Method of fabricating an integrated circuit

A transistor (12) and method of making an integrated circuit (10) uses a chromium based sacrificial gate (22A) to align, dope and activate source and drain portions (36, 38, 52, 53,) of the transistor. The transistor is subjected to a high temperature to activate the source and drain, which would damage a high permittivity gate dielectric. The sacrificial gate is removed by etching with ceric ammonia nitrate. A high permittivity gate dielectric (72) and a final gate electrode (74) are formed over a channel (30) of the transistor. Electrodes (76, 78) are formed for coupling to the source and drain.

FIELD OF THE INVENTION

The present invention relates in general to semiconductor devices, and more particularly to transistors fabricated on high density integrated circuits.

BACKGROUND OF THE INVENTION

There is a continuing demand for higher density integrated circuits with smaller transistor dimensions. For example, the dimensions of future transistors are expected to be scaled down to one hundred nanometers or less, and to be fabricated with gate dielectrics made with high permittivity materials.

However, integrated circuits are subjected to temperatures exceeding one thousand degrees Celsius in order to activate the transistor's source and drain diffusions. Most if not all high permittivity materials are unable to withstand such high temperatures without degradation. This problem can be avoided by using a sacrificial or dummy gate to align a transistor's source and drain. The dummy gate typically is formed with polysilicon or silicon dioxide, which can withstand the high temperature activation. After activating the source and drain, the dummy gate is removed and the high permittivity material is deposited to form the gate dielectric.

A dummy gate process has a disadvantage that removing the dummy gate causes extraneous material to be removed as well, which reduces the control over critical transistor dimensions. Moreover, voids often are left, which increases stress in the transistor and degrades the reliability of the integrated circuit.

Hence, there is a need for a structure and method of fabricating a transistor which maintains good control over critical transistor dimensions and which does not leave voids which degrade reliability.

DETAILED DESCRIPTION OF THE DRAWINGS

In the figures, elements having the same reference numbers have similar functionality.

FIG. 1 shows a top view of an integrated circuit 10 formed on a semiconductor substrate 14 . A transistor 12 has a source 36 and a drain 38 coupled to electrodes 76 and 78 , respectively. An electrode 74 operates as a gate of transistor 12 , altering the conductivity of substrate 14 to control current between source 36 and drain 38 . Electrodes 74 , 76 and 78 are used for coupling electrical signals to other internal or external electrical components (not shown). The distance from source 36 to drain 38 typically is one hundred nanometers or less. In one embodiment, transistor 12 operates as an N-channel metal-oxide-semiconductor field effect transistor.

Substrate 14 comprises a semiconductor material. In one embodiment, substrate 14 is formed from silicon doped to a p-type conductivity. Substrate 14 alternatively may comprise germanium, gallium arsenide or other semiconductor material. As a further alternative, substrate 14 may be doped to an n-type conductivity.

The fabrication of integrated circuit 10 using a sacrificial, i.e., replacement or dummy, gate process is described in detail as follows.

FIG. 2 shows a cross-sectional view of integrated circuit 10 at a first step of fabrication, including substrate 14 over which is formed a first blocking layer 20 , a gate layer 22 , a second blocking layer 24 , a masking layer 26 and a photoresist pattern 28 .

First blocking layer 20 comprises silicon dioxide formed by deposition or oxidation to overlie substrate 14 as shown. First blocking layer 20 typically is formed to a thickness of seventy-five angstroms.

Gate layer 22 is formed over first blocking layer 20 . Gate layer 22 comprises a material that includes chromium. In one embodiment, gate layer 22 comprises a chromium based material such as chromium or chromium nitride (Cr x N y ) formed to a thickness of two thousand angstroms.

Second blocking layer 24 is disposed over gate layer 22 . Second blocking layer 24 typically comprises amorphous silicon formed to a thickness of one thousand angstroms.

Masking layer 26 is disposed over second blocking layer 24 . Masking layer 26 typically comprises silicon dioxide formed to a thickness of one thousand angstroms.

Photoresist is deposited over masking layer 26 and exposed to produce photoresist pattern 28 for defining a conduction channel of transistor 12 in substrate 14 .

FIG. 3 is a cross-sectional view of transistor 12 after a second step of fabrication. Masking layer 26 is patterned with photoresist pattern 28 and etched with a suitable silicon dioxide etchant such as hydrogen fluoride to produce a hard mask 26 A. Photoresist pattern 28 is removed and a suitable silicon RIE etch is used to etch second blocking layer 24 to produce etch stop pattern 24 A. Gate layer 22 is etched with a reactive ion etch containing oxygen to produce a dummy gate or sacrificial gate 22 A. A suitable silicon dioxide etchant is used to etch first blocking layer 20 to produce etch stop pattern 20 A.

The result of the successive etches is to produce a patterned stack 31 that includes hard mask 26 A, etch stop pattern 24 A, sacrificial gate 22 A and etch stop pattern 20 A as shown in FIG. 3 . N-type dopants masked by stack 31 are introduced into substrate 14 to form a lightly doped source 32 and a lightly doped drain 34 . In effect, lightly doped source 32 and lightly doped drain 34 are self-aligned to stack 31 to allow integrated circuit 10 to be fabricated with small physical dimensions. In one embodiment, the distance from lightly doped source 32 to lightly doped drain 34 is less than one hundred nanometers.

