Method for bonding two silicon substrates, and a correspondeing system of two silicon substrates

The present application claims priority to and the benefit of German patent application no. 10 2011 089 569.8, which was filed in Germany on Dec. 22, 2011, the disclosure of which is incorporated herein by reference.

The present invention relates to a method for bonding two silicon substrates, and to a corresponding system of two silicon substrates.

Although various types of micromechanical substrates can be used, the present invention and the problem underlying it are explained on the basis of wafer substrates on silicon basis.

In the related art, e.g., in US 2008/0283990 A1, eutectic bond connections between substrates are described, for which aluminum/silicon/copper are used.

U.S. Pat. No. 7,442,570 B2 discusses a method for bonding two silicon substrates, in which an aluminum layer (not further described) is provided on an oxide layer, which is situated on a top surface of the substrate. This aluminum layer is eutectically bonded to a germanium layer situated on a further substrate. No aluminum spiking occurs in this process.

It is already known from the field of microelectronics to use aluminum with added silicon and copper for aluminum circuit tracks. Copper increases the current-carrying capacity of the circuit tracks, that is to say, improves the electrical migration resistance, while the addition of silicon is to prevent the known Al-spiking into the substrate. The formation of undesired aluminum spikes into the silicon substrate occurs mainly during thermal treatments, which are meant to improve/reduce the electrical contact resistance between aluminum circuit tracks and contact points of electrical components in the silicon substrate. Flat pn-junctions may be short-circuited in the process, which ultimately may lead to a breakdown of the component.

The spiking is a migration process of silicon and aluminum atoms, i.e., aluminum atoms migrate to the silicon substrate, while silicon atoms migrate to the aluminum. To prevent this behavior, normally, in the case of aluminum circuit tracks, silicon is added in the amount needed to reach the solubility limit thereof in aluminum. In practice this is achieved in that aluminum-silicon-copper targets suitable for sputtering are used during the deposition process of the aluminum circuit track layer.

The present invention provides a method for bonding two silicon substrates as described herein, and a corresponding system of two silicon substrates as described herein.

Advantageous further refinements constitute the subject matter of the respective further descriptions herein.

An aspect underlying the present invention is to provide aluminum layers which have no silicon component and selectively lead to spikes during the eutectic bonding of aluminum-germanium, at the connection surfaces of the silicon substrates, the spikes containing aluminum as a minimum. The aluminum bonding layer is a pure aluminum layer or an aluminum layer having a relatively low copper component.

Depending on the aluminum layer used (with/without Cu), it is also possible that Cu atoms are present in the eutecticum and in the Al spikes. In addition, Si atoms may be present here as well, which were dissolved out of the substrate during the Al spike formation. It is then possible that they attach to Ge atoms during the eutectic bonding and form SiGe clusters in the bonding interface.

The method according to the present invention allows the use of an aluminum-germanium eutecticum for bonding two or more silicon substrates, which makes it possible to produce a particularly good connection to the silicon substrates, in particular silicon wafer substrates.

The present invention utilizes the aluminum spiking into the silicon substrate, which usually is undesired in other applications, for the mechanical anchoring, and thus for improving the adhesion, of the eutectic bonding layer to the bonding surfaces of the silicon substrates. As a result, the present invention provides for selective spiking, which occurs because of the use of the pure aluminum layer or the aluminum copper layer without silicon component at the bonding surface.

Since spiking is a diffusion process, it is temperature- and time-dependent. The eutectic bonding process using aluminum and germanium may be carried out in the range between 425° C. and 500° C.

The selective spiking leads to virtual meshing between the eutectic bonding layer and the particular silicon substrate, which enlarges the bonding surface and improves the absorption of lateral and vertical forces.

Additional features and advantages of the present invention are elucidated in greater detail below, based on specific embodiments and with reference to the drawings.

FIGS. 1 a and 1 b show schematic cross-sectional views to explain a method for bonding two silicon substrates, and a corresponding system of two silicon substrates according to a first specific embodiment of the present invention, that is, prior to the bonding, FIG. 1 a and following the bonding, FIG. 1 b.

FIGS. 2 a and 2 b show schematic cross-sectional views to explain a method for bonding two silicon substrates, and a corresponding system of two silicon substrates according to a second specific embodiment of the present invention, that is, prior to the bonding, FIG. 2 a and following the bonding, FIG. 2 b.

