Single-pole component manufacturing

The invention relates to a vertical-type single-pole component, comprising regions (34) with a first type of conductivity (P) which are embedded in a thick layer (32) with a second type of conductivity (N). Said regions are distributed over at least one same horizontal level and are independent of each other. The regions also underlie an insulating material (70).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the manufacturing of single-pole components in vertical monolithic form. The following description more specifically relates to components of Schottky diode type made in vertical form in silicon substrates.

2. Discussion of the Related Art

FIG. 1illustrates a conventional Schottky diode structure. Such a structure includes a semiconductor substrate1, typically made of heavily-doped single-crystal silicon of a first conductivity type, generally N type. A cathode layer2covers substrate1. It is N-type doped, but more lightly than substrate1. A metal layer3forms a Schottky contact with N-type cathode2.

The thickness of layer2is chosen to determine the reverse breakdown voltage of the Schottky diode.

FIG. 2illustrates the variation of the electric field E across the thickness of the structure shown inFIG. 1, along an axis A-A′. For clarity, the different portions of curve10ofFIG. 2have been connected by dotted lines to the corresponding regions of FIG.1.

In such a homogeneous structure, the field variation per thickness unit is proportional to the doping level. In other words, the field decreases all the faster as the doping is heavy. It thus very rapidly drops to a zero value in substrate1. Since the breakdown voltage corresponds to the surface included between the axes and curve10, to obtain a high breakdown voltage, the doping of layer2must be minimized and its thickness must be maximized.

In the manufacturing of single-pole components, opposite constraints have to be considered. Single-pole components, such as the diode shown inFIG. 1, must indeed have as small a resistance (Ron) as possible, while having as high a breakdown voltage as possible when reverse biased. Minimizing the on-state resistance of a single-pole component imposes minimizing the thickness of the most lightly doped layer (layer2) and maximizing the doping of this layer.

To optimize the breakdown voltage without modifying resistance Ron, structures of the type of that inFIG. 3have been provided. InFIG. 3, a vertical Schottky diode includes a single-crystal silicon semiconductor substrate31, heavily doped of a first conductivity type, for example, type N, and coated with a layer32. Layer32is formed of the same semiconductor material as substrate31and is of same doping type, but more lightly doped. Layer32is intended for forming the cathode of the Schottky diode. A metal layer33covers layer32. The metal forming layer33is chosen to form a Schottky contact with N-type silicon32.

Layer32includes very heavily-doped P-type silicon regions or “islands”34. Islands34are distributed over at least one horizontal level (over two levels in the example of FIG.3).

Islands34are separate and buried in layer32. The islands34of different horizontal levels are substantially distributed on same vertical lines.

FIG. 4illustrates the variation profile of electric field E across the thickness of a structure similar to that in FIG.3. More specifically, the profile ofFIG. 4is observed along axis A-A′ of FIG.3.

As appears from the comparison ofFIGS. 2 and 4, the insertion of heavily-doped P-type “islands”34in the structure ofFIG. 3modifies the variation of field E per thickness unit. Since islands34are much more heavily doped than N-type layer32, there are more negative charges created in islands34than there are positive charges in layer2. The field thus increases back in each of the horizontal areas including islands34. By setting the doping and the number of islands34, the space charge area can be almost indefinitely widened. In reverse biasing, the cathode formed by layer32and islands34thus generally behaves as a quasi-intrinsic layer. In average, the electric field variation per thickness unit thus strongly decreases. Thus, for a given doping level of layer32, the breakdown voltage is increased, as illustrated by the increase of the surface delimited by the axes and the curve ofFIG. 4as compared to the corresponding surface of FIG.2.

Accordingly, the structure ofFIG. 3enables obtaining single-pole components of given breakdown voltage with a resistance Ron smaller than that of a conventional structure.

The practical implementation of such a structure with islands is described, for example, in German patent 19,815,907 issued on May 27, 1999, in patent applications DE 19,631,872 and WO99/26,296, and in French patent 2,361,750 issued on Mar. 10, 1978. These different documents provide obtaining a structure similar to that inFIG. 3by performing implantations/diffusions during a growth epitaxy of layer32.

The repeated interruptions of the epitaxial growth are a disadvantage of such an implementation. Indeed, thick layer32thus obtained has an irregular structure. Such structure irregularities alter the performances of the final component.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel method for manufacturing single-pole components of vertical type having a determined breakdown voltage and having a reduced on-state resistance. The present invention also aims at the obtained components.

To achieve these objects, the present invention provides a single-pole component of vertical type, including regions of a first conductivity type buried in a thick layer of a second conductivity type, said regions being distributed over at least one same horizontal level and being independent from one another, and the independent regions of which underlie an insulating material.

