DE102004052039A1 - Determination of melting depth during melting of a metal layer with one or more layer sites by means of a reflected test beam useful in metallurgical melting operations providing relaible and reproducible depth determination - Google Patents

Abstract

Determination of melting depth during melting of a metal layer wih one or more layer sites, which on impact of a test beam emit different discharge beam characteristics. During melting the melting regions are impacted with the test beam (10), and from the reflected test beam the melting of the layer sites can be known by detection of the specific discharge characteristics from which the melting depth can be deduced. An independent claim is included for a substrate for use in determination of the melting depth as described above.

Description

The Invention is in the field of manufacturing electrical circuits, especially for power semiconductor modules, one on a carrier (Substrate) applied metal layer for mounting and / or contacting purposes is melted. For this purpose, the metal layer is at least partially by heat input - e.g. by means of an energetic radiation - until heated to its melting point.

Because of the usual small dimensions and thus small quantities of material to be melted Metal is the dosage of the heating intensity or heating time a significant importance too. For reliable Mounts or electrical connections must be reproducible predetermined melting depth of the metal layer can be achieved. Too low a melting point can lead to incomplete or faulty connections to lead, while one too big Melting depth or excessive heating to thermal loads and mechanical stresses - worst case to destruction the circuit - can lead.

Of the The present invention is therefore based on the object, a method indicate with the reliable reproducible during the melting of a metallization determines the melting depth can be. Furthermore the task is to specify a suitable substrate.

These Task is according to the invention in terms of the method a method according to claim 1 and with respect to the substrate solved by a substrate according to claim 5. Embodiments and developments are the subject of the dependent claims and embodiments.

The Method for determining the melting depth during the melting of a Metal layer provides that to be connected or contacted Metal layer on one or more layers (s) applied becomes. This layer (s) is characterized or distinguished by that they each face the metal layer and possibly opposite the other layer layers when exposed to a test radiation each layer-specific, to the metal layer (and possibly to the other layer layers) different characteristic discharge radiation emit or emit. The metal layer can also be a comparatively small metallized area, e.g. for load current decrease used in a power semiconductor module.

h:

Plancksches Wirkungsquantum h = 6,2·10^–34 Js

f:

Frequenz der reflektierten Strahlung

λ_char:

charakteristische Wellenlänge der reflektierten Strahlung (Entladungsstrahlung)

c:

Lichtgeschwindigkeit

During the melting, the melting area is exposed to a test radiation. The proportion of the test radiation which is reflected back to an evaluation device is also referred to below as reflection radiation in the context of the invention. The reflection radiation contains a characteristic of the respective reflective material discharge radiation. The frequency or wavelength of the discharge radiation is dependent on the so-called work function. This in turn is determined by the irradiated, reflective material. Generally, the work function W _a and the frequency or wavelength of the discharge radiation for a given material are as follows: W a (material) = h · f = h · c / λ char (Material) (1). With:

H:

Planck's constant h = 6.2 · 10 ^-34 Js

f:

Frequency of the reflected radiation

λ _char :

characteristic wavelength of the reflected radiation (discharge radiation)

c:

Speed of Light

you recognizes that the wavelength of the Discharge radiation characteristic of the reflective material or whose work function is. Depending on the molten metal layer is / are therefore one or more specific discharge radiation detectable in the reflection radiation.

Thus, when a layer consisting of a metal melts, a characteristic work function W _a or maximum wavelength λ _{char, Me} results according to the equation W a, Me = h · f = h · c / λ charm (2)

If the metal is, for example, copper (Cu), the corresponding characteristic work function will be referred to below as W _{a, Cu} and the characteristic wavelength as λ _{char, Cu} .

Becomes now at least one more layer below the melted Reaches metal layer, i. begins to melt this layer, is a - of the characteristic wavelength the copper layer different - characteristic maximum wavelength the discharge radiation in dependence detectable by their material composition.

According to a preferred embodiment of the invention it is provided that the metal layer and the layer layer (s) are made of the same base material, wherein the layer layer (s) for producing different characteristic Entla radiation is doped differently in layers.

Becomes as base material (starting material) in a preferred embodiment used in the process of copper, this can be used to produce a layer-specific characteristic discharge radiation in layers, e.g. with aluminum and magnesium be doped.

It would thus result for a first layer layer whose base material (copper) is doped by introducing a dopant μ (eg aluminum) and which has a work function W _{a, μ} , according to the equation W a, μ = h · f = h / λ char, μ (3) a characteristic discharge radiation having the wavelength λ _{char, μ} , which is superimposed on the remaining radiation components in the reflection radiation.

Out The reflection radiation is thus the successive melting of the Layer (s) detected by the fact that depending on the melting depth in the reflection radiation different radiation components (namely the layer-specific characteristic discharge radiation) become. This will in a simple way to the current melting depth closed. The evaluation device for analyzing the reflection radiation or their spectral components can be a per se known spectrometer be.

