DE10301873B4 - Capillary evaporator structure and cooling system - Google Patents

Abstract

capillary Evaporator structure (4) consisting of a substrate (14), in which Numerous adjacent coolant leading channels (8) extend to a Steam room (5) are open and through the intermediate webs (9) are spaced apart, characterized in that in the webs (9) numerous transverse grooves (10) run, which also to the vapor space (5) are open and at least at one end in the channels (8) wherein the average width of the transverse grooves (10) is less than one Third of the average width of the channels (8).

Description

The The present invention relates to a capillary evaporator structure according to the generic term of the attached Claim 1. Furthermore, the invention relates to an improved cooling system for heat dissipation of high power density devices. Such a cooling system can as a heat pipe and be designed as a capillary pumped two-phase cooling circuit and allows the heat dissipation of heat sources especially high power density.

Semiconductor devices need high power to comply with the maximum permissible Operating temperature suitable cooling devices. Optoelectronic devices, such as diode laser bars and CCD sensors also require a constant operating temperature.

The transition to ever higher Packing densities in semiconductor technology leads to ever higher heat outputs per area be dissipated have to. To a heat flow Of some 10 W / cm2 to be able to dissipate from a component, you need special cooling systems.

A possibility for cooling highly stressed components is the forced liquid cooling. The component to be cooled gets on a carrier applied, which contains a microchannel system. Through the microchannels is a liquid coolant pumped the heat receives. In the microchannels arises a high pressure loss, which must be compensated by the pump. cooling systems with microchannel heat exchangers are complex and sluggish in terms of temperature control.

When Alternative systems come into question with the evaporation of a liquid Coolant work. Because of the high heat of vaporization of the refrigerant is one compared to liquid cooling lower coolant flow necessary.

active Evaporative cooling systems, as an impingement cooler be used for the repatriation of liquid a pump.

passive Two-phase cooling systems with a liquid return through capillary forces need no pump. The aforementioned systems are known, for example from Faghri, A; Heat Pipe Science and Technology, ISBN 1-56032-383-3; 1996. They work either on the principle of the heat pipe or as a capillary pumped cooling circuit (referred to in the English literature as "capillary pumped loop").

At the heat pipe it is a hermetically closed system, whose inner walls with a capillary structure are provided. The capillary structure is with a liquid Heat transfer medium filled while in the remaining cavity a gaseous Heat transfer medium located. At a predetermined point of the heat pipe is doing a outside heat source attached by the heat pipe a certain heat flow is supplied. By the heat supply vaporizes liquid coolant from the capillary structure into the cavity. At another point becomes the heat pipe from the outside cooled. In the interior condenses gaseous heat transfer medium in the capillary structure at this point and reflects the heat absorbed during the evaporation from. Due to the resulting pressure gradient in the cavity, the steam flow is maintained. The liquid Heat transfer medium is in the capillary by the acting capillary force of liquid transported from the condensation zone back to the evaporation zone, so that there is a closed passive cooling circuit.

Of the capillary pumped cooling circuit looks like a similar Principle. The difference to heat pipes is that a heat pipe is a consistent Capillary structure, while the capillary pumped cooling circuit a capillary structure only in the evaporation zone and in the condensation zone is present. The liquid transport between the condensation zone and the capillary evaporator structure takes place then in a complete liquid-filled pipeline, which connects both zones.

Both Systems can be miniaturized. Microware pipes are from publications known. In principle, both types of small passive two-phase cooling systems can be used advantageous for cooling of semiconductor devices. The problem is that the systems work reliably only with a low heat output and when exceeded suddenly fail at a certain maximum power. If the maximum power is exceeded, The capillary structure in the evaporation zone dries out suddenly. The temperature on the side of the heat source increases abruptly because no liquid is left in the evaporation zone, which can evaporate. The component to be cooled can be due to overheating Get damaged.

