Method for manufacturing semiconductor substrate

An epitaxial layer is formed on a high-resistance semiconductor substrate containing interstitial oxygen at a high concentration, and then a heat treatment is performed to the semiconductor substrate at a high temperature in an oxidizing atmosphere. Accordingly, a stratiform region of SiO2 is formed by deposition at an interface between the epitaxial layer and the semiconductor substrate. As a result, an apparent SOI substrate for an SOI semiconductor device can be manufactured at a low cost.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for manufacturing a semiconductor substrate that is generally used for a semiconductor device including a composite IC and an LSI.

2. Description of the Related Art

An SOI (Silicon On Insulator) semiconductor device has a semiconductor layer that is disposed on a semiconductor substrate through an intermediate insulating layer. Such an SOI substrate is suitably used for a device such as a composite IC, a high withstand voltage IC or an LSI for a portable instrument that is required to have high speed and low consumption power, in which several kinds of elements such as bipolar, MOS, and power elements are mounted on one chip.

To manufacture the SOI semiconductor device, an SOI substrate is required, which includes a high-quality crystalline semiconductor layer that is formed on a layer made of an insulating material such as SiO2with extremely high resistance. Known conventional methods for manufacturing the SOI substrates include a bonding method, a SIMOX method, a method that combines bonding and ion implantation by utilizing hydrogen brittleness, and the like.

However, the SOI substrate manufactured by conventional techniques in any of the above-described methods is several to several dozen times more expensive than an ordinary bulk substrate. This is the biggest reason for preventing the SOI semiconductor device from being practically used, regardless of its inherent high performance and high functionality.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems. An object of the present invention is to provide a method for manufacturing a semiconductor substrate suitably used for an SOI semiconductor device, with high quality at low cost.

Briefly, according to a first aspect of the present invention, after an epitaxial layer is formed on a semiconductor substrate, an insulating layer is formed by deposition at an interface between the epitaxial layer and the semiconductor substrate by performing a heat treatment in an oxidizing atmosphere. Thus, the semiconductor substrate for an SOI semiconductor device can be manufactured easily at low cost. A thickness of an SOI layer required for the semiconductor device can be determined by the thickness of the epitaxial layer.

According to a second aspect of the present invention, an apparent SOI substrate can be formed by epitaxially growing a semiconductor layer on a semi-insulating substrate having a high resistance. Preferably, before the semiconductor layer is epitaxially grown on the substrate, a heat treatment is performed in a hydrogen atmosphere to improve crystallinity on a surface of the semiconductor substrate. Accordingly, the crystallinity of the semiconductor layer is further improved.

According to a third aspect of the present invention, a base wafer and a bonding wafer are prepared, one of which is composed of a semiconductor substrate containing oxygen at a high concentration or a semi-insulating semiconductor substrate having a high resistance. An oxide film is formed on one of the base wafer and the bonding wafer. Then, the base wafer and the bonding wafer are bonded together with the oxide film interposed therebetween. After that, a back surface of the bonding wafer at an opposite side of the base wafer is ground and polished to form an SOI layer on the base wafer through the oxide film.

According to fourth aspect of the present invention, first, an element is ion-implanted into a high resistance semiconductor substrate, containing oxygen at a high concentration, to form a deposition nuclear layer by the element. The deposition nuclear layer has a plurality of nuclei for deposition and extends at a depth from a surface of the semiconductor substrate. Then, a heat treatment is performed to the semiconductor substrate to form an oxide layer in the semiconductor substrate by making the oxygen, contained in the semiconductor substrate, deposit using the plurality of nuclei in the deposition nuclear layer.

According to the present invention described above, in any case, the semiconductor substrate for an SOI semiconductor device can be manufactured with high quality at significantly reduced low cost.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method for manufacturing a semiconductor substrate for an SOI semiconductor device to which a first preferred embodiment of the invention is applied is explained referring toFIGS. 1A and 1B.

In the first embodiment, a high-resistance semiconductor substrate1having a mirror-finished surface and including interstitial oxygen at a high concentration is used as a substrate for epitaxial growth. While silicon single crystal that has been grown by a CZ method contains oxygen of about 1017atoms/cm3among lattices therein, a mirror wafer containing interstitial oxygen at a higher concentration of, for example, more than 1×1018atoms/cm3is used as a start material in this embodiment. The mirror wafer can be manufactured by the CZ method similarly to ordinary mirror wafers.

