Direct etch for thin film resistor using a hard mask

Abstract: A method of etching a thin film resistor material, such as NiCr or CrSi, for producing a thin film resistor, a non-photoresist hard mask is deposited on an exposed surface of thin film resistor material, a delineated portion of the hard mask is etched with a hydrogen peroxide etchant that does not affect the thin film resistor material to expose the material underneath, and the exposed thin film resistor material is etched with a second etchant that does not affect the hard mask. The second etchant may be sulfuric acid heated to greater than 125°C for NiCr or a mixture of phosphoric acid, nitric acid and hydrofluoric acid for CrSi. The hard mask preferably comprises TiW.

Description: The present invention relates to methods of etching thin film resistor material, by cleanly etching NiCr to form thin film resistors in semiconductor devices. Thin film resistors are well known and may comprise a variety of resistive materials, with the chromium-based resistive materials nickel chromium (NiCr) being common. Thin film resistors may include an electrical interconnect contacting the resistive material for passing a current therethrough. The efficiency and accuracy of the thin film resistor depends upon the condition of the resistive material and the cleanliness of the portion of the resistor contacting the electrical interconnect. Resistive material that is corroded or contaminated ( e . g . has highly oxidized chromium) is undesirable because such conditions adversely affect the sheet resistance of the resistor. A "dirty" contact between the resistive material and the electrical interconnect adversely affects the contact resistance of the resistor. Known methods of manufacturing thin film resistors are considered to be "dirty" because they contaminate and corrode the resistive material and increase the oxidation of the chromium, thereby adversely affecting the sheet resistance of the resistor. Known methods also leave "dirty" contacts between the resistive material and the electrical interconnect, thereby adversely affecting the contact resistance of the resistor. One of the known methods of manufacturing NiCr thin film resistors includes depositing a photoresist (PR) mask over NiCr film and removing selected portions of the PR mask to expose portions of the NiCr. The exposed NiCr is then directly etched with an etchant that does not affect the PR mask, such as ceric sulfate. Ceric sulfate is known to be a "dirty" etchant which leaves residual ceric oxide contaminants across the wafer surface where the NiCr was removed. However, subsequent processing to remove the ceric oxide has an adverse affect on the exposed NiCr. Furthermore, the PR mask and the PR strip process also leave residual contaminants, such as carbon and sulfur, on the surface of the exposed NiCr which has an undesirable corrosive affect on the NiCr film. The combined effect of these contaminants increases the oxidation of the Cr within the NiCr film, thereby adversely affecting the sheet resistance of the NiCr resistor and degrading the contact between the NiCr and an electrical interconnect, thereby degrading the performance of the NiCr resistor. Another known method of producing NiCr thin film resistors uses a PR lift-off process rather than directly etching the NiCr. The lift-off process includes depositing a NiCr film over a patterned PR mask having open areas so that portions of the NiCr film are deposited into the open areas. After the NiCr is deposited the PR mask is lifted-off to remove the portions of the NiCr not in the open areas. The lift-off process, however, leaves behind unwanted NiCr and PR residue which is difficult to remove and which adversely affects the contact between the NiCr and the electrical interconnect. As in the direct etch method, the lift-off process leaves residual contaminants, such as carbon and sulfur, on the surface of the NiCr. The unwanted contaminants adversely affect the contact resistance between the NiCr and the electrical interconnect. They also have a corrosive effect on the NiCr ( e . g . increase the oxidation of the chromium) and thus adversely affect the sheet resistance. US-A-4035208 describes for a method of patterning platinum over a chromium layer without affecting the chromium layer the deposition of a tantalum mask in the same pump-down to that used with the deposition of chromium platinum. The tantalum layer is then patterned using a photolithographic masking and a CF 4 plasma-vapor etch, whereby the chromium layer is left intact due to the plasma-vapor etch. US-A-3634159 describes an electrical circuit assembly using plated gold conductors on a substrate having a resistive coating made of nickl chromium (NiCr) to provide resistors in areas where the gold layer is not removed with an etchant. US-A-5043295 describes thin film resistors made of silicon chronium (SiCr) which are etched through a delineated titanium-tungsten hard mask (TiW). For etching TiW hydrogen peroxide (H 2 O 2 ) is used. US-A-5374569 describes thin film resistors made of SiCr which are delineated with a TiW hard mask without indicating the etching material used. US-A-4081315 describes resistors of Cermet which are delineated by use of phosphoric acid (P 2 O 5 ) through a molybdenum (Mo) hard mask. US-A-3649945 describes thin film resistors such as NiCr which are delineated by use of Mo. The Mo layer is also used as intermediate layer for the aluminium interconnet layer. Due to the desclosure of this prior patent specification titanium and tungsten are difficult or impossible to etch with an etchant that does not also attack the resistive film. It is an object of the invention to provide a method of etching thin film resistor material that produces uncontaminated thin film resistors and which have clean surfaces for minimal contact resistance with electrical interconnects therewith. The present provides a method of making at least one thin film resistor in a semiconductor device as claimed in claim 1. A further development of the invention is subject of claim 2. The invention will now be described, by way of example, with reference to the following drawings in which: Figures 1a-1f are cross-sectional views illustrating the sequence of steps in an embodiment of the method of the present invention. Figures 2a-2b are cross-sectional views illustrating alternative steps in an embodiment of the method of the present invention. Figures 3a-3b are cross-sectional views illustrating furhter alternative steps in an embodiment of the method of the present invention. The sequence of steps in an embodiment of the method is shown in Figures 1a-1f. Referring to Figure 1a, a thin film resistor material 12, such as NiCr, may be conventionally deposited on a substrate 14 for a semiconductor device and a hard mask 10 may be deposited over the resistor material 12 without breaking vacuum over the resistor material 12. The hard mask 10 is deposited on the resistor material 12 in a deposition chamber without breaking the vacuum within the deposition chamber so that no contaminating materials are between the hard mask 10 and the material 12. The hard mask 10 is a non-photoresist material that is not affected by the etching effects of an etchant that may be used to etch the resistor material 12, namely sulfuric acid heated to greater than about 125°C for a NiCr thin film resistor. The hard mask 10 is comprised of TiW. Figure 1b shows the hard mask 10 delineated by depositing a PR mask 16 on the hard mask 10 and patterning the PR mask 16 so that it remains on the portions of the hard mask 10 overlying the portions of the resistor material 12 that will become the thin film resistors. Thereafter, as shown in Figure 1c, the hard mask 10 is etched with a first etchant to expose the portions of the resistor material 12 that will be etched. The first etchant is comprised of hydrogen peroxide. Figure 1d shows the PR mask 16 which may then be removed by using, for example, a solvent strip, and thereafter, the resistor material 12 is etched with a second etchant to remove unwanted thin film resistor material. The second etchant, that cleanly and uniformly etches the resistor material 12, that does not leave behind residue or contaminants which adversely affect the resistor material 12, or that does not otherwise adversely affect the resistor material 12, and that does not have an etching effect on the hard mask 10, is comprised of sulfuric acid heated to greater than about 125°C. Thereafter, as shown in Figure 1e, the remaining hard mask portions are removed with an etchant that does not adversely affect the resistor material 12, such as hydrogen peroxide, thereby leaving behind clean resistor material 12 for contacting electrical interconnects. As shown in Figure 1f, electrical interconnects 18 are deposited and delineated on the resistor material 12 thereby forming plural resistors on a semiconductor substrate 14. The electrical interconnects 18 may be comprised of any electrically conductive material, with aluminum being preferred. The thin film resistors thereby formed are free of contamination that may affect contact resistance with the interconnect. With reference now to Figure 2a-2b, a photoresist is used to delineate the interconnects 18. The electrical interconnect deposition and delineation may be performed prior to the removal of the delineated hard mask 10 from the resistor material 12 as shown in Figure 2a so that the PR, PR solvent strip and interconnect metallization do not contact and contaminate the surface of the resistor material 12. More specifically, the electrical interconnect 18 is deposited and patterned with a PR mask, the unwanted portions of the electrical interconnect are etched with a suitable etchant, and the remaining PR is then removed with, for example, a solvent strip. Thereafter, as shown in Figure 2b, the portions of the hard mask not underlying the electrical interconnects are etched with an etchant that does not adversely affect the electrical interconnects 18 thereby leaving hard mask portions 20 between the electrical interconnects 18 and the resistor material 12. The hard mask is etched with hydrogen peroxide. In this way, the portions of the hard mask 20 between the resistor material 12 and the electrical interconnects 18 act as barriers and prevent interdiffusion between the resistor material 12 and the electrical interconnects 18 without significant effect on the electrical connection. Furthermore, the thin film resistors thereby formed are free of PR contamination and interconnect metallization across their surface. Alternatively, as shown in Figure 3a, after the resistor material 12 has been etched, portions of the hard mask may be etched leaving behind those portions of the hard mask 22 overlying the portions of the resistor material 12 that are for contacting electrical interconnects. Thereafter, as shown in Figure 3b, the electrical interconnects 18 may be deposited over portions of the remaining hard mask 22 thereby leaving hard mask portions 22 between the electrical interconnect 18 and the resistor material 12. The portions of the hard mask 22 between the resistor material 12 and the electrical interconnects 18 act as barriers and prevent interdiffusion between the resistor material 12 and the electrical interconnects 18 without significant effect on the electrical connection.