Two different processes, which act in the same way for the heat pipe and the capillary-pumped cooling circuit, can lead to this dehydration process. The causes of dehydration of the capillary structure are due to the mode of operation. Due to the coupled thermal power, a certain amount of liquid per unit time in the evaporation zone is evaporated during normal operation of the heat pipe. There will be a replenished corresponding liquid flow through the capillary structure. As a result of the viscosity, the liquid undergoes a certain pressure loss on the way from the condensation zone to the evaporation zone. Similarly, a viscosity-related pressure loss occurs in the steam channel, since the vapor must flow from the evaporation zone to the condensation zone. Both pressure losses are compensated in the normal operation of the heat pipe by the capillary pressure of the capillary structure, so that the coolant circulates. With increasing heat output, however, the viscosity-related pressure losses increase. Above a certain limit power, which is called thermocapillary limit, the capillary pressure of the capillary structure is no longer sufficient to compensate for the pressure losses. As a result, the liquid flow breaks off and the evaporation zone dries out.

The However, dehydration of the capillary structure may already be far below the thermocapillary boundary occur. Especially with micro-heat pipes, in which the capillary structure as a system of the steam room out open channels is formed, the power limit comes into play, which by the Occurrence of boiling bubbles is conditional. In the o.g. publication by Faghri, pages 232 ff., the effect is called "boiling limit." If the coolant exclusively evaporated from the liquid surface, becomes the capillary transport of the underlying liquid not affected in the channel. at a correspondingly high heat output per unit area But it comes to bladder boiling. This occurs in the liquid channel Steam bubbles that flow after of liquid prevent. If only a few vapor bubbles are formed, they can escapes into the vapor space and the respective liquid channel is only for a short time Time is blocked. However, there will be enough vapor bubbles per unit of time formed, individual areas of the evaporation zone become so strong Measures of the hydration decoupled that all liquid evaporated from it. As a result, the heat output on the other still wetted areas of the evaporator zone and distributed effectively available for evaporation standing area gets smaller. The increasing power per area then leads to that now in the still active evaporator areas more bubbles are formed. Thus, the desiccation process continues in a rapid manner over the entire evaporation zone and the evaporation zone heats up abruptly.

The Desiccation of the evaporation zone occurs as a result of boiling bubbles, if the heat output per area, that from the heat source transferred to the heat pipe will exceed a certain value. It also occurs when the capillary pressure is still sufficient would, around the toughness To compensate pressure losses.

By the sudden overheating can that be cooled Component destroyed become or take damage. But also if the component to be cooled by the overheating does not suffer direct damage, dehydration is detrimental. Is that the Evaporation zone once dried out, can be a rewet only after cooling respectively. To return to the normal operation of the heat pipe, must be the power of the to be cooled Component switched off or at least greatly reduced.

Especially with inhomogeneous distribution of the heat flow at the coupling into the heat pipe For example, the limit power that causes dehydration may vary statistically. For performances close to the limit power, the operational safety of the heat pipe impaired because drying out with a certain probability too already below the performance can occur at the security dehydration begins.

It are several approaches known the capillary structure for to optimize high performance. The most important previous knowledge will be summarized below.

In the US 5, 769, 154A Labyrinth-like capillary structures are shown which, in contrast to rectilinear channels, allow liquid to flow from several directions.

In the DE 197 44 281 A1 Capillary structures are presented which have a small pore diameter and contain channels with a larger cross-sectional area, in which the liquid transport is less inhibited by the viscosity-related flow resistance. The capillary structure is formed of sintered material or of other porous materials, such as braids, fabrics, foams or fiber felts. The larger channels referred to in the literature (eg GP Peterson: An Introduction to Heat Pipes, ISBN 0-471-30512-X, page 243) as fluid arteries are the subject of DE 197 44 281 A1 formed as grooves in the inner wall of the housing. An embodiment described therein shows the arteries as cylindrical cavities within the fine-pored structure. Another embodiment shows a multi-layered capillary structure, which consists of a lower-side coarse-pored layer and a vapor-space-side fine-pored layer.

Similar and some other structures have been known for some time. An overview was published in the above publication by Faghri (pages 24 ff.). In one example, the cross section of a heat pipe with a porous capillary structure is shown, containing cylindrical fluid arteries. A multilayer capillary structure consisting of a bottom coarse-pored layer and a vapor space-side microporous layer is also shown. In the publication, there is also the image of a structure in which the arteries are formed as grooves in the housing wall, which are covered by a fine-meshed wire mesh.

Following the structures mentioned US 5,769,154 A . DE 197 44 281 A1 and the above-mentioned reference is common that they have a higher liquid transport performance compared to homogeneous capillary structures and thereby the thermal capillary limit performance is higher. However, the thermal capillary limit performance can not be exploited in many cases by far, since the evaporator structure dries out as a result of nucleate boiling even at much lower heat output.