This semiconductor substrate1undergoes a pre-cleaning treatment including, for example, an immersion treatment into SC-1 solution (mixture composed of NH4OH, H2O2, and H2O, APM solution), an immersion treatment into SC-2 solution (mixture composed of HCl, H2O2, and H2O, HPM solution), an immersion treatment into dilute HF solution, super-pure water substitution, and drying. Then, an epitaxial layer (semiconductor active layer)2is formed in accordance with a required thickness by epitaxial growth involving HCl etching, H2gas substitution, and the like within an epitaxial apparatus.

After that, the semiconductor substrate1on which the epitaxial layer2is formed is heated at a high temperature of, for example, 1150° C. or more, in oxidizing atmosphere. Accordingly, oxygen in the high-resistance semiconductor substrate1containing interstitial oxygen at a high concentration is deposited using as nuclei a distortion layer at the interface between the epitaxial layer2and the semiconductor substrate1. In consequence, a stratiform region (oxide film)3of SiO2is formed at the interface between the epitaxial layer2and the semiconductor substrate1, thereby forming an SOI structure. The stratiform region3of SiO2formed at the interface is about 100 nm in thickness. However, since the semiconductor substrate1used has high resistance, the stratiform region3can electrically isolate elements in cooperation with trench isolation, and realize performances equivalent to those of an ordinary SOI substrate.

The method for manufacturing the semiconductor substrate for an SOI semiconductor device described above can dispense with many steps such as preparation of two mirror wafers for bonding, bonding of the two mirror wafers, heat treatment for bonding, edge treatment for obtaining a required SOI thickness, surface grinding, re-polishing for mirror finish, and several checks for voids, SOI thickness, and the like, in comparison with a conventional bonding method. In consequence, significant cost reduction can be achieved. Also, in comparison with a SIMOX method, the SIMOX method necessitates an expensive apparatus and its throughput is low because oxygen must be ion-implanted into a semiconductor substrate with high energy to have a high concentration (1×1018cm−3). To the contrary, in the present invention, the SiO2stratiform region3can be formed by using the semiconductor substrate containing interstitial oxygen at a high concentration, and performing only the ordinary epitaxial growth for the active layer and the heat treatment in oxidizing atmosphere. Therefore, significant cost reduction can be realized in the present embodiment.

FIG. 2shows a second preferred embodiment of the present invention. In this embodiment, a semi-insulating semiconductor substrate11is used in place of the semiconductor substrate1used in the first embodiment. An epitaxial layer12can be formed on the semiconductor substrate11similarly to the first embodiment. Accordingly, an apparent SOI structure can be constructed without performing an oxygen deposition heat treatment at a high temperature in oxidizing atmosphere after the epitaxial layer12is formed.

For example, a substrate having a lifetime of a minority carrier (minority carrier lifetime) less than about 1×10−8sec and a carrier concentration less than about 1×1014cm−3can be used as the semi-insulating semiconductor substrate11in this embodiment. This is because, in a state where elements are formed with impurity layers in the epitaxial layer12and the semiconductor substrate11adjacently to each other, hFE of a parasitic transistor formed by the impurity layers of the adjacent elements and the semiconductor substrate11and the minority carrier lifetime τ g have a relation as shown inFIG. 3. That is, referring toFIG. 3, it is preferable that hFE of the parasitic transistor is less than about 10−1to negligibly decrease the effect by the parasitic transistor, and hFE of the parasitic transistor becomes less than about 10−1when the minority carrier lifetime τ g is less than abut 1×10−8sec. Therefore, the minority carrier lifetime is determined as described above.

The carrier concentration of the semiconductor substrate11is not limited, and for example, may be 1×1014cm−3or less. The semi-insulating semiconductor substrate11may be a substrate containing an impurity that forms a deep trap level in a bandgap of high concentration interstitial oxygen, carbon, or the like.

FIG. 4shows a third preferred embodiment of the present invention. In this embodiment, a semi-insulating semiconductor substrate21doped with a dopant, a conductivity type of which is opposite to that of an epitaxial layer22is used in place of the semiconductor substrate11in the second embodiment. As shown in the figure, specifically, when the epitaxial layer22is n type, the semiconductor substrate21is p type, and when the epitaxial layer22is p type, the semiconductor substrate21is n type. Accordingly, because a PN junction is provided between the epitaxial layer22and the semiconductor substrate21, electrical insulation can be achieved more securely than in the second embodiment.

