Patent Publication Number: US-6214743-B1

Title: Method and structure for making self-aligned contacts

Description:
TECHNICAL FIELD 
     The present invention relates generally to the fabrication of semiconductor devices, and more particularly the formation of contacts holes and/or contact structures in a semiconductor device. 
     BACKGROUND OF THE INVENTION 
     Integrated circuits can typically include various layers of conductive, semiconductive, and/or insulating materials. For example, an integrated circuit may include a substrate in which a number of active devices (such as transistors) may be formed. Such active devices may then be connected to one another by one or more conductive or semiconductive layers. The interconnecting conducting layers may be separated from one another by insulating layers. 
     Insulating layers typically provide electrical isolation between conductive layers. While insulating layers can be formed from a variety of materials, one consideration in selecting an insulating material can be the dielectric constant of the material. If a high dielectric constant material isolates a conductive line from a substrate, “parasitic” capacitance and/or transistor effects can occur. 
     Conductive or semiconductive layers may be formed from a single layer of material, or alternatively, include one or more conductive (or semiconductive) materials. As just a few examples, such a layer can include a conventionally doped polycrystalline silicon (polysilicon) and “silicide” (silicon-metal alloy). Alternatively, a conducting layer can include a titanium(Ti)-tungsten(W) alloy layered onto bulk aluminum, with an underlying barrier layer comprising Ti, Ti-nitride (TIN), or a Ti alloy. Similarly, insulating layers can also be composites. As just one example, an insulating layer may include a “doped” silicon dioxide (“oxide”) and an “undoped” silicon oxide (undoped silicate glass or “USG”). The doped silicon oxide can include dopant elements, such as boron and phosphorous, while the undoped silicon oxide will be essentially free of dopant elements. Phosphorous doped silicon dioxide (phosphosilicate glass or “PSG”) can provide advantageous ion gettering and step coverage properties. Boron and phosphorous doped silicon dioxide (borophosphosilicate glass or “BPSG”) can also provide such advantages, and can be formed at lower temperatures. 
     Different conductive or semiconductive layers can be connected to one another by contacts and/or vias. Contacts and/or vias can include contact holes that extend through one or more insulating layers. Conventionally, contacts can connect a substrate to a conductive or semiconductive layer, while a via can connect two different conductive or semiconductive layers to one another. 
     A conventional way of forming a contact hole in one or more insulating layers can include lithography and etch steps. Lithography can be used to form a pattern over an insulating layer (that includes the location of contact holes). An etch step can transfer the pattern onto the insulating layer. 
     One concern with certain contact structures can be the alignment of a contact with a lower conducting layer. Because a contact is usually formed by etching a hole through an insulating layer to an underlying conductive or semiconductive, it is desirable for the etched hole to be situated directly over the desired contact location in the lower conducting layer. Further, it may be desirable to ensure that a contact hole can be sufficiently insulated from other conductive lines. For example, in the case of semiconductor devices having insulated gate field effect transistors (IGFETs), such as metal-oxide-semiconductor FETs (MOSFETs), it is desirable to have a contact hole aligned with a source or drain, but at the same time, be sufficiently insulated from a gate conductor. 
     Another concern regarding contacts and/or vias can be the area of the contact. The area of a contact can be of concern as a substrate on which a contact is formed can also include other important features, such as transistor channels, transistor isolation structures, transistor diffusion regions, and/or device wells. Thus, reductions in contact size can provide more area for other features and/or reduce the overall size of an integrated circuit device. Further, in many processes contact area may have a minimum requirement in order to ensure a low enough contact resistance value. Thus, it is desirable that a contact forming process be capable of meeting a minimum contact resistance value. 
     Various factors may contribute to reducing contact area. One such factor is that of contact aspect ratio (AR). An aspect ratio can describe the ratio between a contact height and width, when viewed in cross section. The higher the aspect ratio, the more difficult it may be to form the contact. 
     To better understand the formation of certain integrated circuit structures, including contacts and contact holes, a conventional self-aligned contact (SAC) approach is set forth in FIGS.  5  and  6 A- 6 G. FIG. 5 is a flowchart illustrating the general steps involved in forming a self-aligned contact for an integrated circuit that includes MOSFETs. FIGS. 6A-6G set forth a number of side cross-sectional views of a portion of an integrated circuit following the various steps described in FIG.  5 . 