Claims: A method of making at least one thin film resistor in a semiconductor device with a hard mask layer being deposited on a thin film resistor layer in a deposition chamber without breaking a vacuum in the deposition chamber, the method is characterized by the steps of: a) depositing NiCr as said thin film resistor layer on the surface of the semiconductor device and depositing as hard mask layer TiW thereon; b) providing a photoresist masking layer on said TiW hard mask layer and delineating said photoresist layer to a pattern corresponding to said thin film resistor so that it remains on the portions of the hard mask overlying the portions of the thin film resistor, c) etching the exposed TiW hard mask layer with hydrogen peroxide that does not affect the thin film resistor layer, to thereby expose said thin film resistor therebeneath; d) selectively etching the exposed thin film resistor layer with sulfuric acid etchant at a temperature greater than 125°C to leave the thin film resistor beneath the delineated TiW hard mask; e) removing the remaining TiW mask; and f) applying electrical interconnects to the resistor. A method according to claim 1 wherein in step e) the step of removing comprises etching the remaining portion of the TiW hard mask layer except the portions of the hard mask overlying selected portions of the thin film resistor for contacting electrical interconnects.

Inventors: GASNER JOHN T, GAUL STEVE, LINN JACK H, MCCARTY CHRIS, GASNER, JOHN T., GAUL, STEVE, LINN, JACK H., MCCARTY, CHRIS
Applicants: HARRIS CORP, HARRIS CORPORATION
Application year: 1995
Main classification: H01L21
IPC codes: H01L27, H01L21, H01L23, H05K3