FIGS. 3 a and 3 b show schematic cross-sectional views to explain a method for bonding two silicon substrates, and a corresponding system of two silicon substrates according to a third specific embodiment of the present invention, that is, prior to the bonding, FIG. 3 a and following the bonding, FIG. 3 b.

FIGS. 4 a and 4 b show schematic cross-sectional views to explain a method for bonding two silicon substrates, and a corresponding system of two silicon substrates according to a first specific embodiment of the present invention, that is, prior to the bonding, FIG. 4 a and following the bonding, FIG. 4 b.

FIGS. 5 a and 5 b show schematic cross-sectional views to explain a method for bonding two silicon substrates, and a corresponding system of two silicon substrates according to a first specific embodiment of the present invention, that is, prior to the bonding, FIG. 5 a and following the bonding, FIG. 5 b.

FIGS. 6 a and 6 b show schematic cross-sectional views to explain a method for bonding two silicon substrates, and a corresponding system of two silicon substrates according to a first specific embodiment of the present invention, that is, prior to the bonding, FIG. 6 a and following the bonding, FIG. 6 b.

FIGS. 7 a and 7 b show schematic cross-sectional views to explain a method for bonding two silicon substrates, and a corresponding system of two silicon substrates according to a first specific embodiment of the present invention, that is, prior to the bonding, FIG. 7 a and following the bonding, FIG. 7 b.

In the figures, identical reference symbols denote identical or functionally equivalent elements.

FIG. 1 a ), b ) show schematic cross-sectional views to explain a method for bonding two silicon substrates, and a corresponding system of two silicon substrates according to a first specific embodiment of the present invention, that is, prior to the bonding, FIG. 1 a ) and following the bonding, FIG. 1 b ). An exemplary field of use for the subject matter of the present invention is the bonding of a silicon sensor wafer to a silicon cap wafer, e.g., as required for rate-of-rotation sensors or acceleration sensors or pressure sensors.

In FIG. 1 a ), b ), reference numeral 1 a denotes a first silicon substrate having a first bonding surface Va, on which a first bonding layer 20 a of pure aluminum or of aluminum-copper having a copper component of between 0.1 and 5%, which may be 0.5 and 1%, has been deposited.

Reference numeral 1 b denotes a second silicon substrate having a second bonding surface Vb, on which a germanium layer 10 b has been deposited.

Starting from the process state according to FIG. 1 a ), silicon substrates 1 a , 1 b are bonded in such a way that the first and the second bonding surfaces Va, Vb are situated opposite each other, i.e., layers 20 a , 10 b lie on top of each other. This is followed by a thermal treatment step to form a eutectic bonding layer 100 made of aluminum-germanium or containing aluminum germanium as main component, between first silicon substrate 1 a and second silicon substrate 1 b . The thermal treatment step may be performed in a range between 400° C. and 550° C., which may be between 425° C. and 500° C.

During the thermal treatment step, spikes S form on first bonding surface Va, where first bonding layer 20 a of pure aluminum or of aluminum copper is situated, spikes S extending into first silicon substrate 1 a . Spikes S contain aluminum with traces of silicon as main component, but spikes S may also contain germanium or copper in addition, depending on the bonding layer 20 a actually used.

FIG. 2 a ), b ) show schematic cross-sectional views to explain a method for bonding two silicon substrates, and a corresponding system of two silicon substrates according to a second specific embodiment of the present invention, that is, prior to the bonding, FIG. 2 a ) and following the bonding, FIG. 2 b ).

In the second specific embodiment, a bonding layer 20 a made of pure aluminum or of aluminum-copper having the mentioned copper component is situated on first bonding surface VA, and a bonding layer 20 b , which likewise is made of pure aluminum or of aluminum-copper with the mentioned copper component, is situated on second bonding surface Vb. An additional bonding layer 10 a made of germanium is deposited on bonding layer 20 a , and another bonding layer 10 b of germanium is deposited on bonding layer 20 b.

As illustrated in FIG. 2 b ), the same thermal treatment step as in the first specific embodiment takes place, whereupon spikes S form not only on first bonding surface Va, but also on second bonding surface Vb, since a spike-promoting bonding layer 20 b of pure aluminum or of aluminum-copper is provided here as well.

