Abstract:
A thin film resistor ( 60 ) is contained between two metal interconnect layers ( 40, 100 ) of an integrated circuit. Contact may be made to the resistor ( 60 ) through vias ( 95 ) from the metal layer ( 100 ) above the resistor ( 60 ) to both the thin film resistor ( 60 ) and the underlying metal layer ( 40 ) simultaneously. The resistor ( 60 ) may include portions of a hard mask ( 70 ) under the vias ( 95 ) to protect the resistor material ( 60 ) during the via ( 95 ) etch. This design provides increased flexibility in fabricating the resistor ( 60 ) since processes, materials, and chemicals do not have to satisfy the conditions of both the resistor ( 60 ) and the rest of the integrated circuit (especially the interconnect layer  40 ) simultaneously.

Description:
This is a divisional application of Ser. No. 09/452,691 filed Dec. 2, 1999 which is a non-provisional application of provisional application No. 60/112,731 filed Dec. 18, 1998. 
    
    
     FIELD OF THE INVENTION 
     The invention is generally related to the field of integrated circuits and more specifically to thin film resistors and methods for manufacturing the same. 
     BACKGROUND OF THE INVENTION 
     Thin film resistors are utilized in electronic circuits in many important technological applications. The resistors may be part of an individual device, or may be part of a complex hybrid circuit or integrated circuit. Some specific examples of thin film resistors in integrated circuits are the resistive ladder network in an analog-to-digital converter, and current limiting and load resistors in emitter follower amplifiers. 
     Film resistors can comprise a variety of materials including tantalum nitride (TaN), silicon chromium (SiCr), and nickel chromium (NiCr). These resistor materials are generally evaporated or sputtered onto a substrate wafer at a metal interconnect level and subsequently patterned and etched. The thin film resistors require an electrical connection to be made to them and generally the performance of the resistors is related to the condition and cleanliness of the resistor surface and the integrity of the electrical connection. It is well known that contaminants incorporated in the resistor material and around the electrical interconnects can have adverse effects on the resistor performance. It is important to ensure that during the manufacturing process, the resistor surface is not exposed to materials and chemicals likely to leave behind contaminants on the resistor surface that will adversely affect either the bulk sheet resistivity or the subsequent interconnect areas. 
     A well known method of ensuring that the resistor does not come into contact with potential contaminants during processing is to deposit a sacrificial barrier layer, such as titanium-tungsten (TiW) or other suitable material over the resistor just after it has been deposited. This barrier layer is often referred to as a “hard mask ”. After the barrier layer and resistor material are patterned and etched, the metal for the metal interconnect is deposited, patterned and etched. The “hard mask” protects the resistor during this processing and is eventually removed by a wet chemical process such as exposure to a hydrogen peroxide (H 2 O 2 ) solution just before an insulation layer or passivation layer is deposited over the resistor to permanently protect it. 
     Maghsoudnia et al (U.S. Pat. No. 5,420,063, issued May 30, 1995) describes the use of the hard mask scheme described above in an embodiment that was designed to allow small line widths to be obtained in the integrated circuit. During processing, the thin film resistor and hard mask are protected by a photoresist layer whilst dry etching is undertaken on the rest of the integrated circuit. Subsequently, when wet etching of the resistor is undertaken, the area previously dry etched is protected by a second photoresist layer. 
     Linn et al (U.S. Pat. No. 5,547,896, issued Aug. 20, 1996) also describes a method of forming a thin film resistor using a hard mask. Wet etchants are utilized that selectively etch either the hard mask material or the thin film resistor material. 
     Morris (U.S. Pat. No. 5,485,138, issued Jan. 16, 1996) describes a method of forming an inverted thin film resistor. The resistor structure is deposited directly on top of the metallic interconnects, thus avoiding the need for a protective hard mask. 