FIGS. 5A to 5Cshow a fourth preferred embodiment of the present invention. In this embodiment, similarly to the first to third embodiments, a high-resistance semiconductor substrate containing interstitial oxygen at a high concentration or semi-insulating substrate is used as a semiconductor substrate31(FIG. 5A). Then, a high temperature heat treatment is performed to the semiconductor substrate31at, for example, 1000° C. or more in hydrogen atmosphere before performing epitaxial growth.

Accordingly, there arise both or either of phenomena that interstitial oxygen atoms contained in the substrate are outwardly diffused and released from the surface of the substrate, and that atoms forming the surface of the substrate are rearranged. Then, a layer32is formed at the substrate surface by outward diffusion of oxygen or/and rearrangement of atoms (FIG. 5B), so that the crystallinity of the substrate surface is improved. Because of this, an epitaxial layer33formed thereafter can have further improved crystallinity.

FIGS. 6A to 6Dshow a fifth preferred embodiment of the present invention. Incidentally, steps shown inFIGS. 6A to 6Care substantially the same as those shown inFIGS. 5A to 5C. In this embodiment, an epitaxial growth substrate that is fabricated by the method described in the fourth embodiment is heated at a high temperature of, for example, 1150° C. or more, in oxidizing atmosphere (FIG. 6D).

Accordingly, oxygen in the high-resistance semiconductor substrate31that contains interstitial oxygen at a high concentration is deposited using as nuclei a distortion layer at the interface between the epitaxial growth layer and the substrate so that a SiO2stratiform region34can be formed by additionally performing the heat treatment in the oxidizing atmosphere as in the first embodiment.

FIGS. 7A to 7Cshow a sixth preferred embodiment of the present invention. In this embodiment, as in the first to third embodiments, a high-resistance semiconductor substrate containing interstitial oxygen at a high concentration or semi-insulating substrate is used as a semiconductor substrate41(FIG. 7A), and a thin semiconductor layer42is epitaxially grown to have a conductive type opposite to that of an epitaxial layer43that is formed in a subsequent step as an active layer (FIG.7B). Successively, the epitaxial layer43is epitaxial grown (FIG. 7C). For example, when the active layer (epitaxial layer43) is formed to be an n−type layer, the semiconductor layer42is formed to be a p−type layer.

According to this manufacturing method, the semiconductor layer42, which is formed at the interface between the semiconductor substrate41and the epitaxial layer43and has the conductivity type opposite to that of the active layer, can be completely depleted to support voltage and to perform insulating isolation in cooperation with the underlying high-resistance semiconductor substrate41. As a result, the semiconductor layer42can provide an apparent SOI structure. As in the first embodiment, a heat treatment may be performed in high-temperature oxidizing atmosphere to form an oxide layer deposited.

FIGS. 8A to 8Dshow a seventh preferred embodiment of the present invention. In this embodiment, before the epitaxial growth in the sixth embodiment is performed, similarly to the fourth and fifth embodiments, the step shown inFIG. 5Bis performed to the semiconductor substrate41. That is, a high temperature heat treatment is performed at, for example, 1000° C. or more in hydrogen atmosphere. Accordingly, both or either of phenomenon that interstitial oxygen contained in the semiconductor substrate41is outwardly diffused to be released from the substrate surface, and atoms constituting the substrate surface are rearranged occur, and a layer44is formed at the substrate surface due to outward diffusion of oxygen and rearrangement of atoms (FIG. 8B). As a result, the crystallinity of the substrate surface is improved.

After that, as shown inFIGS. 8C and 8D, the similar steps to those shown inFIGS. 7B and 7Care performed to form an apparent SOI substrate. Incidentally, also in the present embodiment, a heat treatment may be performed in a high-temperature oxidizing atmosphere to make an oxide layer deposited as in the fifth embodiment.

FIGS. 9A to 9Dshows an eighth preferred embodiment of the present invention. This embodiment uses a high-resistance semiconductor substrate500having a mirror-finished surface and containing oxygen at a high concentration (FIG. 9A). Then, first, a pad oxide film (not shown) having a thickness of about 45 nm is formed on the surface by performing a heat treatment in oxidizing atmosphere. This step is performed in an ordinary semiconductor process to prevent occurrence of channeling components along crystal axes and sputters on the surface by ion implantation.

Next, for example, oxygen ions are implanted into the substrate500through the pad oxide film at about 1×1016cm−2(FIG. 9B). An acceleration voltage in this case was 100 to 180 KeV in this embodiment, which was determined in accordance with a depth of implantation. Accordingly, nuclei for depositing dissolved oxygen in the substrate500can be formed as a deposition nuclear layer501shown inFIG. 9B. Implantation of oxygen ions for forming an SOI substrate is known in the SIMOX method; however, in the case of the SIMOX method, a dose is generally about 1×1018cm−2, which is larger than that of the present embodiment by two digits.

