WO1991003085A1

WO1991003085A1 - Improved laser diode package

Info

Publication number: WO1991003085A1
Application number: PCT/US1990/004175
Authority: WO
Inventors: Marvin D. Bausman, Jr.
Original assignee: Cray Research, Inc.
Priority date: 1989-08-21
Filing date: 1990-07-25
Publication date: 1991-03-07

Abstract

A semiconductor mounting package (10) is provided which permits the active p-n junction (27) of a light emitting semiconductor (18) to be operated in a liquid heat transfer fluid (16). The package (10) provides optical coupling for extracting light energy.

Description

IMPROVED LASER DIODE PACKAGE

DESCRIPTION OF THE PRIOR ART

The invention relates to a package for a semiconductor device and more particularly, to a liquid cooled package for use with a light emitting laser diode. Semiconductor devices are usually operated in sealed packages to prevent contamination of the semiconductor. Various types of packaging have been adopted for this purpose. In operation the package performs several functions. For example, active semiconductors which have electrical power supplied to them typically use the package as a radiating surface to remove heat generated by the semiconductor device. In the case of high power semiconductor diodes it is conventional to mount the P-N junction structure so that the anode is electrically and thermally coupled to a metal stud. The stud then operates as a heat sink and an electrical connection.

In light emitting diodes, the P-N junction is forward biased and light is generated at this junction. The light is emitted at right angles to the junction plane in the case of the LED and parallel to the junction plane in the case of a LASER diode. Depending upon the application, a Laser diode may be supplied in a package with a transparent window or with a fiber optic pig-tail. In either case the transparent medium is aligned with the plane of the p-n junction and is used to extract the optical power from the diode.

Monitoring of the package temperature is conventional in this art. LASER diodes are susceptible to thermal runaway and as a consequence temperature based feedback is used to control drive current. Cooling of the P-N junction in prior art packaged has been through the use of thermal conduction from the semiconductor to a heat sink. In some cases thermoelectric cooling or Peltier effect devices are applied to the heat sink to control the temperature of the junction. SUMMARY OF THE INVENTION In contrast to this teaching the present invention provide a package which provides direct liquid cooling of the P-N junction. Although the package is useful for LED applications, the package is particularly useful when used with LASER diodes. The package includes a closed capsule which contains liquid cooling medium. The package may include a window or other device for coupling the optical energy out of the package.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings in which like reference numerals indicate corresponding parts throughout, FIG. 1 is a diagram of an embodiment of the invention and,

FIG. 2 is a graph of the power output characteristics of invention.

FIG. 3 is a graph computed from the data presented in Figure 2.

FIG. 4 is an alternate embodiment of the invention. FIG. 5 is an alternate embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 shows a package 10 which is generally cylindrical in form. This capsule contains a cooling fluid 16 which substantially fills the interior of the package or cavity. A small port 14 may be provided to fill the capsule. The preferred cooling fluid is liquid fluorocarbon. The preferred coolant is manufactured under the trade name Fluorinert, and this product may be purchased from 3M Company, St. Paul. Minnesota. The preferred member of the Fluorinert family depends upon the power dissipation required by the semiconductor chip 18. It is highly desirable to prevent boiling of the coolant at the P-N junction and this requirement can be used to select the specific heat capacity and boiling point for the Fluorinert. Preferably, the Fluorinert exhibits a specific heat of 0.25 g-cal/g-C and has a boiling point greater than 96 C and less than 216 C. There are numerous advantages derived in preventing the Fluorinert from boiling. If boiling occurs, vapor bubbles formed thereby optically distort beam quality and beam direction due to the lensing effects of the differences in the index of refraction between the liquid and the vapor. The vapor bubbles are created randomly and the distortion induced thereby renders the laser beam and its optical properties uncontrollable. Further, formation of vapor bubbles occurs first and most vigorously on the optical facets, i.e., the surfaces where the laser beam exits the die, as these facets are the highest temperature surfaces on the die. As the vapor bubbles form, they stick to the facets until they grow to a size that allows their buoyancy to overcome surface tension. This creates an insulating vapor layer at the points of highest temperature on the surface of the die, i.e., the facets, which is exactly the thing one wishes to avoid if one is trying to cool the laser efficiently. Additionally, the location of bubble formation makes the optical distortion even more pronounced as the bubbles form at the points the optical beams exit the die.