     SUMMARY OF THE INVENTION 
     Pursuant to the present invention, a thin film resistor is disclosed which is contained between two metal interconnect layers of an integrated circuit. Contact is made to the resistor from vias on the metal layer above the resistor to both the thin film resistor and the underlying metal layer simultaneously. The advantage on this design is that flexibility in processing the resistor is increased since processes, material, and chemicals do not have to satisfy the conditions of both the resistor and the rest of the integrated circuit (especially the interconnect layer) simultaneously. 
     This and other advantages will be apparent to those of ordinary skill in the art having reference to the specification in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a cross-sectional diagram of an integrated circuit having a thin film resistor according to the invention; and 
     FIGS. 2-8 are cross-sectional diagrams of the integrated circuit of FIG. 1 at various stages of fabrication. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention or its application or uses. The present invention discloses a process for manufacturing a thin film resistor in an integrated circuit with the resistor film sandwiched between dielectric layers, which are themselves located between two metal interconnect layers. Contact from the upper metal layer to the resistor and to the lower metal layer may be made simultaneously. 
     A thin film resistor  60  according to the invention is shown in FIG. 1. A first dielectric layer  30  is formed over a semiconductor body  10 . Semiconductor body  10  may, for example, comprise a silicon substrate with transistors and other devices formed thereon. A lower metal interconnect layer  40  is shown as contacting a diffused region  20  of semiconductor body  10 . The line of metal interconnect layer  40  shown extends into the page. Lower metal interconnect layer  40  typically comprises aluminum. However, other suitable metals are known in the art. 
     Separating the lower metal interconnect layer  40  and the upper metal interconnect layer  100  is a multi-level dielectric. Two levels  50 ,  90  are shown. Dielectrics  50  and  90  may, for example, comprise a spin-on-glass. Other suitable dielectrics, such as HSQ, are known in the art. The combined thickness of dielectric  50  and  90  may be chosen according to the requirements for separating the metal interconnect levels  40  and  100  from etch other. The relative thicknesses of layers  50  and  90  can be determined based on optimizing the formation of the multi-level dielectric. 
     In a preferred embodiment of the invention, the distance from the top of the silicon substrate in semiconductor body  10  to the top of dielectric  50  is chosen such that it&#39;s physical thickness multiplied by its refractive index is equal to an odd integer number of laser quarter wavelengths. This optimizes the accuracy of the laser for laser trimming of the resistor after processing. Because the actual thickness of the dielectric varies somewhat, due to deposition errors for example, dielectric  50  may include an additional layer added after measurement of the dielectric thickness as described in co-pending application Ser. No. 09/406,457, filed Sep. 27, 1999, now U.S. Pat. No. 6,326,256 B1 to Bailey et al and assigned to Texas Instruments Incorporated. 
     Thin film resistor  60  is sandwiched between dielectrics  50  and  90 . Thus, in contrast to the prior art, thin film resistor  60  is located between interconnect levels. Although metal interconnect layer  40  is shown as metal level  1 , it may in fact be any metal level except for the topmost metal level. Likewise, although second metal interconnect layer  100  is shown as metal level  2 , it may be any metal level other than metal level  1 . 
     Conductively filled vias  95  extend from the upper metal interconnect layer  100  to both resistor  60  and lower metal interconnect layer  40 . As discussed further hereinbelow, these vias may be formed simultaneously, with no processing steps being added to contact resistor  60 . Vias  95  preferably contact optional portions of hard mask  70  that remain over the ends of resistor material  60 . However, vias  95  may contact resistor material  60  and/or hard mask  70 . Optional hard mask portions  70  can protect resistor material  60  during the via etch. In this case, hard mask portions  70  comprise a conductive material such as titanium-tungsten (TiW), titanium-nitride (TiN), or molybendum (Mo). Resistor material  60  typically comprises a material such as tantalum-nitride (TaN), silicon-chromium (SiCr), or nickel chromium (NiCr). Resistor material  60  may be, for example, 50-2000 Å, while hard mask  70  may, for example, be 500-3000 Å. 