The structures after DE 197 44 281 A1 are particularly susceptible to the formation of boiling bubbles, because in the arteries located vapor bubbles that impede the liquid transport, difficult to escape into the vapor space. The structures after US 5,769,154 A Although optimized in terms of liquid transport, but have no appreciably higher resistance to drying out by boiling bubbles on.

In the publication US 6,056,044 A mushroom-shaped capillary structures are presented, which have improved evaporation properties. The rewetting of the structure when boiling bubbles occur is improved by the additional edges in the liquid channel. However, overhanging parts of caps make it difficult to escape the boiling bubbles into the vapor space. The vapor space side surfaces of the caps are not wetted with liquid and therefore ineffective for evaporation. The resulting steam must pass through the narrow spaces between the caps in the steam room. This increases the back pressure on the liquid, which in turn limits the maximum heat output.

Of the above-described process of drying out the evaporation zone By boiling bubbles limits the maximum possible power per area, the from the heat pipe can be included. Therefore, high-density devices, where waste heat on a particularly small surface becomes free, not be cooled with known heat pipes. The application of known heat pipes is only possible when the heat flow of the Component before the coupling into the heat pipe spread over a larger area becomes. The heat spread with a good thermally conductive solid, for example Made of copper, silver or diamond, with the disadvantages of the extra Expenses and space requirements afflicted. In addition, the disadvantage of a Temperature difference between the heat source and the heat pipe unavoidable, which is the greater the stronger the heat spreading have to be.

One another problem of known heat pipes, their use for cooling small components with high heat output is contrary, is the thermal resistance. The heat, transmitted through a heat pipe must, at be discharged at a lower temperature than they received on the evaporator side becomes. The temperature difference depends on the transferred Heat output. The relationship the temperature difference to the heat output is called thermal resistance of the heat pipe designated. When the heat because of the small size of the too cooling Component is transferred to a small area, the thermal resistance of the heat pipe essentially by the thermal resistance the evaporation zone determined. As a heat transfer medium in the relevant temperature range liquid Metals, but only dielectric substances, such as water, ammonia, chlorinated hydrocarbons, or fluorinated hydrocarbons, which have a very low thermal conductivity have, the heat conduction takes place predominantly in the solids content of Capillary structure. Known capillary structures based on fiber felts have a disadvantageously high because of the laterally oriented structure Thermal Resistance while porous Although sintered materials have a lower thermal resistance, but the liquid flow one high flow resistance oppose.

Capillary channel structures, such as for example according to the patent US 5,769,154 A have a low thermal resistance of the solid state structure to the top on which the evaporation takes place. However, because of the poor heat conduction in the liquid, the evaporation takes place mainly in the immediate edge zone of the liquid, while a large part of the liquid surface is hardly effective for evaporation. This also results in a high thermal resistance of the evaporation zone.

Of the high thermal resistance the evaporation zone of the known heat pipes affects the operation of the heat pipe disadvantageous because the temperature difference across the heat pipe a lower temperature on the side of the heat sink requires and thus higher Demands on the downstream cooling system.

The use of miniaturized heat pipes for cooling laser bars is known from US Pat. No. 5,453,641 A. As capillary structure open microchannels are used, but the have the above-mentioned disadvantages of limited by Blasensieden heat output per area and a high thermal resistance of the evaporation zone.

task The invention is therefore a capillary evaporator structure for Use in a passive two-phase cooling system, in particular for cooling Semiconductor devices show that over the prior art a higher heat output per area can absorb and has a reduced thermal resistance. Furthermore The operational safety is due to the improved capillary evaporator structure of the cooling system increased near the maximum power become.

The solution These tasks are performed by a capillary evaporator structure with the features specified in claim 1. With the capillary invention Evaporator structure can cooling systems either on the principle of the heat pipe or as a capillary pumped cooling circuit being constructed.

One Advantage of the capillary invention Evaporator structure and constructed with this cooling system is that in comparison to known systems a higher maximum heat output per area from the heat source can be dissipated. Devices for heat spreading between to be cooled Component and the cooling system according to the invention can be less educated or saved altogether.

One Another advantage of the cooling system according to the invention is that a reduced thermal resistance of the system is achieved.

The Operational safety of the system near maximum performance is improved. The cooling system according to the invention is in particular also for the cooling of Higher laser bars Performance suitable.

