Method of forming an integrated circuit thin film resistor

A thin film resistor structure (75) is formed on a dielectric layer (60). A capping layer (90) is formed above said thin film resistor structure (75) and vias (110) are formed in the capping layer (90) using a two step etching process comprising of a dry etch process and a wet etch process. Conductive layers (120) are formed in the vias and form electrical contacts to the thin film resistor structure (75).

FIELD OF THE INVENTION

The invention is generally related to the field of thin film resistors and more specifically to a method of forming a thin film resistor with uniform sheet resistance and low contact resistance.

BACKGROUND OF THE INVENTION

Thin film resistors are very attractive components for high precision analog and mixed signal applications. In addition to a low thermal coefficient of resistance, low voltage coefficient of resistance, and good resistor matching they should provide good stability under stress. Precise resistance control of the thin film resistor is essential for high precision analog circuits such as analog-to-digital converters and digital-to-analog converters. In many instances the resistance is adjusted after resistor fabrication by laser trimming. For ease of laser trimming, the thickness of the thin film resistor is made as thin as possible consistent with the requirement of the sheet resistance need for the particular circuit design. For example for silicon chromium (SiCr) thin film resistors having a sheet resistance of 1000 Ohms/square a film thickness of about 3.4 nm is required. To achieve precise sheet resistance values and reduction of circuit area it is critical to minimize the contact area between the thin film resistor head and the thin film resistor layer. In addition during the formation of the vias used to contact the thin film resistor it is imperative to minimize the loss of the thin film resistor material beneath the via. The dry etch methods current in use to form the thin film resistor vias often results in material loss and damage to the thin film resistor. This results in failed resistor contact and/or high contact resistance. A method is therefore needed to form thin film resistors without damaging the resistor during contact formation. The instant invention provides such a method.

SUMMARY OF THE INVENTION

The instant invention describes a method for forming thin film resistor structures on integrated circuits. A thin film resistor structure is formed on a dielectric layer formed over a semiconductor. A capping layer is formed over the thin film resistor structure and vias are formed in the capping layer by performing a dry etch process followed by a wet etch process. Conductive material is then used to fill the vias to form electrical contacts to the thin film resistor structure. A dielectric layer is formed over the capping layer and conductive material followed by the formation of openings in the dielectric layer over the conductive material. Plugs are then formed in the openings such that the plugs contact the conductive material using to fill the vias.

Common reference numerals are used throughout the figures to represent like or similar features. The figures are not drawn to scale and are merely provided for illustrative purposes.

DETAILED DESCRIPTION OF THE INVENTION

While the following description of the instant invention revolves aroundFIGS. 1-4, the instant invention can be utilized in many semiconductor device structures. The methodology of the instant invention provides a solution to forming thin film resistors having a uniform sheet resistance and a low contact resistance.

An embodiment of the instant invention is shown inFIGS. 1-4. As shown inFIG. 1a first inter-level dielectric layer10is formed over an integrated circuit. The integrated circuit comprises any number of active devices including MOS and/or bipolar transistors as well as any number of metal interconnect levels. The integrated circuit and any metal interconnect levels are not shown for clarity. The first inter-level dielectric layer10can comprise silicon oxide formed using any suitable method including chemical vapor deposition. In a first embodiment the first inter-level dielectric layer10is formed using material selected from the group comprising TEOS silicon oxides, PECVD silicon oxides, silicon nitrides, silicon oxynitrides, silicon carbides, spin-on glass (SOG) such as silsesquioxanes and siloxane, xerogels or any other suitable material. Using standard integrated circuit manufacturing technology a first metal layer20can be formed in the first inter-level dielectric layer10. Following the formation of the first inter-level dielectric layer10and metal layer20, a second inter-level dielectric layer30can be formed over the first inter-level dielectric layer10. The second inter-level dielectric layer30can be formed using any suitable dielectric material and in a first embodiment is formed using TEOS silicon oxides, PECVD silicon oxides, silicon nitrides, silicon oxynitrides, silicon carbides, spin-on glass (SOG) such as silsesquioxanes and siloxane, xerogels or any other suitable material. Following the formation of the second inter-level dielectric layer30an optional high-density plasma (HDP) silicon oxide layer50can be formed on the second inter-level dielectric layer30. A second metal layer40can then be formed in the HDP silicon oxide layer if required. An oxide liner layer60is then formed over the second inter-level dielectric layer30and the optional HDP silicon oxide layer50and second metal layer40if present. In an embodiment the oxide liner layer is a 300 A to 800 A thick TEOS silicon oxide layer. A resistor layer70is then formed over the oxide liner layer60. In subsequent processing the resistor layer70will be etched to form the thin film resistor (TFR). In an embodiment of the instant invention the resistor layer is formed using a silicon chromium (SiCr) alloy, nickel chromium (NiCr) alloy, tantalum nitride, titanium nitride, tungsten, or any other suitable material. A photoresist layer80is formed and patterned over the resistor layer70and will be used to define the TFR during the etching process.

