BACKGROUND OF THE INVENTION

[0002] The present invention relates to lighting modules comprising light emitting diodes (LEDs), and in particular to such modules comprising micro-reflector cavities to improve the illumination efficiency of such lighting modules.

[0003] LEDs are semiconductors that convert electrical energy into light. Since LEDs generate relatively little heat compared to other common forms of lighting, such as incandescent lights, the energy conversion process performed by LEDs is quite efficient. This is a highly desirable trait in lighting systems to be used for illumination, since excessive heat production not only wastes electricity, but may also require extensive heat dissipation efforts, and may even raise safety concerns depending upon the fixture installation. Some of the other advantages that make LEDs desirable for illumination applications include their small size; their relatively high radiance (that is, they emit a large quantity of light per unit area); their very long life, leading to increased reliability; and their capacity to be switched (that is, turned on and off) at very high speeds.

[0004] While visible light LEDs have been applied in a number of fields since their invention in 1960, they have been used for illumination applications only relatively recently. One of the primary limitations in the use of LEDs in this field has been the difficulty of producing white light. White light consists of a mixture of light wavelengths across the visible light spectrum. Traditional LEDs cannot produce white light; instead, each LED can produce only light in one very narrow frequency band. It is well known that the combination of light in the three primary colors of red, green, and blue will produce white light. In fact, any color of light may be produced by the appropriate combination of light in these three colors. While red and green LEDs have been commercially available for decades, the blue LED was not developed until 1993, when it was introduced by the Nichia Corporation of Japan. By combining these traditional red, green, and blue LEDs in a tightly coupled pattern, a crude form of white light could then be produced. By varying the relative intensity of the light emitted by the red, green, and blue LEDs, one could alter the color of light produced, thereby providing a light source that will generate light of any color desired.

[0005] An alternative method of producing white light, developed by the Nichia Corporation in 1996, is the coating of a blue LED with a white phosphor. The blue LED stimulates the phosphor to generate a broad band of visible light emissions, thereby producing white light. This method suffers from the limitation that the frequency band of light produced is fixed, and cannot be altered to produce different lighting effects from the same LED. This method is therefore inappropriate for applications where different colors of light or lighting effects may be desired.

[0006] In addition to the problem with producing white light, the other primary limitation on the use of LEDs for illumination applications has been their brightness, which historically was far below that of typical incandescent and fluorescent light sources. By 1997, however, the Nichia Corporation, along with Texas Instruments Incorporated of Dallas, Tex., were producing LEDs of sufficient brightness for many illumination applications. It thus became possible to provide complete illumination solutions using only LEDs in certain applications, such as relatively small, indoor areas.

[0007] As already explained, a very simple system for producing white light with LEDs could involve the application of a pre-set current to a combination of red, green, and blue LEDs. It would be possible with such a system to emulate, for example, the color of light produced by daylight or by a typical incandescent bulb. Such a simple system would not, however, allow the user to take advantage of the many opportunities for temperature variance made possible by the use of an LED illumination system. (It should be noted that light color is often referred to as its “color temperature” or simply “temperature,” corresponding to the temperature of a black body that would produce light of that color measured in degrees Kelvin.) Since both temperature and intensity of the light produced by an LED illumination system may be varied simply by varying the amount of electrical current applied to the red, green, and blue LEDs in the system, many desirable illumination effects become possible that would not be available with incandescent lights. For example, an illumination system might include settings to emulate ambient lighting conditions at different times of day. Or the system might allow for variance in the light temperature depending upon application, such as applying a “cold” blue-tinged light for reading purposes, while allowing a “warm” red-tinged light setting to be chosen at meal times. Far more subtle and complex effects are possible. In order to take advantage of such flexibility offered by an LED illumination system, however, some form of electronic control system is required.

[0008] The mixing of red, green, and blue LED lights to produce lighting effects is known. For example, U.S. Pat. No. 5,420,482, issued to Phares, teaches a controlled lighting system that includes a set of light elements each having a control unit. The control units are individually addressable along a data bus. Information packets may be sent to each control unit by addressing each packet to match the address of the control unit. The data packets may contain information necessary to manipulate the output level of each of the light elements controlled by a particular control unit. In this way, the temperature and intensity of the light produced by each of the light elements may be manipulated by the use of digital information packets sent along a control bus. The system can thus produce an overall light output of varying temperature and intensity in response to digital signal inputs.