Further Advantages, details and developments of the present invention result from the following description of preferred embodiments, with reference to the drawing. Show it:

1 a simplified perspective view of a cooling system, which works as a heat pipe;

2 a longitudinal sectional view of in 1 illustrated heat pipe;

3 a detailed perspective view of a first embodiment of the capillary structure used in the cooling system;

4 a second embodiment of the capillary structure in a view from above;

5 a third embodiment of the capillary structure in a detailed view from above.

In the 1 and 2 shows the basic structure of a cooling system, which works as a heat pipe.

The heat pipe is as a flat closed hollow body 1 educated. At a first side is near the first end of the hollow body 1 a component to be cooled 2 appropriate. The to the component 2 adjacent area of the hollow body serves on its outer side as an additional cooling surface 3 which is connected to the recooling system (not shown).

The longitudinal section of the heat pipe is in 2 shown. A capillary structure 4 , which contains liquid coolant, is located on the inner wall of the hollow body 2 So inside the cavity 5 , The heat, on a relatively small area of the device 2 is removed, leads to the evaporation of coolant (not shown) in an evaporation zone 6 passing through a front portion of the capillary structure 4 is formed. The steam flows through the cavity 5 and arrives at a condensation zone 7 where it condenses with heat release. The condensation zone 7 is through a rear portion of the capillary structure 4 educated. In modified embodiments, however, the condensation zone can also be formed by a conventional capillary structure. Evaporation zone and condensation zone can merge directly into each other or be formed as separate components, which are optionally spaced by a small gap.

The heat can now be on a contact surface 13 between the condensation zone 7 and the inner wall of the hollow body are removed. The contact surface 13 is dimensioned much larger than the surface portion of the device 2 that with the hollow body 1 directly connected. The recooling system may be, for example, a Peltier element or an air flow. The nature of the recooling system and its coupling to the heat pipe are not relevant to the present invention, so that a detailed description of the recooling system is dispensed with.

For cooling components 2 With a particularly high power density, it may be advantageous to use a diamond substrate 15 or a similar good heat tendes element for heat spreading between the component 2 and the hollow body 1 to arrange.

A section of the capillary structure according to the invention 4 is enlarged as an outbreak in 3 shown. The capillary structure 4 is through adjacent channels 8th formed with eg rectangular cross-section. For example, the width is 100 microns, the depth 150 microns and the distance 200 microns. Between every two adjacent channels 8th remain webs 9 , which have a web width of about 100 microns. The bridges 9 additionally contain narrower transverse grooves 10 for example, 10 microns wide and 20 microns distance, for example, straight, for example, parallel to each other at right angles in each case by one of the wide channels 8th to the adjacent channel. In modified embodiments, the transverse grooves 10 also only on one end face in the channels 8th open and have a closed end face.

The entire capillary structure is in a base substrate 14 brought in. The base substrate 14 may be, for example, silicon or another semiconductor material. For the formation of the capillary structure in the base material, various technologies are known to those skilled in the art, which are used in particular in semiconductor technology. The depth of the narrower transverse grooves 10 is suitably chosen to be much lower than that of the wide channels 8th , for example, about 15 microns. This is the one for the heat conduction in the webs 9 available material cross-section affected only at a small depth.

It is known that the evaporation of the coolant takes place predominantly in the immediate vicinity of the interface between the walls of the capillary structure and the liquid. By the arrangement of narrower transverse grooves 10 For example, the edge zone of the liquid, in which the evaporation mainly takes place, is multiplied many times over a structure containing only the wide channels. For a given total area of the evaporator zone, a higher evaporation capacity is possible in this way. The dehydration by formation of vapor bubbles can occur only at significantly higher heat output per area. By extending the edge zone of the liquid also the thermal resistance of the evaporation zone is significantly reduced.

In addition, in the capillary evaporator structure according to the invention a higher blistering rate is tolerated because the rewet of the wider channels 8th through the in the narrow transverse grooves 10 existing liquid is ensured even with stronger blistering.

In addition, by a suitable geometry of the narrower transverse grooves 10 a reproducible seed bubble source is induced which causes seed bubbles of defined size at the junctions of the narrower transverse grooves 10 in the wider channels 8th be introduced immediately below the liquid surface, which hardly affect the liquid transport in the underlying channel cross-section and still nucleate boiling from the wider channels 8th allow with a high heat transfer rate.