Shown inFIG. 2is the TFR75formed by etching the resistor layer70using the photoresist layer80shown inFIG. 1as a masking layer. The resistor layer can be etched using any suitable dry or wet etching process. Following the formation of the TFR structure a capping layer90is formed over the TFR structure75and any underlying layers. In an embodiment the capping layer is between 600 A and 1500 A and is formed using silicon oxide, silicon nitride, or any other suitable dielectric material. In further embodiments of the instant invention the capping layer can comprise multiple layers formed using layers comprised of the same or differing dielectric material. Following the formation of the capping layer90a patterned photoresist layer100is formed over the capping layer as shown in FIG.2and will be used to pattern the vias that will be subsequently etched through the capping layer90. Electrical contacts to the TFR will be formed through the vias. Following the formation of the patterned photoresist layer100the vias are formed using a multi-step process.

In the first step a dry etching process is used to partially etch the capping layer90. In an embodiment where the capping layer90is formed using silicon oxide, a dry etch can be performed in suitable apparatus such as the TEL-DRM etcher. The etch process is a timed etch designed to stop within 100 A to 300 A of the surface TFR. Therefore if a 1000 A thick capping layer90is used the dry etch process will remove 700 A to 900 A of the capping layer. This is shown inFIG. 2where a portion of the capping layer90that is exposed in the via openings110is removed using a dry etch process. In a further embodiment the dry etch chemistry can comprise 1-5 sccm of O2, 120-300 sccm of Ar, and 3-13 sccm of C4F8at pressures between 50 mT to 140 mT.

Following the partial etching of the vias using a dry etching process, a wet chemical etch will be used to complete the etching of the vias. In a first embodiment the wet chemical etching will be performed following the removal of the patterned photoresist layer100. In a second embodiment the wet chemical etching will be performed prior to the removal of the patterned photoresist100. In the first embodiment where the wet chemical etch is performed subsequent to the removal of the patterned photoresist layer, the patterned photoresist layer100can be removed using a standard process such as an ash. Following the removal of the patterned photoresist layer, a highly selective wet chemical process is used to complete the etching of the vias. In an embodiment where the capping layer is formed using silicon oxide and the TFR is formed using SiCr, a buffered hydrofluoric (HF) acid etch can be used to etch the vias. The etch rate of a silicon oxide capping layer in a buffered HF etch is approximately 175 A/min compared to an approximately 0.1 A/min etch rate for SiCr. Therefore using the above described wet etch process a minimal amount of the SiCr exposed beneath the via will be removed during the via etching process. This will result in a uniform TFR sheet resistance. In addition there will be no residue formed in the via on the exposed surface of the TFR during the etching process. Residue formed on the surface of the TFR during the etching process will increase the contact resistance of the electrical contacts formed in the vias to contact the TFR. In the second embodiment the wet via etching process is performed prior to the removal of the patterned photoresist layer. In this embodiment a buffered hydrofluoric (HF) acid process can be used to etch the via. Following the etching of the vias the remaining patterned photoresist layer100can be removed using standard processes such as an ash.

Following the formation of the vias, conductive layers120are formed in the via openings as shown in FIG.3. In an embodiment of the instant invention the conductive layers120comprise titanium nitride or a titanium tungsten alloy and are formed by first depositing a blanket layer of the conductive material followed by a patterned etch process. A third inter-level dielectric layer130is formed over the structure as shown inFIG. 3followed by the formation of a patterned photoresist layer140on the third inter-level dielectric layer130. Using the patterned photoresist layer140as a mask, openings150are formed in the third inter-level dielectric layer130which will be used to form electrical contacts to the conductive layers120. In an embodiment the third inter-level dielectric layer is formed using TEOS silicon oxides, PECVD silicon oxides, silicon nitrides, silicon oxynitrides, silicon carbides, spin-on glass (SOG) such as silsesquioxanes and siloxane, xerogels or any other suitable material.

Shown inFIG. 4is the completed TFR structure. Following the removal of the patterned photoresist layer140inFIG. 3, a conductive material is used to fill the openings150and form the plugs160that will provide electrical contact to the conductive layers120. In an embodiment of the instant invention this conductive material comprises tungsten, aluminum, titanium, titanium nitride, tantalum nitride, copper, or any suitable conductive material. Following the formation of the conductive plugs160, a dielectric layer170is formed over the third inter-level dielectric layer130. In an embodiment the dielectric layer can comprise HDP silicon oxide. Metal layers180and190can be formed in the HDP silicon oxide layer170to provide electrical contact to the conductive plugs160. Metal layers180and190are used to interconnect the TFR with the other electronic devices that comprise the integrated circuit. Following the formation of the metal layers180and190addition inter-level dielectric layers200can be formed as shown in FIG.4.