[0009] U.S. Pat. No. 6,016,038, issued to Mueller et al. and assigned to Color Kinetics, Inc. of Boston, Mass., teaches a method of controlling the intensity and temperature of an RGB LED system using pulse-width modulated (PWM) signals generated by a microcontroller. PWM is a well-known technique for controlling analog circuits with the output of a microprocessor or other digital signal source. A PWM signal is a square wave modulated to encode a specific analog signal level. In other words, the PWM signal is fixed frequency with varying width. The PWM signal is still a digital signal because, at any given instant of time, the full direct current (DC) supply current is either in the “on” or “off” state. The voltage or current source is thus supplied to the analog load by means of a repeating series of on and off pulses. The on-time is the time during which the DC supply is applied to the load, and the off-time is the period during which that supply is switched off. Given a sufficient bandwidth, PWM can be used to encode any analog value.

[0010] When the power to an LED is rapidly switched on and off, variance of the length of time during the on and off modes gives the effect of variance of the intensity of the light that is produced. As a result, a PWM signal can be used in place of a varying DC current to achieve intensity variance in an LED. PWM has numerous advantages over traditional analog control systems, including less heat production than analog circuits of similar precision, and significantly reduced noise sensitivity. Given the significant advantages that PWM control offers in communications and control systems applications, many microprocessors and microcontrollers produced today include built-in PWM signal generation units that may be directly applied to illumination control systems.

[0011] Another problem with prior attempts to use LEDs for illumination applications is the development of appropriate reflectors to efficiently direct or focus the LED light on the desired area. Traditional indicator or “bulb” LEDs are formed of a small LED semiconductor chip mounted in a reflector cup. The cup and LED chip are entirely encased in epoxy, which also serves as the lens of the device. Electrical leads pass from the cup through the epoxy and out of the base of the device. While sufficient for instrumentation purposes, this arrangement is not feasible for illumination applications, however, due to the high power and heat dissipation requirements necessary to generate sufficient light intensity for illumination. Typical indicator LEDs are limited to operating temperatures no greater than 120° Fahrenheit, and input power no greater than 100 milliWatts.

[0012] Recent improvements in LED design have led to significant advances in power capacity over indicator LEDs. For example, Lumileds of San Jose, Calif. now produces LED lights composed of a large LED semiconductor die mounted on a heat-sink sub-mount formed of copper or aluminum. The LED semiconductor is encased in a soft gel, which is capped with a clear lens formed of a high-temperature plastic material. These types of LEDs are capable of handling input power levels in excess of 1 Watt.

[0013] A limitation on these new high-power LED die designs, however, is that much of the light emitted from the LEDs is either lost or not directed in the desired direction. Although light intensity of LED sources has greatly improved, it still lags behind traditional sources, and the available light output must be maximized in order to provide sufficient light for many practical illumination applications. When an LED is forward biased, a percentage of the injected carriers that recombine in the vicinity of the P-N junction in the device result in the generation of photons. Because of power loss mechanisms such as absorption, Fresnel losses, and internal reflection, not all of the generated light is able to emerge from the interior of the LED semiconductor. In order to create a highly efficient LED based lighting system, all of these power loss mechanisms must be minimized.

[0014] What is desired then is a means of mounting an LED die that maximizes its efficiency by directing its light in a manner optimized for illumination applications. In addition, it would be desirable that such a mounting means be capable of dissipating the heat produced by such devices in an efficient manner so that performance of the LED is not effected by excessive heat build-up. Finally, it would be desirable to develop such an LED mounting means that could be cost-effectively manufactured.

[0015] As will be explained hereafter, the present invention utilizes anisotropic wet etching techniques in silicon wafers to overcome the limitations of prior art LED die mounting and reflecting means. The basic principles of wet chemical anisotropic etching of silicon are well known in the art. This technology is one of the most popular methods of constructing microelectromechanical systems (MEMS), sometimes referred to as “nano-technology.” MEMS are, in essence, mechanical devices that are constructed on a scale similar to that of traditional solid state electronic components. MEMS devices available today include accelerometers, chemical and biological sensors, and microfluidic devices such as valves and pumps. To the inventor's knowledge, however, the wet-etching technique has not been utilized in the formation of LED die submounts, and the potential advantages in doing so have remained unrealized. The present invention overcomes the limitations of the prior art and achieves the objectives set forth herein as described below.