Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Physical stress responsive
Reexamination Certificate
2001-02-21
2002-04-30
Chaudhari, Chandra (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Physical stress responsive
C438S021000
Reexamination Certificate
active
06379987
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of manufacturing a hybrid integrated circuit comprising a semiconductor element and a piezoelectric filter which are provided next to each other on a carrier substrate, with the semiconductor element comprising semiconductor zones provided in a layer of silicon, and the piezoelectric filter comprising an acoustic resonator formed on an acoustic reflector layer, said acoustic resonator including a layer of a piezoelectric material, a first electrode situated between this layer and the acoustic reflector layer, and, on the other side of the layer of piezoelectric material, a second electrode situated opposite the first electrode.
The semiconductor element may be a single transistor but also an integrated circuit formed in a layer of semiconductor material and comprising a large number of transistors to which, if necessary, passive components may be added. Piezoelectric filters, also referred to as “Thin Film Acoustic Wave Resonators” can be manufactured so as to have resonant frequencies in the range between 500 MHz and 5 GHz, quality factors Q above 1,000 and small dimensions, for example a length and a width of 200 &mgr;m. The use of such filters enables hybrid integrated circuits, such as selective amplifiers, to be formed on the carrier substrate, which are particularly suitable for use in equipment for personal wireless communication, such as GSM telephony, with which signals of said frequencies are processed.
In practice, the layer of piezoelectric material may be, for example, a layer of aluminium nitride AlN or zinc oxide ZnO. These layers are applied in a thickness equal to half the wavelength with which acoustic waves of said frequencies propagate in these materials. The acoustic reflector layer on which the resonator is situated is generally composed in practice of several sub-layers of alternately a high and a low acoustic impedance. Use is customarily made of sub-layers of, for example, tungsten having a relatively high acoustic impedance of approximately 100 Gg/m
2
s and, for example, silicon oxide or a synthetic resin having a relatively low acoustic impedance of, respectively, approximately 13 Gg/m
2
s and approximately 2 Gg/m
2
s. These layers are applied in a thickness which is equal to a quarter of the wavelength with which acoustic waves of said frequencies propagate in these materials. For frequencies in said frequency range, both the piezoelectric layers and the reflector layers have thicknesses in the range from 1 to 3 &mgr;m.
DESCRIPTION OF THE RELATED ART
U.S. Pat. No. 3,414,832 discloses a method of the type mentioned in the opening paragraph, where, in a first example, there is started from a carrier substrate of silicon. In this substrate, the semiconductor element, i.e. a bipolar transistor, is formed. Subsequently, the piezoelectric filter is provided next to the semiconductor element. For this purpose, first an acoustic reflector layer is locally formed on the substrate, whereafter the acoustic resonator is provided on this layer. Finally, a metallization is formed which connects the semiconductor element with the filter. In a second example, there is started from an insulating, ceramic substrate on which first a semiconductor crystal including the semiconductor element is provided. From this point, the method is carried out in the same way as described with respect to the first example.
In practice, it has been found to be difficult to provide the acoustic reflector layer next to the semiconductor element. For this purpose, such a layer must be deposited both on and next to the semiconductor element, whereafter it must be patterned by means of an etch treatment, as a result of which the layer is removed again from the semiconductor element. Deposited layers may exhibit local differences in thickness, and etch processes may locally take place at different etch rates. Consequently, in order to make sure that the layer is entirely removed from the semiconductor element, the etch treatment is carried out for a period of time exceeding the time strictly necessary to remove the layer. In practice, an “overetch time” of approximately 20% is normal. As the acoustic reflector layer is thick in comparison with layers used in semiconductor elements, it is possible that layers in the semiconductor element are entirely etched away during the “overetch” time”. This may result in such damage to the semiconductor element that it can no longer be used.
SUMMARY OF THE INVENTION
It is an object of the invention to overcome the above-mentioned drawback. To achieve this, the method in accordance with the invention is characterized in that the semiconductor element is formed at a first side of an auxiliary slice of silicon, after which the layer of piezoelectric material supporting the first electrode is provided, at the same first side, on the auxiliary slice, whereafter the structure thus formed is provided over its free surface area with an acoustic reflector layer and, subsequently, attached to the carrier substrate with this layer, whereafter silicon is removed from the second side of the auxiliary slice at the location of the acoustic resonator.
After the formation of the semiconductor element, a layer of a piezoelectric material is deposited on the auxiliary slice of silicon and subsequently etched in accordance with a pattern. This layer is thin as compared to the acoustic reflector layer. The thickness of the layer of piezoelectric material practically ranges between 1 and 3 &mgr;m. Such a thin layer can be readily etched in accordance with a pattern without layers of the semiconductor element being attacked to such an extent by an “overetch time” that the semiconductor element becomes useless. The acoustic reflector layer which is applied to the semiconductor element and to the acoustic resonator is not etched in accordance with a pattern. The hybrid integrated circuit can thus be formed on the carrier substrate without the acoustic reflector layer having to be etched in accordance with a pattern.
Preferably, the semiconductor zones are formed in a top layer of a slice of silicon, which slice is provided with a silicon oxide layer situated on the top layer, after which the layer of piezoelectric material supporting the first electrode is formed on this layer of silicon oxide, whereafter the free surface of the structure thus formed is provided with an acoustic reflector layer, and the structure is subsequently attached to the carrier substrate with said layer, after which the surface of the second side of the auxiliary slice is subjected to a silicon-removal process which stops just short of the top layer, and subsequently, at the location of the acoustic resonator, silicon is removed down to the layer of silicon oxide. The first silicon-removal step, which takes place throughout the surface, may be carried out by means of a customary mechanical-chemical polishing treatment. In the second step, wherein the layer of silicon oxide is exposed at the location of the acoustic resonator, the silicon oxide layer may serve as an etch-stop layer. In an etch bath containing potassium hydroxide, silicon can be etched very selectively with respect to silicon oxide. During this etching treatment, the auxiliary slice must be masked at the location of the semiconductor element to preclude silicon from being removed at said location.
The silicon can be even more readily removed from the second side of the auxiliary slice if the semiconductor zones are formed in a top layer of a silicon slice provided with a silicon oxide layer buried under the top layer, which top layer is removed next to the semiconductor element, whereafter the layer of piezoelectric material supporting the first electrode is formed on the silicon oxide layer thus exposed, after which the free surface of the structure thus formed is provided with an acoustic reflector layer, whereafter the structure is attached to the carrier substrate with this layer, after which silicon is removed from the surface of the second side of the auxiliary slice down to
Dekker Ronald
Dolmans Wilhelmus Mathias Clemens
Leyten Lukas
Maas Henricus Godefridus Rafael
Chaudhari Chandra
U.S. Philips Corporation
Waxler Aaron
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