     The conventional process set forth in FIG. 5 is designated by the general reference character  500 . The process  500  can begin by forming MOSFET gate stacks having sidewalls (step  502 ). A portion of an integrated circuit following step  502  is set forth in FIG. 6A. A MOSFET gate stack  600  can be formed on a substrate  602 , and include a gate oxide  604 , conductive portions  606 , and an insulating portion  608 . Sidewalls  610  are also set forth in FIG.  6 A. 
     It is noted that a substrate  602  can include isolation structures formed therein by a previous substrate isolation structure forming step. As just two examples, a substrate  602  can include shallow trench isolation structures and/or local oxidation of silicon (LOCOS) structures. Further, a substrate  602  may also include doped portions formed with previous and/or subsequent doping steps, such as transistor sources, drains, channels, and device wells. 
     A conventional process  500  can continue with the deposition of an insulating layer of BPSG and USG (step  504 ). A portion of an integrated circuit following step  504  is set forth in FIG.  6 B. Referring now to FIG. 6B, the integrated circuit portion includes a BPSG layer  612  and an USG layer  614  formed over the BPSG layer  612 . The BPSG and USG layers ( 612  and  614 ) may be formed over MOSFET gate stacks  600  and may be deposited using chemical vapor deposition (CVD) methods. 
     The conventional process  500  can continue with lithography and etch steps. A photoresist layer can be deposited on a USG layer  614  (step  506 ). Referring to FIG. 6C, an example of a portion of semiconductor device following a step  506  is shown in a side cross sectional view. A photoresist layer (“resist”) can include a photoresist material  616  and a bottom antireflective coating (BARC)  618 . 
     The photoresist material  616  can be patterned (step  508 ). Typically, a photoresist material  616  can be patterned by selectively exposing portions of the photoresist material to a radiation source. Typically a photomask can be used to establish the pattern. Radiation sources can include various light sources, including coherent light, from various spectrums including the visible spectrum and ultraviolet spectrum. Other methods can utilize X-rays, electron beam (e-beam), or ion beams to form a pattern in a photoresist material  616 , to name but a few examples. E-beam and ion beam systems may not require a photomask. 
     A BARC  618  can reduce deleterious “corner” effects and/or other undesirable developing artifacts that may occur with smaller geometry devices. 
     Portions of the photoresist material  616  can then be removed according the pattern developed by the exposure to radiation. A portion of a semiconductor device after removal of photoresist material is shown in FIG.  6 D. Once selected portions of the photoresist material  616  are removed, the photoresist material  616  can be “baked” to form an etch mask. In this way, in a conventional process  500 , photoresist material serves as an etch mask. 
     A photoresist material  616  can have a predetermined thickness when deposited. Consequently, a resulting etch mask can also have a predetermined thickness. In the example of FIG. 6D, the photoresist etch mask exposes portions of the BARC  618 , below which a contact hole may be formed. 
     Once an etch mask of photoresist is formed, a conventional process can then remove those portions of the BARC  614  and USG  614  that are situated below openings in the photoresist etch mask (step  510 ). A portion of a semiconductor device following the removal of a BARC  618  and USG layer  614  is shown in FIG.  6 E. 
     A conventional process  500  can proceed with a SAC etch (step  512 ). FIG. 6F shows a portion of semiconductor device following a SAC etch. A SAC etch can form etch holes  620  that extend through a BPSG layer  612  to a substrate  602 . As shown in FIG. 6F, the resulting contact holes  620  may be “self-aligned” with the MOSFET gate stacks  600 . The self-alignment occurs by the insulating portions  608  and sidewalls portions  610  insulating conductive portions  606  from contact holes  620 . As a result, a minimum spacing requirement between the edges of a contact hole  620  and a MOSFET gate stack  600  may not be necessary. 
     A conventional SAC etch can have a high degree of selectivity between a photoresist etch mask and a BPSG layer  612 . That is, the rate at which BPSG can be etched may be substantially faster than the rate at which the photoresist etch mask can be etched. 