According to an embodiment of the present invention, the component includes at least two levels, the independent regions of successive levels being substantially vertically aligned.

According to an embodiment of the present invention, the independent regions are rings.

According to an embodiment of the present invention, the deepest level includes non ring-shaped regions.

The present invention also provides a method for manufacturing a single-pole component of vertical type in a silicon substrate of a given conductivity type, including the steps of:

a) forming openings in a thick silicon layer covering the substrate, doped of the same conductivity type as said substrate, but more lightly;

b) coating the walls and bottoms of the openings with a silicon oxide layer;

c) forming, by implantation/diffusion through the opening bottoms, regions of the conductivity type opposite to that of the substrate; and

d) filing the openings with an insulating material.

According to an embodiment of the present invention, before step d) of filling the openings, steps a) to c) are repeated at least once, the initial openings being continued into the thick silicon layer.

According to an embodiment of the present invention, the silicon layer of the same given type of conductivity as the substrate is intended for forming the cathode of a Schottky diode.

According to an embodiment of the present invention, the silicon layer of same conductivity type as the substrate is intended for forming the drain of a MOS transistor.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated inFIG. 5A, a substrate61is initially covered with a single-crystal silicon layer62, of same doping type, for example N, as substrate61. Layer62, intended for forming the cathode of the Schottky diode, is more lightly doped than substrate61. Layer62is etched, by means of a mask65, to form openings66. Substrate61and layer62are obtained by any appropriate method. For example, layer62may result from an epitaxial growth on substrate61, or substrate61and layer62may initially be a same semiconductor region, the doping differences then resulting from implantation-diffusion operations.

At the next steps, illustrated inFIG. 5B, an insulating layer67, for example a silicon oxide layer (SiO2), is formed on the walls and at the bottom of openings66. Then, a P-type dopant that penetrates into the silicon at the bottom of openings66is implanted, after which a heating is performed to form heavily-doped P-type regions641.

At the next steps, illustrated inFIG. 5C, layer67, regions641, and layer62are anisotropically etched, to form openings68that continue openings66. The upper portion of each of openings68is thus surrounded with a diffused ring641. Then, the walls and bottoms of openings68are covered with a thin insulating layer69, for example silicon oxide.

The implantation operations previously described in relation withFIG. 5Bare then repeated to form heavily-doped P-type regions642.

At the next steps, illustrated inFIG. 5D, openings66-68are filled with an insulating material70. Then, mask65is removed and the structure thus obtained is planarized. Finally, a metal layer63adapted to ensuring a Schottky contact with layer62is deposited over the entire structure.

Before ending, in accordance with the steps described in relation withFIG. 5D, the structure formation by removing mask65, filling openings66εwith material70, and depositing a metal layer63, the steps described in relation withFIG. 5Ccould be repeated several times, to form several horizontal levels of heavily-doped P-type rings similar to rings641.

It should be noted that the intermediary rings and the underlying regions form islands according to the preceding definition. They thus provide the corresponding advantages, previously discussed in relation withFIGS. 3 and 4.

An advantage of the method according to the present invention and of the resulting structure, previously described in relation withFIG. 5D, is the forming of a homogeneous cathode region62.

Those skilled in the art will know how to adapt the number, the dimensions, the positions, and the doping of the different rings641,642to the desired performances. As an example, according to prior art, to obtain a breakdown voltage of approximately 600 volts, a cathode layer (2,FIG. 1) of a thickness of approximately 40 μm and of a doping level on the order of 2.2. 1014atoms/cm3may be used, which results in an on-state resistance of approximately 6.7 Ω.mm2. According to the present invention, by using groups of three P-type rings doped at approximately 3.5·1017atoms/cm3, vertically spaced apart by 10 μm around silicon oxide columns of a 1-μm width, for a same breakdown voltage of 600 V with an epitaxied layer (62,FIG. 5D) of a same thickness on the order of 40 μm, the cathode doping could be increased to a value on the order of some 1015atoms/cm3, which results in an on-state resistance of approximately 3 Ω.mm2.

It should be noted that it has been chosen to describe as a non-limiting example the present invention in relation withFIG. 6applied to the forming of silicon islands in the cathode of a Schottky diode. It would however be possible to implement a method aiming at forming in the drain of a MOS transistor, around vertical columns of an insulating material, very heavily-doped P-type silicon rings, similarly to the method previously described in relation withFIGS. 5A-D.

Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the operations described in relation withFIG. 5Dcan be carried out according to any appropriate sequence. Thus, after filling openings66-68, layer65may be removed and the structure may be planarized in a single step by means of a chem-mech polishing (CMP) method.

Further, the present invention applies to the forming in vertical form of any type of single-pole component, be it to reduce its on-state resistance for a given breakdown voltage, or to improve its breakdown voltage without increasing its on-state resistance.