     A method for forming thin film resistor  60  according to the invention will now be discussed with reference to FIGS. 2-7. Referring to FIG. 2, a semiconductor body  10  is provided having an active region  20  formed therein. Active region  20  may, for example, be a N-type of P-type diffusion region of a transistor. Semiconductor body  10  is typically a silicon substrate processed through the formation of isolation structures, transistors, and other devices (all not shown). Deposited over semiconductor body  10  is a dielectric layer  30 . Next, a lower metal interconnect layer  40  is formed. Lower metal interconnect layer  40  may, for example, comprise aluminum. Methods for forming metal interconnect layers are well known in the art. 
     Dielectric layer  50  is formed next. Dielectric layer  50  may be the first layer of a multi-layer dielectric. Dielectric layer  50  is preferably a planarized layer and may be formed in any of a number of ways. Some examples include: deposition followed by CMP (chemical-mechanical-polishing), resist etch back, deposition of a flowable oxide such as HSQ, dep-etch-dep, deposition of a spin-on-glass (SOG) and etchback. Dielectric  50  may be any planarized dielectric suitable for interlevel dielectric layers, such as SOG, BPSG, PSG, USG and HSQ. 
     After dielectric  50  has been formed, a layer of resistor material  60  is deposited. Suitable materials for resistor material  60  are known in the art, such as TaN, SiCr, or NiCr. As an example, sputter deposition may be used. Resistor material  60  may be, for example, 50-2000 Å thick. Next, a hard mask  70  is deposited over resistor material  60 . The function of hard mask  70  is to protect resistor material  60  from contaminants during subsequent processing. Suitable materials include TiW, TiN, and Mo. The thickness of hard mask  70  may be, for example, 500-3000 Å. 
     Referring to FIG. 3, a photoresist mask  80  is formed over hard mask  70 . Photoresist mask  80  masks those portions of resistor material  60  that will become the thin film resistor. The exposed portions of hard mask  70  and resistor material  60  are then removed using a suitable etchant or combination of wet and dry etchants. Suitable wet etchants for NiCr, such as ceric sulphate, are known in the art. Photoresist mask  80  is then stripped as shown in FIG.  4 . 
     If it is desired to leave portions of hard mask  70  over areas where interconnect to resistor  60  is to be made (i.e., at the ends of resistor material  60 ), a second photoresist mask  85  is formed to produce patterns over the edges of the resistor  60 , as shown in FIG.  5 . The exposed portions of hard mask  70  are then removed with a suitable etchant that does not adversely affect the thin film resistor material  60 , as shown in FIG.  6 . In the case of a TiW hard mask  70 , a suitable etch is a solution of hydrogen peroxide (H 2 O 2 ). Photoresist mask  85  is then stripped. Leaving portions of hard mask  70  in this manner is preferred, but optional. The remaining portions of hard mask  70  will protect resistor material  60  during the subsequent via etch. 
     Referring to FIG. 7, the remaining portion of the multi-level dielectric layer, dielectric  90 , is formed. Dielectric  90  is formed over dielectric  50 , resistor material  60 , and any remaining portions of hard mask  70 . Dielectric  90  may comprise a range of materials including, but not limited to, silica, silicate glasses, and spin on glasses (USG, PSG, BPSG, HSQ, SOG, etc.). 
     Next, vias  95  are formed in dielectric  90  and dielectric  50  using a pattern  87  and etch, as shown in FIG.  8 . Vias  95  extend through dielectric  90  to hard mask  70  (or resistor material  60  if hard mask portions do not remain) and through both dielectric  50  and  90  to the lower metal interconnect layer  40 . All vias  95  may be formed during the same patterned etch step, so no additional steps are needed to connect to the resistor  60 . Alternatively, vias  95  to the lower interconnect layer  40  and vias  95  to the resistor  60  may be formed separately. The upper metal interconnect layer  100  is formed and the vias  95  are filled with conducive material. Finally, dielectric layer  110  and passivation layer  120  are deposited over the device, resulting in the structure shown in FIG.  1 . 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.