Coherent light generators – Particular temperature control
Reexamination Certificate
2001-03-02
2003-03-18
Leung, Quyen (Department: 2878)
Coherent light generators
Particular temperature control
C372S035000, C372S036000
Reexamination Certificate
active
06535533
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention relates to a mounting substrate and a heat sink for high-power diode laser bars of the type disclosed in U.S. Pat. No. 5,848,083.
b) Description of the Related Art
High-power diode laser bars (HDB) are extended semiconductor laser components of high optical output power (mean line power densities of >1 watt/mm component width, approximately 10 to 100 watts cw per component). Their operation requires high currents of a few 10 to 100 amperes, for which preference should be given in the connecting technique between the HDB and its substrate to hard solders which are highly stable in the long term, rather than to soft solders, which are prone to electromigration. However, by contrast with soft solders, hard solders have the disadvantage that they require a substrate whose lateral coefficient of thermal expansion (in the case of extended chips) must be adapted to better than 1 ppm/K to the material of the HDB, for example GaAs, if it is desired to avoid the influence of damaging mounting stress on the HDB. Such a substrate is termed expansion-matched. Its mounting surface should have a planarity of better than 1 &mgr;m/cm, in order not to worsen the beam formation of the optical emission profile of the HDB by excessive bending of the emitting active zones.
HDB are very sensitive in thermal terms. Their operating temperature should not exceed 55° C. to 60° C., if lifetimes of more than 10,000 hours are required. Consequently, substrates for HDB are designed as heat sinks. For an efficiency of 50%, the power loss occurring during operation is exactly as great as the optical power, and so HDB heat sinks require thermal resistance of from 0.2 to 0.5 K/W (Kelvin per Watt). In principle, HDB heat sinks comprise a heat-spreading mounting substrate and a heat-dissipating cooling element. Copper or diamond, for example, comes into consideration as material for the mounting substrate. A solid copper block on a Peltier element (conductive cooling), a microchannel cooler flowed through by water (MCC, forced or active convective cooling) or a micro heatpipe (MHP) filled with water vapor, free or passive convective cooling) can serve as heat sink. In any case, the mounting substrate can by all means be integrated in the heat sink, as is the case with microchannel heat sinks (MCHS). Thus, microchannel coolers, in particular, have been developed for active (forced) convective cooling with a low thermal resistance, and the use of mounting substrates made from exceptionally highly thermally conductive material—that is to say material exceeding the thermal conductivity value of all known metals—(for example diamond) has been investigated. However, the latter so far have had only led to unsatisfactory life times, because the soldering tensile stress transmitted by the solder onto the HDB are also damaging to the HDB. There have therefore already been many attempts to find highly thermally conductive heat sinks with expansion match. Various arrangements of two or more materials with different coefficients of expansion can achieve expansion matching in at least one direction (parallel to the largest dimension of the laser):
In the case of lateral expansion match, the layering of the materials is performed in a fashion lined up next to one another parallel to the axis of the expansion-matched lateral (width) direction. It is approximately pure in the case of vertically thin layers of large lateral width (EP 0 590 232).
In the case of vertical expansion match, the layering of the materials is performed in a fashion stacked one above another perpendicular to the axis of the expansion-matched lateral (width) direction. It is likewise approximately pure in the case of vertically thin layers of large lateral width (DE 195 06093, DE 196 05 302, U.S. Pat. No. 5,299,214).
In the case of mixed vertical-lateral expansion match, both aspects come into play, specifically by virtue of the fact that both the extent of the expansion-matched direction is limited, as is also the maximum aspect ratio of width to thickness of the material layers. This is the general case, and it goes over into the two previously mentioned types in the limiting case when appropriate extreme values of the layer dimensions are adopted. Typical arrangements for the expansion matching which is clearly of a mixed complexion are
a) in the case of regular structures, interruptions in the layers in the case of vertical match in order to weaken its mechanical relevance in comparison to the continuous layers (DE 198 21 544, DE 196 51 528, EP 0 590 232, U.S. Pat. No. 5,848,083 and WO 94/24703).
b) in the case of irregular structures, the homogeneous spatial distribution of particles of low thermal expansion (aluminum nitride, diamond) in a matrix made from a material of high thermal expansion (copper, aluminum) (U.S. Pat. No. 5,455,738 and EP 0 898 310).
The use of expansion-matched heat sink materials made from CuW, CuMo or, as in U.S. Pat. No. 5,455,738, from CuC with C as diamond is disadvantageous because of the still excessively low thermal conductivity and the difficulty of mechanical processing.
An asymmetric two-layer system which consists of continuous diamond and copper (U.S. Pat. No. 5,299,214) is ruled out of application because of its susceptibility to bending. Multilayer systems made from copper/molybdenum/copper (DE 196 05 302) and copper/aluminum nitride/copper (DE 195 06 093) are certainly symmetrical, but still continue to use relatively small thermally conducting metal layers.
Consequently, with reference to mounting substrates made from a multilayer system, the top layer denotes the layer facing the heat source (diode bar laser), and the bottom layer denotes the layer facing the heat sink (metal block, MCC, MHP).
A diamond body with openings having mounting stresses occurring in cutouts transverse to the main directions is presented in DE 197 01 680 for the purpose of decreasing stresses. Mounting it on a microchannel cooler is an example of its integration. The concept of expansion match with such a diamond body is not set forth completely in conjunction with the microchannel cooler. A disadvantage of the diamond body on its own is the still lacking expansion match, and a disadvantage of its combination with a heat sink is the individual match required of the heat sink, tailored to the cooling technique, for bar soldering.
The chip arrangements presented in DE 196 51 528 are based on a (cooling) substrate onto which an HDB is soldered or bonded via a connecting device made from diamond parts spaced apart from one another. A similar arrangement is also found with heat sinks which are presented in DE 198 21 544 and already form per se the expansion-matched component required for HDB mounting. Both arrangements have the advantage that an additional mounting step can be omitted after the soldering of the bar laser. However, there is a disadvantage of a large number of different heat sinks which have to be prepared for the HDB soldering in a fashion depending on the type of heat sink, and on the volume of substrate material which has to be heated for the HDB soldering. Also disadvantageous, furthermore, is the mechanically strongly asymmetric character of the design, which tends to give rise to bending in production.
On the one hand, EP 0 590 232 presents a mounting substrate which has one vertical layer and consists laterally of alternating layers of materials of high and low thermal conductivity. The thicknesses of these layers can be dimensioned in such a way that the substrate effectively has a coefficient of expansion matched laterally to the semiconductor material. There are certainly few bends in such a mechanically symmetrical substrate, but it has no cooling element for its side averted from the heat-producing component. The indispensable cooling element is, however, a mechanically very influential constituent of each HDB component.
On the other hand, EP 0 590 232 presents a design with two vertical layers and comprising a substrate, which is laterally lay
Dorsch Friedhelm
Lorenzen Dirk
Jenoptik Aktiengesellschaft
Leung Quyen
Reed Smith LLP
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