     A conventional process  500  can continue by removing a photoresist etch mask (step  514 ). A photoresist etch mask can be removed with a plasma etch that etches photoresist at a substantially faster rate than other exposed semiconductor device materials. Such a removal of a photoresist etch mask with a plasma etch is often referred to as “ashing.” 
     While the described conventional process may produce desired contact holes at certain geometries (i.e., certain contact widths), as semiconductor device manufacturing processes provide smaller and smaller features, such conventional contact approaches may be inadequate. Smaller contact feature sizes may result in larger aspect ratios. 
     As just one example, a semiconductor device fabricated according to a conventional approach may include contacts of about 0.30 μm. Such a contact formed adjacent to some MOSFET gate stacks can have an aspect ratio of about 2.5:1. However, when a contact hole is considered with its corresponding photoresist etch mask, its aspect ratio can be larger. This can best understood with reference to FIGS. 6F and 6G. In FIG. 6G a contact hole  620  that does not include the photoresist mask can have an aspect ratio of about 2.5:1. If a photoresist etch mask is included as part of a contact hole, as shown in FIG. 6F, a contact hole  620  can have an aspect ration of about 5:1. 
     Accordingly, if contact size decreases while photoresist etch mask thickness remains the same (or does not decrease proportionally), contact hole aspect ratios may increase to the point where a contact hole cannot be formed. This is illustrated in FIG. 7 which shows a side cross sectional view of an attempted formation of contact holes  720 . Contact holes  720  (when photoresist mask portions are included) have too high a desired aspect ratio. In the particular example of FIG. 7, contact holes  720  are formed partially through a BPSG layer  712  and fail, as they do not expose a portion of a substrate  700 . A contact hole may also fail by not providing sufficient contact area (i.e., exposing a sufficient amount of substrate area). 
     Alternate conventional etch processes can form contact holes without etch masks formed from photoresist. Such approaches include “hard” etch masks. Conventional hard etch masks may be etch masks formed from a deposited material that itself can be etched into a desired pattern. Nitride and polysilicon are two conventional hard mask materials. Polysilicon can be undesirable as a hard mask because it can be difficult to remove after it is used. Nitride can be undesirable as a hard mask because it can introduce a relatively high dielectric constant material between conductive layers. 
     In light of the conventional approach set forth above, it would be desirable to provide a way of forming contacts that can overcome the drawbacks described above. 
     It would also be desirable to form such contacts through an insulating material that includes PSG and/or BPSG, and therefore include the advantages of such materials. 
     It would further be desirable that such contacts be self-aligned contacts for an IGFET, as IGFET contact size can play an important role in reducing the overall size of an integrated circuit. 
     It would further be desirable that such contacts have relatively small geometries, for example 0.25 μm or less, or more desirably, 0.18 μm or less. 
     SUMMARY OF THE INVENTION 
     According to the disclosed embodiments of the invention, methods and structures for forming self-aligned contacts are set forth in a series of charts and diagrams. The disclosed methods and structures can provide hard etch masks for self-aligned contacts that can result in more favorable contact hole aspect ratios. According to selected embodiments of the invention, a self-aligned contact hard etch mask can be formed from undoped silicon dioxide (SiO 2  or USG). USG can introduce a material having a relatively low dielectric constant (as compared to silicon nitride, for example). 
     According to one aspect of an embodiment, a USG hard etch mask can be formed over a layer of borophosphosilicate glass (BPSG). A subsequent etch can have a selectivity between BPSG and USG. 
     According to another aspect of an embodiment, a USG hard etch mask can be formed over a layer of phosphosilicate glass (PSG). A subsequent etch can have a selectivity between PSG and USG. 
     According to another aspect of an embodiment, the USG hard etch mask can function as a mask for contact holes having geometries of 0.25 μm or less. 
     According to another aspect of an embodiment, the USG hard etch mask can function as a mask for contact holes having geometries of 0.18 μm or less. 
     According to another aspect of an embodiment, contact holes that include the USG hard etch mask can have aspect ratios of less than 8:1. 
     According to another aspect of an embodiment, contact holes that include the USG hard etch mask can have aspect ratios of less than 6:1. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow diagram of a first embodiment. 
     FIGS. 2A to  2 E are side cross-sectional views of a semiconductor device according to a first embodiment. 
     FIG. 3 is a flow diagram of a second embodiment. 
