) to effect bacterial kill or other treatment of the fluid being treated. Conventional ultraviolet radiation lamps are so-called “low pressure” mercury lamps and “medium pressure” lamps.
The art in low pressure mercury lamps has evolved with the development of the so-called Low Pressure, High Output (LPHQ) and amalgam UV radiation lamps. These lamps have found widespread use in UV radiation water treatment systems, particularly those used for treatment of municipal drinking water and wastewater. As used herein, the term “low pressure” UV radiation lamp is intended to encompass conventional UV radiation lamps, LPHO UV radiation lamps and amalgam UV radiation lamps.
In recent years, an interest has evolved in light emitting diodes (LEDs) as an alternate source of ultraviolet radiation.
With respect to UV LEDs, the prior art approaches have revolved around grouping individual LEDs into lighting systems that would be used as a light source.
For example, International Publication Number WO05/031881 [Jensen] teaches a tubular LED light source that involves substitution of a cylindrical group of LEDs for a standard cylindrical lamp in a lamp sleeve in conventional fluorescent lighting.
A similar design for the UV LED light source in a portable water disinfection system is taught by International Publication Number WO04/028290 [Maiden].
United States Patent Application Publication US2005/0000913 [Betterly] teaches a fluid treatment system in which this scheme is inverted—i.e., a scheme wherein an outside cylinder of LEDs directs UV light inwards toward a transparent pipe where the water to be disinfected flows.
The disadvantage of the approaches taught by Jensen, Maiden and Betterly is the difficulty in achieving heat extraction from the individual LEDs arranged in their respective geometries. A further disadvantage of the approaches taught by Jensen, Maiden and Betterly is the low UV power densities possible with individual light sources. The low power density possible in a practical disinfection device such as that of Maiden results in low possible flow rates for disinfecting water. That is why this system is a small personal use system. An alternate reactor geometry of Betterly has a rectangular array of LEDs that are shown protruding from a rectangular weir (see FIG. 4 of Betterly)—this makes cleaning very difficult once fouling occurs in the disinfection reactor. FIG. 4 of Betterly is based on a standard epoxy encapsulant that would surround the LED chip giving the illustrated bullet-shaped profile. Epoxy encapsulation for a UV LED is not feasible due to the fact that a conventional epoxy encapsulant is susceptible to degradation over time upon prolonged exposure to UV radiation.
International Publication Number WO 05/31881 [Scholl] teaches a disinfecting lamp using semiconductors with AlGaN alloys. No practical demonstration of these lamps is presented. Scholl teaches that the current effectiveness of AlGaN LEDs “can be improved from the current 20% to 40%”. Scholl further teaches that the power density of a hypothetical LED of 1 W UV power output and 40% effectiveness with a 1 mm2 area is 40 W/cm2. This would compare favorably to standard mercury lamps with power densities of 0.04-1.5 W/cm2. However, there are two problems with this statement. First, the power density would only be 40 W/cm2 directly above the LED chip. Over the entire area of the device of a sample area of 1 cm2, the power density would drop to an average of 0.4 W/cm2. Second, the state of the art UV for output in UV LEDs emitting in the germicidal wavelength region of ˜240-280 nm is only ˜1 mW at 280 nm (see J. P. Zhang et al., “AlGaN-based 280 nm light-emitting diodes with continuous-wave power exceeding 1 mW at 25 mA”, Applied Physics Letters 85, p. 5532-5534, 2004).
Thus, the state of the art average power density for a UV LED is therefore ˜1 mW/cm2, which is much less than that of mercury lamps. In Zhang et al, the efficiency of the state of the art 280 nm LED is only 0.85% versus the 20-40% mentioned above.
Thus, despite the purported advances made in the art, there is an ongoing need for an actual UV LED that can be used as an efficient and effective disinfection light source.
It is an object of the present invention to obviate or mitigate at least one of the above-mentioned disadvantages of the prior art.
In one of its aspects, the present invention provides an ultraviolet radiation device comprising a base portion, a plurality of semiconductor structures connected to the base portion and an ultraviolet radiation transparent element connected to the plurality of semiconductor structures.