In operation the LASER diode chip 18 is forward biased by supplying electrical power to the terminals 20 and 21. A photo diode 22 may be provided to monitor the optical power output. This sensor may also form a sensor to for a drive current control system. As shown in figure 1 the diode chip is mounted so that it is aligned with a window 24. The support 28 may form an electrical connection for the diode as shown in this Figure. The optical energy is emitted from the chip normal to the plane of P-N junction. The figure illustrates that the P- N junction 27 is separated from the heat sinking support 28 by the bulk P channel material 29. However the junction 27 is substantially completely surrounded by the Fluorinert which fills the cavity. In general the optical power output is a function of the forward current. Figure 2 is a graph drawn from experimental data depicting the optical power output as a function of forward drive current. The first curve 40 is taken from a Mitsubishi ME-4402 LASER diode which has the P-N junction operated in a gaseous atmosphere. The second curve 42 is taken with the same diode however the P-N junction is submerged in Fluorinert. The graph set forth as Figure 3 is computed form the Figure 2 data set and displays a curve 44 which shows the difference in output power as a function of the input current supplied to the LASER diode. This figure demonstrates that the output power from the diode is increased by submerging the diode in the Fluorinert. Experimental work suggest that this observed effect is not the result of improved optical coupling but is directly attributable to operating the P-N junction in the Fluorinert.

The figure also shows an increase in the amount of drive current required to start the LASER action in comparison with the p-n junction operated in air. It is believed that this effect is caused by the higher index of refraction exhibited by the Fluorinert which causes the loss of photons out of the laser cavity. As a consequence it is believed that more current is required to achieve the LASER threshold. Figure 4 shows an alternate embodiment of the present invention. In this figure the diode is mounted on stud 26 which forms a heat sink for the device. The stud encloses a cylindrical cavity formed by wall 12 and window 24. The anode of the diode is connected to a stud which functions as a support and an electrical connection for the semiconductor laser chip 18. Figure 5 shows a pig-tail version of the package shown in figure 4. In this embodiment an optical fiber 46 is positioned within the cavity to couple light energy from the chip 18 to the fiber. The fiber itself may be covered with a protective sheath 48 which is connected to the capsule with a strain relief 50.

It is apparent from the foregoing description of illustrative embodiments of the invention that many alterations could be made to the package without departing from the scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A package for an optical semiconductor comprising: a closed capsule defined by a plurality of walls; light emitting semiconductor means for converting electrical energy into optical wavelength energy; mounting means for positioning said light emitting semiconductor means within said capsule means; and liquid means in said capsule means for mediating the transfer of energy out of said capsule means, said liquid means being of a composition that prevents boiling.

2. The package of claim 1 wherein said capsule means further comprises: a transparent wall means for permitting the transfer of optical energy out of said capsule.

3. The package of claim 1 wherein said liquid means further comprises: means for thermally coupling said semiconductor means to said capsule means.

4. The package of claim 3 wherein said liquid means exhibits a specific heat of .25 g-cal/g-C .

5. The package of claim 3 wherein said liquid means exhibits a boiling point greater than 96 C and less than 216 C .

6. The package of claim 1 wherein said liquid means is substantially in direct contact with said semiconductor means.

7. The package of claim 2 wherein said semiconductor means comprises a laser diode.

8. The package of claim 2 wherein said semiconductor comprises a light emitting diode.

9. A method of cooling the junction of a light emitting diode comprising the step of submerging said junction in a coolant.

10. A method of cooling the light emitting junction of a semiconductor diode comprising the step of surrounding said junction with a liquid coolant.