One An essential aspect of the present invention is therefore to that the reflection radiation is material-specific characteristic Contains discharge radiation with characteristic wavelengths.

One Advantage of the invention is that the Aufschmelztiefenbestimmung in real time during of the current melting process. This can be the measurement result act directly on the control of the reflow process. This allows extremely precise desired melting depths reproducible and reliable adjust and thus the heat input optimize. This allows a secure connection through adequate Ensuring melting and yet minimized material stress become.

at a manufacturing technology preferred embodiment of the method according to the invention provided that for melting the metal layer and for generating the test radiation the same laser device is used.

The The invention further relates to a substrate for use in a method according to any one of the preceding claims.

According to the invention this substrate is a carrier with several metallic layers of a base material, each layer having an individual doping such that when exposed to a test radiation a layer-specific emitting characteristic discharge radiation. The basic material is preferably copper.

Further Preferably, the doping of the layers is selected so that the respective layer-specific Work function increases toward the top layer. This advantageously allows an improved resolution in the spectral analysis of the contained radiation components (discharge radiation).

The Invention will be further exemplified with reference to a drawing explains; show it:

1 a substrate used in carrying out the method according to the invention,

2 Construction and beginning of the process according to the invention,

3 a process state in which a desired melting depth is reached, and

4 a process state in which the desired melting depth is exceeded.

1 shows a substrate 1 that as a base or carrier 2 has a ceramic plate. There are three layers on it 3 . 4 . 5 applied, the base material, for example, each copper. But there are also other metals as basic material conceivable. The lowest (ie the base) layer layer 3 consists of magnesium-doped copper and has, for example, a thickness of 100 microns. The layer layer formed thereon 4 consists of aluminum-doped copper and has a thickness of for example 50 microns. The upper metal layer on top 5 is made of copper and is 150 μm thick. On this layer is an example of an even thicker copper layer area 6 formed, for example, in an interconnection of a power semiconductor module as a load terminal 6a can serve. In principle, it is also possible to provide further correspondingly individually doped lower layer layers, which can be further increased by the accuracy or resolution in the melting depth determination described below in detail.

As explained above, cause the under different doping of the layers 3 . 4 . 5 different work functions. According to the relationship between work function and characteristic wavelength, irradiation of the layers with a test radiation (eg laser radiation) results in discharge radiation each having a specific, layer-specific wavelength λ _{char (3)} , λ _{char (4)} , λ _{char, Cu} . The choice of material or doping is chosen so that the work function of the lowest layer layer 3 to the top layer 5 respectively. 6 increases. As a result, an improved spectral resolution of the discharge radiation or the spectral components of the reflection radiation is possible.

2 shows a structure for carrying out the method according to the invention and the situation at the beginning of the procedure. You can see that in 1 shown substrate 1 with the metal layer to be melted 5 . 6 and the underlying layers 3 . 4 , An area 7 is with a laser beam 8th a laser device 9 applied. The laser beam brings highly concentrated and locally limited high energy into the layer material, whereby this is heated above its melting point, that is melted. Part of the laser radiation serves as test radiation 10 in that a part of the radiation on the layer 5 . 6 is reflected and as reflection radiation 11 to an evaluation device 12 is thrown back. The evaluation device 12 is a common glow discharge spectrometer (GDOS). This may have a detection range of 100 nm to 1000 nm wavelength. The detection or output signal 14 of the spectrometer 12 can serve as a control signal for the control of the laser device. A particular advantage of this embodiment is that for the actual melting and for the test radiation the same radiation source, namely here the laser device 9 , is used.

The melting can be used for laser welding, for example, a connection line indicated only by dashed lines 15 at the load connection 6a to fix.

In the in 2 As shown, the reflow process has only begun; the already molten area 7 is still completely in the metal layer 5 respectively. 6 , Therefore, in the reflection radiation, the discharge radiation of copper can be detected as the maximum wavelength. This is in the lower part of the 2 represented, in which the intensity I is plotted against the detected wavelength λ schematically. Here, the detected maximum wavelength is the characteristic wavelength of copper, λ _{char, Cu.} This indicates that the target reflow depth has not yet been reached, because the underlying layer layer serving as an indicator of an advanced reflow depth 4 not yet exposed and therefore can not generate any reflection yet.

In the in 3 - using the same reference numerals as in 1 and 2 for identical or identical elements - the situation shown is the reflow process in the area 7 ' advanced. It can be seen that the material of the (doped) copper layer layer 4 from the laser beam 8th has been reached and melted. Therefore, the reflection radiation contains 11 ' both (further) the discharge radiation λ _{char, Cu of} the layer 5 respectively. 6 as well as the discharge radiation λ _{char (4) of} the layer layer 4 , Since now - as in the lower part of the 3 shown in the intensity-wavelength diagram - the two specific characteristic, layer-specific wavelengths λ _{char (4)} and λ _{char, Cu} are detected by the spectrometer, it can be concluded in a simple manner that a melting depth up to the layer 4 is reached. In the exemplary embodiment was assumed (see. 1 ) that the metal layer 5 made of copper is 150 microns thick. If this is the desired minimum reflow depth of the layer layer 5 is, so that the achievement of the desired melting point is displayed and the melting process can be terminated here. At least a portion of the layer thickness (in this example 100 μm) of the layer remains as the residual melting depth 4 ,