     FIGS. 4A to  4 M are side cross-sectional views of a semiconductor device according to a second embodiment. 
     FIG. 5 is a flow diagram of a conventional self-aligned contact process. 
     FIGS. 6A to  6 G are side cross-sectional views of a semiconductor device according to the conventional approach of FIG.  5 . 
     FIG. 7 is a side cross-sectional view of a conventional semiconductor device having a failed self-aligned contact etch. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Various embodiments will now be described in conjunction with a number of charts and diagrams. The embodiments set forth approaches to forming self-aligned contact holes with a hard etch mask formed from undoped silicate glass (SiO 2  or “USG”). A hard etch mask formed of USG can result in contact holes having lower aspect ratios than conventional approaches for forming contact holes of like size. 
     Referring now to FIG. 1, a flow diagram is set forth showing a method of forming a contact hole according to a first embodiment. The first embodiment is designated by the general reference character  100 , and can include forming a structure with a sidewall (step  102 ). A sidewall may insulate a structure in a lateral direction from a contact hole. One example of a portion of an integrated circuit following a step  102  is set forth in a side cross-sectional view in FIG.  2 A. 
     FIG. 2A includes a substrate  200  on which a structure  202  may be formed. A substrate  200  can provide a surface on which structures may be formed and/or in which active devices (such as transistors) may be formed. A substrate  200  may include a generally monocrystalline semiconductor substrate that can have portions insulated from one another (e.g., by local oxidation of silicon (LOCOS), shallow trench isolation (STI), or silicon-on-insulator (SOI), to name but a few limited example). 
     Some of the possible ways in which a structure  202  may be formed can include etching a substrate  200 , and/or depositing one or more layers and then etching such deposited layers. A structure  202  may include a conductive (or semiconductive) portion  204  and a top insulating portion  206 . A top insulating portion  206  may isolate a conductive (or semiconductive) portion  204  from a contact hole that overlaps a structure  202 . 
     Also included in FIG. 2A is a sidewall  208 . A sidewall  208  may isolate a conductive (or semiconductive) portion  204  from a contact hole that can be formed next to, or overlap a structure  202 . One way in which a sidewall  208  may be formed, can be by depositing an insulating layer over the sides of a structure  202 , and then anisotropically etching the insulating layer. Such a sidewall (also referred to as a “spacer”) can preferably comprise a material such as silicon dioxide (SiO 2 ) and/or silicon nitride (Si 3 N 4 ). 
     A first embodiment  100  may also include depositing a first insulating layer (step  104 ). A first insulating layer can insulate a structure  202  from subsequently deposited conductive (or semiconductive) layers. A first insulating layer can include silicon dioxide, preferably silicon dioxide that is doped with phosphorous (phosphosilicate glass or “PSG”). More preferably, the first insulating layer can be doped with boron and phosphorous (borophosphosilicate glass or “BPSG”). A first insulating layer may be deposited using chemical vapor deposition (CVD), or plasma enhanced and plasma assisted CVD (PECVD and PACVD), or high density plasma (HDP) deposition, to name but a few examples. A PSG or BPSG layer can be subject to a “reflow” and/or planarization step. 
     FIG. 2B shows a portion of a semiconductor device following the deposition of a first insulating layer  210 . A first insulating layer  210  can insulate a substrate  200  and/or a structure  202  from subsequently deposited conductive (or semiconductive) layer. 
     A first embodiment  100  may also include depositing a second insulating layer over the first insulating layer (step  106 ). A second insulating layer can form all or part of a “hard” etch mask for a first insulating layer  210 . A second insulating layer  210  can include silicon dioxide, preferably undoped silicon dioxide (undoped silicate glass or “USG”). A second insulating layer may be deposited using chemical vapor deposition (CVD), or plasma enhanced and plasma assisted CVD (PECVD and PACVD), or high density plasma (HDP) deposition, to name but a few examples. A second insulating layer  210  may also be planarized. 
     FIG. 2C shows a portion of a semiconductor device following the deposition of a second insulating layer  212 . A second insulating layer  212  can be deposited over (but not necessarily on) a first insulating layer  210 . 
     Once a second insulating layer  212  is deposited, a hard etch mask can be formed that can include a second insulating layer  212  (step  108 ). A step  108  can include forming hard etch mask openings in a second insulating layer  212 . One way to form such hard etch mask openings can include conventional lithographic and etch techniques. 