In one of its aspects, the present invention provides an ultraviolet radiation device comprising a base portion, an ultraviolet radiation transparent element disposed in spaced relation with respect to the base portion and at least one light emitting diode interposed between the base portion and the ultraviolet radiation transparent element; wherein: (i) the at least one light emitting diode is in direct contact with the ultraviolet radiation transparent element, or (ii) there is a spacing between the at least one light emitting diode and the ultraviolet radiation transparent element, the spacing being substantially completely free of air.
In another of its aspects, the present invention provides a fluid treatment system comprising a fluid inlet, a fluid outlet and a fluid treatment zone disposed therebetween, the fluid treatment zone comprising an ultraviolet radiation light emitting diode device, the ultraviolet radiation device comprising an ultraviolet radiation transparent element having a radiation emitting surface configured to be in direct contact with fluid in the fluid treatment zone.
a is a schematic of a direct contact UV LED array device, which comprises a series of LEDs;
a shows how the surface of the sapphire substrate could be modified to include a series of sapphire dome elements; and
A schematic for the basic structure of an ultraviolet (UV) light emitting diode (LED) 100 is illustrated in
The nature of the semiconductor material is not particularly restricted. Non-limiting examples of suitable semiconductor materials include GaN, InN, InGaN, AlInGaN, AlN, AlGaN and mixtures thereof. Throughout this specification, when reference is made to a specific semiconductor material, it is to be understood that this is for illustrative purposes only and should not be used to construe or limit the scope of the invention.
Typically, the insulating substrate may be constructed from sapphire (aluminum oxide). Metallic (or other electrically conductive) contacts are made for the negative (n) terminal 120, and the positive (p) terminal 125. Preferably, part of the semiconductor structure is etched away to contact the n-side of the device, since the substrate is insulating. Using the p-metallic contact 125 as a mirror, the light emitted from the active layer towards the p-side of the device can be reflected back so that it can be emitted out through the transparent substrate 115. The solid arrow in
In order to make electrical contact without blocking a portion of the emitted radiation produced by UV LED 100, it is preferred to use a “flip chip” contacting system—an example of such a system is shown in
This mounting system allows the UV radiation created by the LED to be emitted in an upwards direction through the substrate while concurrently allowing for electrical contacts.
In order for the device to perform at optimum efficiency, it is preferred that the active layer of the device create the UV light efficiently and that the radiation also escape the device through substrate 115. Since the refractive indices of the materials in the device are different, Snell's law applies and the maximum angle from the vertical Θc where light can be emitted is expressed as
Θc=sin−1 (n1/n2) [1]
for substances with indices of refraction n1 and n2. At angles greater than Θc, the light is trapped in the first layer by total internal reflection. In
If the critical angle is rotated about a line in the vertical direction, an escape cone is defined where all radiation inside the cone can escape into the upper layer. For the UV LED, radiation can be lost outside the escape cone defined by Θc1 and Θc2, although the radiation internally reflecting inside the sapphire substrate (angles >Θc2) can escape through the sides of the substrate provided the radiation is then directed upwards by an external mirror 112 through the window of the LED.
The extraction efficiency η of a LED is determined by the equation
η=R×[1−(1−(n1/n2)2)1/2]/2×ΣC, [2]
where R is the transmission across the interface which accounts for reflection loss, the central expression is the fractional solid angle formed by the escape cone, and the last expression accounts for the number of escape cones.
The solid angle (Ω) of the outer surface of a cone with radius R from the cone tip to the outer surface and angle θ from the line at the cone center to the edge of the cone is
Ω=area of cone surface/R2=2π(1−cos θ)
The fractional solid angle over 4π steradians in a sphere is therefore (1−cos θ)/2. From Equation 1 above, the cone formed by the critical angle has sin Θc=n1/n2. Using Pythagorean Theorem, cos Θc=(1−(n1/n2)2)1/2. Substituting this expression into the fractional solid angle yields
[1−(1−(n1/n2)2)1/2]/2
which appears in Equation 2 above. For the fractional solid angle emitted from the sapphire substrate to the air or water, the maximum solid angle will be a half-sphere or 2π steradians. The fractional efficiency will therefore be [1−(1−(n1/n2)2)1/2].