After that, a heat treatment is performed to the semiconductor substrate500at a temperature of, for example, 1100° C. or more in nitrogen or oxygen atmosphere for 18 to 35 hours (FIG. 9C). Accordingly, dissolved oxygen in the semiconductor substrate500is deposited using implanted oxygen ions as nuclei in the layer501so that an oxide layer, i.e., a SiOx layer502is formed as shown inFIG. 9C, thereby forming an SOI substrate504. Here, a value of x was about 2 at the heat treatment conditions described above.

In this embodiment, because the substrate500is composed of a high-resistance semiconductor substrate, a high-resistance semiconductor layer underlies the deposited oxide layer502, and a depletion layer is formed when a voltage is applied across the oxide layer. As a result, a larger withstand voltage than that determined by the thickness of the oxide film can be exhibited. In this embodiment, although oxygen is ion-implanted as an element for forming deposition nuclei, other elements such as nitrogen, silicon, carbon, and fluorine can be used in place of oxygen, which are liable to combine with oxygen to be deposited.

As shown inFIG. 9D, when a semiconductor layer503having a predetermined conductive type and a thickness is epitaxially grown on the SOI substrate504manufactured as above, an SOI substrate can be formed with desirable film thickness, conductive type, and concentration.

FIGS. 10A to 10Dshow a ninth preferred embodiment of the present invention. In this embodiment, a high-resistance semiconductor substrate containing interstitial oxygen at a high concentration or semi-insulating substrate as disclosed in the first to three embodiments is prepared as a base wafer51, and an ordinary mirror wafer is prepared as a bonding wafer52(FIG. 10A). Then, an oxide film53is formed on a mirror-finished principal surface of at least one of the base wafer51and the bonding wafer52(FIG. 10B), and the two wafers are bonded together at the principal surfaces thereof in clean atmosphere by an ordinary wafer bonding method, and a high-temperature heat treatment is performed to thereby form a combined wafer54(FIG. 10C). After that, the back surface of the combined wafer54at the side of the bonding wafer52is ground and polished for mirror finishing so that an SOI layer have a predetermined thickness. As a result, an SOI substrate is manufactured (FIG. 10D).

In this embodiment, unlike a conventional manufacturing method, because the high-resistance semiconductor substrate containing interstitial oxygen at a high concentration or semi-insulating substrate is used as the base wafer, an SOI substrate having a high withstand voltage of, for example, 200 V or more can be attained with a thin embedded oxide film thickness of about several hundreds nm that is about 1/10 thinner than that of a conventional one.

FIGS. 11A to 11Eshows a tenth preferred embodiment of the present invention. In this embodiment, a high-resistance semiconductor substrate containing interstitial oxygen at a high concentration or semi-insulating substrate as described in the first to third embodiments is prepared as a bonding wafer61, while an ordinary mirror wafer is prepared as a base wafer62(FIG. 11A). Then, an oxide film63is formed on a mirror-finished principal surface of at least one of the bonding wafer61and the base wafer62(FIG. 11B), and the two wafers are bonded together at the principal surfaces thereof in clean atmosphere by an ordinary wafer bonding method, and a high-temperature heat treatment is performed to thereby form a combined wafer64(FIG. 11C).

After that, the back surface of the combined wafer64at the side of the bonding wafer61is ground and polished for mirror finishing. As a result, an SOI substrate is manufactured with an SOI layer having a required thickness (FIG. 11D). Further, oxygen on the SOI layer surface is outwardly diffused by a heat treatment performed at a high temperature in hydrogen atmosphere. Accordingly, oxygen remains at the bonding interface, and gettering sites are formed at that portion (FIG. 11E). The gettering sites formed in the SOI layer can take heavy metal contaminants in when an oxide film is formed on the SOI layer, and therefore lengthen the lifetime of the oxide film.

For example, the SOI substrate manufactured as described in this embodiment can be used for a device shown inFIG. 12. This device is formed with an LDMOS70, a bipolar transistor80, a CMOS90, and a diode100.

The LDMOS70is composed of a p type base region71formed at a surface portion of the n−type SOI layer (bonding wafer61), an n+type source region72formed in a surface portion of the p type base region71, an n+type drain region73formed in a surface portion of the SOI layer remotely from the p type base region71, a gate insulating film74formed at least on the p type base region71, a gate electrode75formed on the gate insulating film74, a source electrode76electrically connected to the n+type source region72, and a drain electrode77electrically connected to the n+type drain region73.