4 shows - using the same reference numerals as in 1 to 3 for identical or similar elements - the farther in the range 7 '' advanced melting process. Here is the material of the layer layer 4 in the melting area 7 '' completely melted and the laser beam 8th reaches the layer position 3 , Therefore, the reflection radiation contains 11 '' both (further) the discharge radiation λ _{char, Cu of} the layer 5 respectively. 6 and the discharge radiation λ _{char (4) of} the layer layer 4 as well as the discharge radiation λ _{char (3) of} the layer layer 3 , Since now - as in the lower part of the 4 shown in the intensity-wavelength diagram J / λ - the specific characteristic, layer-specific wavelengths λ _{char (4)} , λ _{char (3)} and λ _{char, Cu)} are detected by the spectrometer, this indicates in a simple manner that the melting depth to to the layer position 3 enough. Thus, according to the exemplary embodiment (assuming a desired melting depth of 150 μm), it can be seen that the desired melting depth with the actual melting depth of at least 250 μm (sum of the thicknesses of the layers 4 and 5 ) is exceeded.

It can also be seen in the intensity-wavelength diagram J / λ that λ _{char (4)} <λ _{char (3)} ; thus the work function of the layer 4 is greater than that of the layer 3 , which allows for improved spectrometric resolution.

Instead of indicating the exceeding of the desired reflow depth, the layer layer could 3 and optionally further underlying layers to an even finer resolution of the determination of Melting depth can be used. The principle of the invention is thus obviously not limited to three differently doped layers.

With the described method can by simple means during the laser welding of the layer 5 . 6 continuously - quasi online - the current melting depth can be precisely monitored and controlled with relatively little effort.

The invention has been described above with reference to preferred embodiments. In particular, here are examples of the different layers 3 . 4 . 5 . 6 called certain materials. However, the inventive principle is not limited to the use of said materials but extends to any combination of materials of the different layer layers, wherein the top layer layer 5 and or 6 of the carrier 2 which is to be melted, preferably formed from a metal or an alloy.

I

intensity

λ

detected wavelength

λ _{char, Me}

wavelength

λ _{char, Cu}

wavelength

λ _{char (3)}

wavelength

λ _{char (4)}

wavelength

W _{a, Me}

work function

W _{a, Cu}

work function

1

substratum

2

carrier

3

layer sheet

4

layer sheet

5

layer sheet

6

Copper layer region

6a

load connection

7

Area

7 '

Area

7 ''

Area

8th

laser beam

9

laser device

10

probing

11

reflected radiation

11 '

reflected radiation

11 ''

reflected radiation

12

evaluation

14

output

15

connecting cable

Claims (7)

Method for determining the melting depth during the melting of a metal layer ( 5 . 6 ), in which - under the metal layer ( 5 ) one or more layer (s) ( 3 . 4 ) is / are provided, which when exposed to a test radiation in each case a layer-specific, different to the metal layer characteristic discharge radiation (λ _{char (4)} , λ _{char (3)} ), emit / emit, - during the melting of the melting area ( 7 . 7 ' . 7 '' ) with a test radiation ( 10 ) and - from the reflected test radiation ( 11 ) the melting of the layer layer (s) ( 5 . 4 . 3 ) is detected by detecting the layer-specific characteristic discharge radiation (λ _{char, Cu} , λ _{char (4)} , λ _{char (3)} ) and from this the melting depth is concluded. Method according to claim 1, in which - the metal layer ( 5 . 6 ) and the layer layer (s) ( 3 . 4 ) are produced from the same starting material, the layer layer (s) ( 3 . 4 ) is doped differently in layers in order to produce different characteristic discharge radiation (λ _{char, Cu} , λ _{char (4)} , λ _{char (3)} ). Method according to one of claims 1 or 2, in which - the reflected test radiation ( 11 ) by means of a spectrometer ( 12 ) is monitored. Method according to one of the preceding claims, in which - for melting the metal layer ( 5 . 6 ) and for generating the test radiation ( 10 ) the same laser device ( 9 ) is used. Substrate for use in a method according to any one of the preceding claims, wherein - on a support ( 2 ) several metallic layers ( 5 . 4 . 3 ) are applied from a base material, each layer ( 5 . 4 . 3 ) has an individual doping such that when exposed to a test radiation ( 10 ) emits a layer-specific characteristic discharge radiation (λ _{char, Cu} , λ _{char (4)} , λ _{char (3)} ). The substrate of claim 5, wherein - the basic material Copper is. Substrate according to claim 5 or 6, wherein - the doping of the layers ( 5 . 4 . 3 ) is selected such that the respective layer-specific work function to the uppermost layer ( 5 ) increases.