For a UV LED, the semiconductor material layer and substrate layer are optically thin, so multiple reflections are possible and the transmission value of R is ˜1. Also, there are 2 cones since the LED can emit light from the active layer emitting directly downwards from the substrate, or from light reflected from the p contact metal so ΣC=2. Equation 2 then reduces to
η˜[1−(1−(n1/n2)2)1/2]. [3]
It should be noted that this is also the same formula for the extraction efficiency from the sapphire layer to air or water if the internally reflected light in the sapphire cannot be collected, since the fractional efficiency is multiplied by 2 since the emission is into a hemisphere, and there is only one cone.
By way of example, the refractive index of GaN as a function of wavelength (Wurtzite structure—stable alloy) is shown in
From
Table 1 shows calculated values of the critical angles for the interfaces listed, as well as the calculated fractional solid angles at these same interfaces. The fractional efficiency can then be calculated for the entire three layer structure, where this value was set to the fractional solid angle of the GaN/dielectric interface if Θc2 is >45° (coded italics in the table). In this case, the escape cone for the top and the sides overlap and all the photons can escape through the interface. If Θc2 is <45° (coded underscored in the table), then the fractional solid angle of the two interfaces are multiplied together to give the fractional efficiency. These calculations were performed for either sapphire or SiO2 dielectrics; and air, water or epoxy upper layers. For a UV LED, a conventional epoxy cannot be used as an upper layer since epoxy may not be transparent to short wavelength light, and will degrade upon prolonged exposure to this light. Thus, the calculations with epoxy were included to compare results with a standard visible LED.
Table 1 illustrates that, for a GaN LED with a sapphire substrate, a 29% efficiency is expected for a visible LED using an epoxy coating. The present inventors have determined that, for a UV LED with a sapphire substrate, the same efficiency can be expected for the device where the upper layer is water, but is only 4.9% when the device is in air. It can be seen that there is a large advantage to be gained in efficiency if the upper layer in the structure is water. Thus, an aspect of the present invention relates to the provision of a UV LED in which the outer sapphire (or other ultraviolet radiation transparent surface) is in direct contact with water—i.e., elimination of the air gap between the layered LED structure and the water layer being treated.
A conventional UV light emitting diode device 10 is illustrated in
The prior art has taught that improvement can be made to the basic design by incorporating a light reflecting assembly 18 around the LED chip as shown in
There are a number of disadvantages for a UV LED made using this prior art approach.
First, a specially machined miniature reflector that effectively reflects UV light will be costly to produce. It is well known in the semiconductor industry that packaging costs are a major part of the total cost of the device and the need for a miniature reflector will significantly increase this cost. Thus, an alternate method of collecting the light outside of the escape cone of the sapphire substrate would be highly desirable.
Second, the principal manner by which heat may escape the device is downwards through the sub mount and bottom of the device container. A design for an LED that would allow an additional path for heat to be removed from the device would be very advantageous.
Finally, a device that can offer greater disinfection power density in UV watts would allow for higher flow rates of water to be disinfected. The actual device shown in
As described above, the escape cone from sapphire to water is much larger than the escape cone to air, with a 29% versus 4.5% fractional efficiency assuming all the light can escape to the water from sapphire with a >45° escape cone angle. A device which has the sapphire substrate in direct contact with the water to be disinfected would also provide an additional path for heat to escape the LED. A schematic of a direct contact UV LED array device 200 is shown in
The output of many UV LEDs 100 are available using UV LED array device 200, so that a higher total UV output power is achieved. The provision of many UV LEDs 100 will result in the production of a significant amount of heat. An advantage of UV LED array device 200, is that this excess heat may be efficiently dissipated by virtue of heat transfer via two separate pathways: heat transfer to the fluid (Arrow D) (e.g., a liquid such as water, although other liquids and gaseous fluids may be used in the context of the invention) and heat transfer to the bottom of the device (Arrow E). An additional feature of this device is that the provision of a substantially flat outer surface for sapphire substrate 115a facilitates cleaning thereof to removing fouling materials that may accumulate thereon at the surface fluid interface. More specifically, the substantially flat outer surface for sapphire substrate 115a will easily allow a wiping mechanism (chemical, mechanical or combined chemical/mechanical) to pass over the surface to remove fouling materials that would otherwise reduce the UV output of UV LED array device.