Note that lightly doped source 32 and lightly doped drain 34 may be electrically activated by contact probing. Moreover, hard mask 26 A and/or etch stop pattern 24 A can be removed to enable electrical contact to sacrificial gate 22 A. In some specific embodiments etch stop pattern 24 A, which includes amorphous silicon, can be doped to be sufficiently conductive to perform the contact probing while retaining pattern 24 A in position as shown in FIG. 4 . Hence, a conduction channel 30 can be enabled in substrate 14 to produce a functioning transistor 12 A which can operate as an in-line process monitor.

FIG. 4 is a cross-sectional view of transistor 12 after a third step of fabrication. Spacers 40 and 42 and liners 41 and 43 are formed adjacent to sacrificial gate 22 A and etch stop pattern 24 A, as shown, to mask N-type dopants being introduced into substrate 14 to form source 36 and drain 38 . Spacers 40 and 42 typically comprise silicon nitride, and liners 41 and 43 typically comprise silicon dioxide.

A planarizing layer 44 is formed over substrate 14 to operate as an interlayer dielectric. A chemical mechanical polish or other planarizing step may be performed if further planarization is desired. If not previously done, hard mask 26 A and etch stop pattern 24 A are removed in succession.

Recall that sacrificial gate 22 A comprises chromium nitride. As a feature of the present invention, sacrificial gate 22 A is removed by etching with ceric ammonia nitrate, which selectively etches chromium nitride at a high etch rate while etching silicon dioxide and silicon nitride at a much lower rate. For example, in one embodiment, ceric ammonia nitrate etches chromium nitride five hundred times faster than it etches either silicon dioxide or silicon nitride.

The highly selective etch of ceric ammonia nitrate results in sacrificial gate 22 A being completely removed, with little or no material removed from either etch stop pattern 20 A or liners 41 and 43 . Hence, sidewalls 50 and 51 remain substantially perpendicular to substrate 14 , which facilitates control over critical transistor dimensions, e.g., the gate length. The selectivity of ceric ammonia nitrate has a further benefit of protecting substrate 14 from process induced defects because etch stop pattern 20 A remains substantially intact.

As yet a further benefit, the invention virtually eliminates voids during the sacrificial gate removal due to the high selectivity of ceric ammonia nitrate to chromium nitride versus silicon oxide and silicon nitride. Such voids are common with previous methods, particularly in corners and edges exposed to the etchant, such as seams 52 and 53 . Hence, the present invention provides integrated circuit 10 with improved reliability.

Etch stop pattern 20 A is removed with hydrogen fluoride or other suitable etchant.

The resulting structure is shown in FIG. 5 , which is a cross-sectional view of transistor 12 after a fourth step of fabrication. Substrate 14 is subjected to a temperature exceeding eight hundred degrees Celsius to activate dopants in source 36 and drain 38 . The region of conduction channel 30 may be doped if desired to provide a transistor threshold adjustment.

FIG. 6 is a cross-sectional view of transistor 12 after a fifth step of fabrication. A gate dielectric 72 is disposed over substrate 14 as shown. Gate electrode 74 is formed over gate dielectric 72 to operate as a control electrode. Electrodes 76 and 78 provide electrical contact to source 36 and drain 38 , respectively.

Gate dielectric 72 comprises a high permittivity material. In one embodiment, gate dielectric 72 is formed with strontium titanate having a relative permittivity greater than one hundred. It can be shown that similar control over transistor operation can be obtained with a thicker layer of high permittivity material than would be possible with a lower permittivity material. Hence, in one embodiment gate dielectric 72 is formed with a thickness of one hundred angstroms, substantially thicker than a gate dielectric of similar performance made with a low permittivity material.

Note that gate dielectric 72 is not subjected to high temperatures because the high temperature activation of source 36 and drain 38 is completed before gate dielectric 72 is formed.

Electrodes 74 , 76 , and 78 are formed by depositing and patterning a conductive material such as aluminum, polysilicon, copper or the like.

In brief, the present invention provides a device and method of making a transistor with improved dimension control and reliability. A dummy gate comprising a chromium based material is disposed over a semiconductor substrate for aligning source and drain regions of the transistor. The dummy gate subsequently is removed with an etch of ceric ammonia nitrate and replaced with a final gate electrode. Because of the highly preferential etch of ceric ammonia nitrate, little or no extraneous material is removed and voids in the transistor are virtually eliminated. Hence, high reliability is achieved. Moreover, the sides of the final gate electrode are substantially vertical, which improves the control over the transistor's critical dimensions, especially where the dimensions are less than one hundred nanometers.

While we have shown and described specific embodiments of the present invention, further modifications and improvements will occur to those skilled in the art. We desire it to be understood, therefore, that this invention is not limited to the particular forms shown and we intend in the appended claims to cover all modifications that do not depart from the spirit and scope of this invention.