The bipolar transistor80is composed of a p type base region81formed on a surface portion of the SOI layer, an n+type emitter region82formed in a surface portion of the p type base region81, an n+type collector region83formed in a surface portion of the SOI layer remotely from the p type base region81, and a base electrode84, an emitter electrode85, and a collector electrode86electrically connected to these regions, respectively.

The CMOS90is composed of an n type well layer91and a p type well layer92, which are formed in a surface portion of the SOI layer, p+type source93aand drain94aformed in the n type well layer91separately from each other, n+type source93band drain94bformed in the p type well layer92separately from each other, gate insulating films95a,95band gate electrodes96a,96brespectively provided above channel regions between the respective sources93a,93band the respective drains94a,94b, source electrodes97a,97brespectively connected to the sources93a,93b, and drain electrodes98a,98brespectively connected to the drains94a,94b.

The diode100is composed of a p type region101and a p+type contact region102formed in a surface portion of the SOI layer, an n+type region103provided remotely from the p type region101, and anode and cathode electrodes104,105electrically connected to the respective regions101,103.

In this device, because gettering sites are formed in the. SOI layer of the SOI substrate manufactured in this embodiment, the following effects can be attained when the SOI substrate is used for the LDMOS70, the CMOS90and the diode100.

Specifically, in the case of elements such as the LDMOS70and the CMOS90having the gate insulating films74,95a,95b, because the gettering sites take heavy metal contaminants in, the gate insulating films74,95a,95bcan be improved in lifetime. This results in improved reliability of the elements.

Besides, in the case of the CMOS90in which both the n type well layer91and the p type well layer92are formed, it is preferable to isolate the layers from each other by a trench in consideration of latch up prevention. However, there is a case where the trench isolation is not provided to reduce the size of the device. Even in such a case, the gettering sites can prevent latch up.

Further, when an operational state is switched from ON to OFF in the diode100, holes injected into the n−type SOI layer from the anode electrode return into the anode electrode to generate current flow in an inverse direction. However, if gettering sites exist, the gettering sites trap holes as trap sites, and make the holes recombine with electrons. As a result, the holes disappear apparently, and no current flows in the inverse direction. The diode100can be improved in recovery property. Incidentally, though it is not shown inFIG. 12, since an IGBT can have current flow in an inverse direction similarly to the diode100, the SOI substrate shown in this embodiment can be used for formation of the IGBT to improve the recovery property of the IGBT.

The above-described embodiments exemplify oxygen arranged among lattices other than lattice points; however, oxygen may be arranged at other positions to provide the same effects as described above. Especially, oxygen contained in the semiconductor substrates1,21,31,41, and51may not be interstitial oxygen. Also, in the above-described embodiments, the semiconductor substrates1,21,31,41, and51are respectively composed of high-resistance substrates; however, the substrates can provide the same effects as described above even when they do not have high resistance.

In the first and fifth embodiments, although it is explained that the oxide film is deposited by the heat treatment performed in oxidizing atmosphere, it is possible to deposit other insulting layers. For example, a nitride layer can be deposited using as nuclei partially existing nitrogen in a substrate or the like. Thus, the insulating layer can form an apparent SOI substrate. In this case, the semiconductor substrate has no need to contain oxygen therein.

Incidentally, various insulating isolation structures can be formed by the substrates as manufactured in the above-described embodiments. Examples are shown inFIGS. 13A to 13E, in which a substrate having a PN junction as described in the third embodiment is used, but the other substrates in the other embodiments can also be used as well.

For example, as shown inFIG. 13A, a well-isolation structure is formed by forming a well layer110in the n−type epitaxial layer with an inverse conductive type to that of the epitaxial layer22to contact the semi-insulating substrate21. Otherwise, as shown inFIG. 13B, a trench isolation structure can be formed by forming a trench111in the epitaxial layer22so that the trench reaches the semi-insulating substrate21, and by filling the trench111with an insulating film112. Otherwise, as shown inFIG. 13C, a well-trench isolation structure can be formed by combining the structures shown inFIGS. 13A and 13B. As shown inFIG. 13D, a double-trench isolation structure may be formed, in which two trenches each of which is similar to that shown inFIG. 13Bare formed adjacently to each other.FIG. 13Eshows a double-trench isolation structure in which a region interposed between two trenches is made a well layer113having the same conductivity type as that of the semi-insulating substrate21, and the well layer113is grounded for parasitic removal.