In
b illustrates a perspective view of UV LED array device 200 with sapphire substrate 115a removed for clarity.
c illustrates an enlarged isometric view of a single LED device in UV LED array device 200 illustrated in
The UV LED array can be manufactured by a semiconductor processing method. Initially, the AlGaN semiconductor (or other semiconductor) layers are deposited on a continuous sapphire substrate. Next, portions of the AlGaN layers are etched down all the way to the sapphire substrate to create an array of semiconductor structures comprising the AlGaN semiconductor (or other semiconductor) layers. The semiconductor structures created by this process are physically separated from one another but still bonded to a continuous sapphire substrate which will act as the radiation emitting window for the entire array. Metallic contacts are then formed on the appropriate layers and, if desired, large areas are then separated into individual arrays. A sub mount which contacts all the p and n contacts of the individual chips is then attached to the chips for a multi “flip-chip” design.
A possible modification to direct contact UV LED array device shown in
In the event that there is still an appreciable amount of light that is trapped within the sapphire substrate due to total internal reflection, the surface of the sapphire substrate could be modified as shown in
The modification to the surface could also be present in a top quartz layer 145 in contact with sapphire layer 115a as shown in
The improvements in efficiency, heat extraction and cleaning associated with the present ultraviolet radiation device have been mentioned above.
A further improvement to the total possible power density also accrues from the present ultraviolet radiation device. Zhang et al. teach that processing of a conventional UV LED yields 1 mW power for a chip with an area of 2×104 cm2. Thus, 50% coverage of a 1 cm2 area would give an array with 2,500 LED chips. At 1 mW power each, the UV LED array would have a power of 2.5 W and power density of 2.5 W/cm2. This compares favorably with the power densities of mercury lamps, and does not assume any increase in the electrical efficiency of the electrical device beyond the original 0.85%. Improvements in efficiency using the features of this invention will increase the power density. Practically, the total power consumption of such an array would be ˜6V×0.12 A/chip [6]×2,500 chips=300 W. This would be a large amount of heat to extract for a 1 cm2 area, so reduction of the chip density to 5% would give a total UV power of 250 mW (250 mW/cm2 power density) and a power consumption of 30 W. Extraction of heat resulting from 30 W is reasonably achievable, and the power density of 250 mW/cm2 still compares favorably to that of low pressure mercury lamps. It is believed that improvements in efficiency will increase the UV output at a constant power consumption of 30 W. The embodiment of the present ultraviolet radiation device in the form of an array could be put into practice using a number of discrete UV LEDs with the substrate window or the substrate/quartz window in contact with the water to give increased light coupling efficiency and heat extraction, but the array embodiment would be preferred to maximize the UV power density.
The present ultraviolet radiation device may be used in a fluid treatment system that capitalizes on the higher UV power densities possible with UV LED arrays and the more efficient heat extraction possible with the present ultraviolet radiation device. Also, it is believed that the efficiency of this array should be better than that of a system using discrete LEDs in a container, since a direct contact between the light emitting surface of the LED and the fluid (e.g., water) exists.
All publications, patents and patent applications referred to herein are incorporated by reference in their entirety to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety.
67.00
41.47
83.40
50.55
33.75
56.44
The present application is a continuation of U.S. patent application Ser. No. 12/131,485, filed Jun. 2, 2008, now U.S. Pat. No. 7,842,932, issued Nov. 30, 2010, which claims the benefit under 35 U.S.C. §119(e) of provisional patent application Ser. No. 60/924,849, filed Jun. 1, 2007, the contents of all incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60924849 | Jun 2007 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12131485 | Jun 2008 | US |
| Child | 12955212 | US |