Preferably, in the webs 9 relatively many cross grooves 10 arranged to the described advantageous effect of preventing dehydration of the channels 8th to use on a large scale. For an optimal use of space between the cross grooves 10 remaining material areas (transverse webs) narrower than the width of the transverse grooves 10 be designed. It is also useful if the transverse grooves 10 deeper than wide, like this 3 is apparent.

In a modified, not shown embodiment of the capillary structure The narrower transverse grooves also run straight, but not at right angles to the wider channels. This embodiment, for example, with an angle of 109.5 °, is particularly advantageous when used as substrate (110) -oriented silicon. Then For example, the structure can be made by anisotropic etching be with the angle between the channels and the transverse grooves of the Angle of the crystal lattice planes corresponds.

The capillary structure according to the invention can generally be produced by known etching methods in silicon or another semiconductor material. For this purpose, first a first masking layer is produced on the substrate, by which both the image of the wider and the narrower channels are structured. On this first masking layer, a second made of a different material is applied, in which only the image of the wider channels is structured. In a first deep etching step, the wider channels are etched to slightly above the desired depth. Thereafter, the top masking layer is selectively removed and a second depth etching step is performed to the target depth of the narrower channels. Examples of suitable masking layers to be removed are metals, silicon oxide and silicon nitride, as well as various paints. The etching steps can each according to eg as plasma etching DE 42 41 0454 or as a wet-chemical anisotropic etching process.

At the in 4 the embodiment of the channel structure shown are the wider channels 8th made such that the width thereof in the direction of liquid flow, as viewed at the beginning 11 of the evaporator zone 6 greater than at the end 12 , The narrowing channels cause a capillary pressure increasing toward the end of the evaporator. The narrower transverse grooves 10 become gradually longer towards the end of the evaporator, which corresponds to an advantageous extension of the edge zone of the liquid.

At the in 5 In the embodiment shown, the wider channels run 8th not parallel, but close in the evaporator zone 6 each an acute angle. This embodiment is particularly advantageous when the heat sink has a greater width than the heat source.

Generally, it should be noted that the cross sections of the wider channels 8th as well as the narrower transverse grooves 10 do not have to be rectangular. Advantageously, for example, trapezoidal, triangular or circular segmental cross sections can be used. Oval cross-sections, in particular in manufacturing processes based on weakly anisotropic etching processes, are also suitable.

In an advantageous embodiment is the capillary structure according to the invention in a semiconductor substrate, such as silicon, gallium arsenide, Indium phosphide or silicon carbide realized, which is the evaporation zone represents the cooling system. On the side facing away from the capillary structure of the substrate can be structured in this case, a semiconductor device, which effectively by the cooling system according to the invention is cooled.

The Overall system can be seen in further development of the described embodiments also executed in this way be that heat dissipation on the opposite side Side or on both sides of the flat hollow body takes place. The return transport the liquid into the capillary evaporator structure then takes place for example via a known capillary structure or a porous material on the insides the walls.

A another variant of the aforementioned embodiment includes webs inside the cavity, which increase the stability of the arrangement. These Stages can also from a porous material be prepared or coated with a porous material or channels contain a liquid transport between top and bottom allow.

In a further modified embodiment are in the webs 9 in addition to the cross grooves 10 even more auxiliary grooves formed in the transverse grooves 10 open out. The advantages of the arrangement of the transverse grooves 10 in terms of increased reliability of the wider channels 8th bring with it, become in this way also for the transverse grooves 10 provided by the auxiliary grooves. The transverse grooves 10 are in turn fed from the auxiliary grooves with coolant, so that even at this point drying of the transverse grooves is prevented.

at the capillary evaporator structure according to the invention can about that In addition, measures can be applied, the previously known evaporator structures for Improvement of the operation are known. For example, smaller recesses or supernatants on the sidewalls or on the bottom of the channels, the transverse grooves and / or the auxiliary grooves are formed.

It has already been pointed out that the inventive capillary Evaporator structure also used advantageously in capillary pumped cooling circuits can be. The basic structure of such cooling circuits is known in the art. The previously described capillary structure may be known instead Structures advantageously used in the evaporation zone of such a cooling circuit become. The channels for the Liquid or for the Steam transport between the evaporator and the condenser are separated in such cases executed. If necessary, in the liquid line near the condensation zone a Zone for subcooling the liquid provide the risk of formation of vapor bubbles in the fluid channel turns away. The connecting pipes between evaporator and condenser can each as a separate tube for the Steam and for the liquid accomplished be.

