Composite masking self-aligned trench MOSFET

This disclosure relates to MOSFET devices, and more specifically, to a high density trench MOSFET device and a method for fabricating the same.

Field Effect Transistors (FETs) are semiconductor transistor devices in which a voltage applied to an electrically insulated gate controls flow of current between source and drain. One example of a FET is a metal oxide semiconductor FET (MOSFET), in which a gate electrode is isolated from a semiconducting body region by an oxide insulator. Generally, MOSFETs may have a planar gate structure or a trench gate structure. A MOSFET device that includes a trench gate structure offers important advantages over a planar transistor for high current, low voltage switching applications. A trench gate of a MOSFET device usually includes a trench extending from the source to the drain and having sidewalls and a floor that are each lined with a layer of thermally grown silicon dioxide. The lined trench may be filled with doped polysilicon. The structure of the trench gate allows less constricted current flow, and consequently, provides lower values of specific on-resistance. Another feature making the trench MOSFETs attractive is that the current flows vertically through the MOSFET channel extending along the vertical sidewalls of the trench from the bottom of the source across the body of the transistor to the drain below. This makes possible a smaller cell pitch and a higher cell density. One issue of increasing cell density by fabricating the transistors closer together is the alignment tolerance. The alignment tolerance is the excess amount needed to compensate for variations in the alignment. The alignment tolerance of the current lithographic processing lies in the range of 300 Å-500 Å or 30 nm-50 nm.

In addition, a high density trench MOSFET device usually includes a contact trench for making contact to source and body regions. Conventional processes for manufacturing of the high density trench MOSFET devices have used two independent masks for making gate trenches and contact trenches. Specifically, a gate trench mask is used to make a vertical gate trench. In a separate step, a trench contact mask is used to make a contact trench on a same substrate after the gate trench has been formed. However, a mask overlay issue occurs when the two masks are used to form a vertical MOSFET structure because a well-controlled spacing between the gate trench and nearby contact trench is required for high density MOSFET devices, which have increasingly smaller dimensions.

It is within this context that embodiments of the present invention arise.

Objects and advantages of aspects of the present disclosure will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which:

FIGS. 1A-1T are cross-sectional views illustrating a process of making gate trench and contact trench of a MOSFET device according to an embodiment of present disclosure.

Several methods based on self-alignment processes have been proposed to solve mask overlay issue. One proposed method involves using various constructs of alternating oxide and nitride blocking planar or sidewall spacers to create a contact trench that is self-aligned to the gate trench. Another proposed method using one mask to define the gate and contact trenches protects the contact trench with resist during the etching processing of the gate trench. In order for the resist to fill the contact trench and to avoid getting into the gate trench, the gate and contact trenches cannot be placed too close, and the pitch between the gate and contact trench may be at least two alignment tolerances. The cell pitch in this method is thus limited by alignment tolerance between the contact trench and the gate trench.

Aspects of the present disclosure present a process that uses a single mask to define both gate and contact trenches. In addition, aspects of the present disclosure allow for a pitch between contact and gate trenches limited only by the capabilities of the photolithography tools. An exemplary fabrication process according to an embodiment of the present disclosure is shown in FIGS. 1A-1T .

As shown in FIG. 1A , the process uses a semiconductor substrate 102 as a starting material. In some embodiments, the substrate 102 can be an N-type silicon wafer with an N-type epi layer grown on it for an N-channel device or a P-type silicon wafer with a P-type epi layer grown in it for a P-channel device. A hard mask 104 is formed on a surface of the semiconductor substrate 102 using known techniques. In some embodiments, the hard mask 104 can be an oxide-nitride-oxide (ONO) layer. More generally, the hard mask 104 includes alternating layers of two different insulator materials that are each resistant to an etch process that etches the other. The ONO layer 104 comprises from bottom to top: a bottom oxide layer 104 a , a nitride layer 104 b and a top oxide layer 104 c . A photoresist (not shown) is then applied on the ONO layer 104 and patterned to define both the contact and gate trenches. The patterned photoresist includes openings at locations of the contact trenches and gate trenches. As shown in FIG. 1B , an ONO etch is performed to etch away portions of the ONO layer 104 that are exposed to an enchant through openings in the photoresist. After the photoresist is removed, the remaining portions of the ONO layer 104 act as a mask and the uncovered portions of the underlying semiconductor substrate 102 are etched down to simultaneously form the contact trenches 106 and gate trenches 108 as shown in FIG. 1C . It is understood by those skilled in the art of semiconductor fabrication, a wider trench opening results in a deeper trench than a narrower trench opening due to the nature of the silicon etch loading factor. Since gate trench openings are wider than contact openings, the resulting gate trenches 108 are etched deeper than the contact trenches 106 as shown in FIG. 1C . Next, a screen oxide layer 110 , an insulating layer, is grown along the inner surface of the trenches as shown in FIG. 1D . The screen oxide layer 110 acts as an etch stop for an isotropic nitride etch in the subsequent process.