It should be mentioned that in the second specific embodiment, one of bonding layers 10 a , 10 b of germanium may also be omitted.

FIG. 3 a ), b ) show schematic cross-sectional views to explain a method for bonding two silicon substrates, and a corresponding system of two silicon substrates according to a third specific embodiment of the present invention, that is, prior to the bonding, FIG. 3 a ) and following the bonding, FIG. 3 b ).

The third specific embodiment differs from the second specific embodiment in that the sequence of layers 10 b , 20 b is reversed, that is to say, bonding layer 10 b of germanium is situated on second bonding surface Vb, and bonding layer 20 b of pure aluminum or aluminum-copper is situated on top of it. As illustrated in FIG. 3 b ), the formation of spikes S in this specific embodiment takes place in the same way as in the first specific embodiment, but on first bonding surface Va only.

FIG. 4 a ), b ) show schematic cross-sectional views to explain a method for bonding two silicon substrates, and a corresponding system of two silicon substrates according to a first specific embodiment of the present invention, that is, prior to the bonding, FIG. 4 a ) and following the bonding, FIG. 4 b ).

In contrast to the first specific embodiment, in the fourth specific embodiment an additional bonding layer 30 a of aluminum-silicon-copper has been deposited on bonding layer 20 a , the silicon component, in addition to the aforementioned copper component, lying between 0.1 and 5%, which may be between 1 and 2%.

In this specific development as well, spikes form on first bonding surface Va during the thermal treatment step due to the presence of bonding layer 20 a of pure aluminum or aluminum-copper having the aforementioned copper component. The eutectic bonding layer in this specific embodiment contains aluminum, germanium, copper and silicon and therefore is denoted by reference numeral 100 ′.

FIG. 5 a ), b ) show schematic cross-sectional views to explain a method for bonding two silicon substrates, and a corresponding system of two silicon substrates according to a first specific embodiment of the present invention, that is, prior to the bonding, FIG. 5 a ) and following the bonding, FIG. 5 b ).

In the fifth specific embodiment, a bonding layer 10 a of germanium has been deposited on bonding layer 20 a situated on first bonding surface Va.

A bonding layer 30 b of aluminum-silicon-copper having the aforementioned copper component or silicon component, is deposited on second bonding surface Vb. As in the first specific embodiment, spikes S form only on first bonding surface Va during the thermal treatment step.

FIG. 6 a ), b ) show schematic cross-sectional views to explain a method for bonding two silicon substrates and a corresponding system of two silicon substrates according to a first specific embodiment of the present invention, that is, prior to the bonding, FIG. 6 a ) and following the bonding, FIG. 6 b ).

In the sixth specific embodiment, in contrast to the fifth specific embodiment, a bonding layer 10 b of germanium is deposited on second bonding surface Vb, and the afore-described bonding layer 30 b of aluminum-silicon-copper is deposited thereon.

Here, too, the development of spikes S in the thermal treatment step occurs only on first bonding surface Va.

FIG. 7 a ), b ) show schematic cross-sectional views to explain a method for bonding two silicon substrates, and a corresponding system of two silicon substrates according to a first specific embodiment of the present invention, that is, prior to the bonding, FIG. 7 a ) and following the bonding, FIG. 7 b ).

In comparison to the second specific embodiment, in the seventh specific embodiment an additional bonding layer 20 b ′ of pure aluminum or of aluminum-copper having the aforementioned copper component has been deposited on bonding layer 10 b of germanium, above second bonding surface Vb.

Although the present invention has been described with the aid of exemplary embodiments, it is not limited to these. The mentioned materials and topologies in particular have merely exemplary character and are not restricted to the examples elucidated.

It should be mentioned specifically that the Al spike formation is able to be initialized in parallel with, but also prior to the formation of the eutectic bond. Like in many processes, there is a temperature and time-dependency here as well. If one stays below the eutectic bonding temperature, the Al-spiking process may already be started without a simultaneous initialization of a bonding connection. On the other hand, when using a temperature ramp which rapidly leads to the bonding temperature above the eutectic point, then the spiking process takes place virtually simultaneously with the eutectic bonding process.