In a further embodiment is the case double-walled in the zone between the evaporation zone and the condensation zone executed, wherein the inner steam channel through the inner wall of the shell-side fluid channel is separated.

Around existing cooling systems To make it more effective, it is possible to use the capillary evaporator structure as an independent Form part to make this component as an evaporation zone in different cooling systems use. The evaporator structure is then using a joining technology in the hollow body attached. The hollow body can on its interior walls with a known capillary structure or a porous layer be provided. It may be appropriate, a decoupling the evaporation zone of the adiabatic zone of the cooling system in which between these two zones a narrow space which is left for even distribution the cooling liquid contributes.

A modified embodiment of he According to the invention capillary evaporator structure consists of a metal, preferably copper, silver, aluminum or titanium, which is produced by means of a negative mask made of plastic.

1

hollow body

2

to cooling module

3

additional cooling surface

4

capillary

5

cavity

6

Evaporation zone

7

condensation zone

8th

channels

9

Stege

10

transverse grooves

11

Beginning the evaporator zone

12

The End the evaporator zone

13

contact area

14

base substrate

15

diamond substrate

Claims (15)

Capillary evaporator structure ( 4 ) consisting of a substrate ( 14 ), in which numerous adjacent channels ( 8th ) leading to a vapor space ( 5 ) are open and through the intermediate webs ( 9 ) are spaced apart, characterized in that in the webs ( 9 ) numerous cross grooves ( 10 ), which also to the steam room ( 5 ) are open and at least at one end in the channels ( 8th ), the average width of the transverse grooves ( 10 ) less than one third of the average width of the channels ( 8th ) is. Capillary evaporator structure according to claim 1, characterized in that the transverse grooves ( 10 ) have a smaller depth than the channels ( 8th ). Capillary evaporator structure according to claim 1 or 2, characterized in that in the webs ( 9 ) between the transverse grooves ( 10 ) continue to run auxiliary grooves, which also to the vapor space ( 5 ) are open and in the transverse grooves ( 10 ). Capillary evaporator structure according to one of claims 1 to 3, characterized in that the channels ( 8th ) and / or the transverse grooves ( 10 ) have small recesses or projections on the side walls or on the channel bottom. Capillary evaporator structure according to one of claims 1 to 4, characterized in that the wider channels ( 8th ) and / or the narrower transverse grooves ( 10 ) each have a decreasing in the flow direction of the coolant width. Capillary evaporator structure according to one of claims 1 to 5, characterized in that the wider channels ( 8th ) and / or the narrower transverse grooves ( 10 ) each have a rectangular, trapezoidal, triangular, circular segment, oval or pillow-shaped cross-section. Capillary evaporator structure according to one of claims 1 to 6, characterized in that the substrate ( 14 ) consists of a semiconductor material. Capillary evaporator structure according to claim 7, characterized in that it is in the semiconductor material ( 14 ) is introduced by successive etching steps. Capillary evaporator structure according to one of claims 1 to 7, characterized in that they by galvanic molding a photoresist pattern was prepared. Capillary evaporator structure according to one of claims 1 to 9, characterized in that channels ( 8th ) have a width between 20 μm and 500 μm and that the transverse grooves ( 10 ) have a width between 0.1 μm and 20 μm. Cooling system ( 1 ) with an evaporation zone ( 6 ) and a conformation zone ( 7 ) for removing heat from a component ( 2 ), characterized in that at least the evaporation zone has a capillary evaporator structure ( 4 ) according to one of claims 1 to 10. Cooling system according to claim 11, characterized in that the capillary evaporator structure ( 4 ) is formed as a separate component. Cooling system according to claim 12, characterized in that between the capillary evaporator structure ( 4 ) and the capillary structure of the adiabatic zone of the cooling system, a narrow gap is formed. Cooling system according to one of claims 11 to 13, characterized in that between the component to be cooled ( 2 ) and the capillary evaporator structure ( 4 ) a diamond substrate ( 15 ) is arranged for heat spreading. cooling system according to one of the claims 11 to 14, characterized in that it is pumped as a heat pipe or capillary Cooling circuit is trained.