Next, a nitride layer 112 is deposited as shown in FIG. 1E where the narrow trenches (i.e., contact trenches 106 ) are completely filled up, but not the wide trenches (i.e., gate trenches 108 ). Depending on the width of the contact trench, the thickness of the nitride layer 112 has to be thick enough to fill the contact trench but not completely fill the gate trenches 108 . In some embodiments, the thickness of the nitride layer 111 can be approximately 500 Å to 2000 Å for a contact trench with a depth of 0.1 μm to 0.5 μm. After nitride deposition, an isotropic etch is carried out, as shown in in FIG. 1F to remove all nitride from the wide trenches (i.e., gate trenches 108 ) but leaves the narrow trenches (i.e., contact trenches 106 ) filled with nitride 112 a . The screen oxide layer 110 acts as an etch stop for the nitride. Thereafter, the screen oxide layer 110 is removed in FIG. 1G . An etch on the gate trenches are conducted in FIG. 1H . The gate trenches 108 are etched deeper into the semiconductor substrate 102 while contact trenches 106 are protected from the gate trench etch with the nitride 112 a in the contact trenches 106 . Again, although the foregoing example involves an oxide layer 110 and nitride material 112 a , those skilled in the art will appreciate that what matters is that one insulating layer 110 is resistant to an etch process that etches the other insulating layer 112 and vice versa.

Next, a sacrificial oxide layer (not shown) may be grown and then removed to improve the silicon surface. A gate oxide layer 114 is then formed on the sidewalls and the bottoms of trenches 108 as shown in FIG. 1I . A conductive material is deposited to fill in the trenches. In some embodiments, the conductive material can be in-situ doped or undoped polysilicon. The thickness of the conductive material is such that it will completely fill up the gate trenches 108 . The conductive material is then etched back forming the gate poly structures 116 as shown in FIG. 1J . In some embodiments, the surface of the gate poly structure 116 can be recessed approximately 0.05 μm to 0.2 μm below the top of the semiconductor substrate 102 .

Body implant and body diffusion take place as shown in in FIG. 1K and FIG. 1L , respectively. A body mask (not shown) is applied for body implantation. In FIG. 1K , dopants are then implanted into the top portions of the semiconductor substrate 102 . The dopant ions are of the opposite conductivity type to the doping of the substrate 102 . In some embodiments, the dopant ions can be Boron ions for an N-channel device. In some embodiments, Phosphorous or Arsenic ions can be used for P-channel devices. Heat is then applied to activate dopant atoms and drive dopant diffusion to form a body region 118 as shown in FIG. 1L .

Next, source implant and source diffusion take place as shown in FIG. 1M and FIG. 1N , respectively. In source implant depicted in FIG. 1M , a large tilt angle implant may be performed for source implantation with a source mask (not shown). In some embodiments, Arsenic ions can be implanted to form the source regions for an N-channel device. Alternatively, Boron ions can be implanted to form the source region for a P-channel device. Standard diffusion processes may then be carried out to form the source regions 120 within body regions 118 as shown in FIG. 1N .

A dielectric layer 122 , such as an oxide, can be deposited over the semiconductor substrate 102 as shown in FIG. 1O . In some embodiments, the dielectric layer 122 is formed by a low temperature oxide process and a layer of Borophosphorosilicate Glass (BPSG).

A contact photoresist (not shown) may be applied on the dielectric layer 122 and pattern with openings that uncover the contact trenches 106 . The uncovered portions of the dielectric layer 122 is then etched back as shown in FIG. 1P , and the etching stops at the surface of the nitride 112 a which acts as an etch stop. In FIG. 1Q , the nitride 112 a in the contact trenches is then removed. Standard processes of contact implant is followed to form contact regions 124 proximate to the bottoms of the contact trenches 106 shown in FIG. 1R so that the contact regions 124 become more heavily doped. A subsequent etch may be performed to remove the screen oxide layer 110 along the inner surface of the contact trenches 106 as shown in FIG. 1S .

Next, as depicted in FIG. 1T , a barrier metal 126 is first lined with the inner surface of the contact trenches 106 . In some embodiments, the barrier metal 126 may be Titanium (Ti) or Titanium Nitride (TiN). A conductive material 128 , such as Tungsten (W), may then be blanket deposited in the contact trenches 106 followed by an etch back up to the surface of the dielectric layer 122 to form a conductive plug 128 as shown in FIG. 1T . Finally, a metal layer 130 is deposited on top of the semiconductor substrate 102 as in FIG. 1T . In some embodiments, the metal layer 130 may be Aluminum (Al) or Aluminum Copper (AlCu).

Aspects of the present disclosure uses a single mask to simultaneously form a contact trench 106 and a gate trench 108 into a semiconductor substrate as shown in FIGS. 1B and 1C . In addition, aspects of the present disclosure eliminate the need for additional protection of the contact trench 106 during additional gate trench etch because the narrower contact trench 106 remains filled with a dielectric material (nitride 112 a ) while the dielectric material is removed from the gate trench 108 with an isotropic etch back in FIGS. 1E and 1F . Accordingly, aspects of the present disclosure permits a smaller pitch between the contact trench and the gate trench limited only by the resolution of photolithography, and allows for making high density MOSFET devices. The resolution of the current photolithographic technique is in the range of 5 nm to 20 nm.

While the above is a complete description of the preferred embodiments of the present invention, it is possible to use various alternatives, modifications, and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead be determined with reference to the appended claims, along with their full scope of equivalents. Any feature, whether preferred or not, may be combined with any other feature, whether preferred or not. In the claims that follow, the indefinite article “A”, or “An” refers to a quantity of one or more of the item following the article, except where expressly stated otherwise. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means for.” Any element in a claim that does not explicitly state “means for” performing a specified function, is not to be interpreted as a “means” or “step” clause as specified in 35 USC § 112, ¶6.