     FIG. 2D shows one example of a semiconductor device following a formation of a hard etch mask that includes a second insulating layer  212 . A hard etch mask is designated by the general reference character  214 , and is shown to include a mask portion  216  and a mask opening  218 . A mask portion  216  can include portions of a second insulating layer  212 . A mask opening  218  may be formed by removing portions of a second insulating layer  212 . A mask opening  218  can be situated over a location where a contact hole may be formed. 
     With a hard etch mask  214  formed over a first insulating layer  210 , a first embodiment  100  can continue by etching a contact hole (step  110 ). Contact holes can be etched through a first insulating layer  210  to a conductive (or semiconductive) layer below, such as a substrate  200 . A contact hole may be anisotropically etched. Anisotropic etching is believed to provide better control over the aspect ratio of the contact hole then an isotropic etch. A contact hole may be preferably formed with an etch having a high degree of selectivity between a first insulating layer  210  and a second insulating layer  212 . 
     A contact hole can preferably be self-aligned with a structure  202 , more preferably, self-aligned with a conductive (or semiconductive) structure  202  having a sidewall  208 . A contact hole may expose a sidewall  208 , but not necessarily result in a sidewall  208  being entirely exposed. None or only a portion of a sidewall  208  may be exposed by a step  110 . 
     FIG. 2E shows one example of a semiconductor device following a formation of a contact hole  220 . A contact hole  220  may be considered “self-aligned”, as there may be no minimum spacing requirements between a contact hole  220  and an adjacent structure  202 . 
     In the process described by the first embodiment  100 , a hard etch mask  214  can result in a contact hole  220  having a lower aspect ratio than a conventional approach, such as those that may utilize an etch mask that includes photoresist. This may allow the formation of smaller contacts than conventional approaches. 
     If a hard etch mask  214  includes USG, such a hard etch mask  214  does not necessarily have to be removed after a contact hole  220  is formed. USG can have a lower dielectric constant than other hard etch mask materials, such as silicon nitride, for example. Consequently, the potential for parasitic effects, can be less than other conventional approaches that can introduce higher dielectric constant materials. 
     If a hard etch mask  214  of USG is formed over a first insulating layer of BPSG and/or PSG, the hard etch mast  214  can further function as a “cap” oxide. A cap oxide may stabilize a BPSG and/or PSG layer and/or prevent the out-diffusion of BPSG and/or PSG dopants. 
     Referring now to FIG. 3, a flow diagram shows a third embodiment  300 . A third embodiment  300  includes a method for forming a self-aligned contact in an integrated circuit that may include insulated gate field effect transistors (IGFETs). One example of an IGFET is a metal-oxide-semiconductor (MOS) FET. A second embodiment  300  may include a first step of forming gate structures (step  302 ). 
     Forming gate structures may include depositing one or more conductive (or semiconductive) gate layers over a gate insulator. A top insulating layer can also be deposited over the conductive gate layer(s). 
     FIG. 4A shows a side cross sectional view following a deposition of gate materials. A semiconductor device includes a substrate  400  on which a gate insulator  402 , a first gate conductive layer  404 , and a second gate conductive layer  406  may be formed. A top insulating layer  408  may be formed over a second gate conductive layer  406 . 
     Forming a gate structure may further include forming a pattern in deposited gate conductive layers ( 404  and  406 ) and/or a top insulating layer  408 . Such a pattern forming step may include conventional lithography and etch steps. 
     FIG. 4B shows a side cross sectional view following a patterning of gate materials with conventional lithographic and etch steps. A gate etch mask  410  can be formed over a top insulating layer  408 . A gate etch mask  410  may be formed from photoresist as just one example. A gate etch mask  410  can include gate mask portions  412  and gate mask openings  414 . Portions of the gate conductive layers ( 404  and  406 ) and top insulating layer  408  exposed by gate mask openings  414  can be removed by a gate etch. Preferably, the gate etch can be a reactive ion etch (RIE). A gate etch mask  410  may then be removed. 
     FIG. 4C illustrates a semiconductor device after a gate etch mask  410  has been removed. Gate stacks  416  can be formed over a substrate  400 . 
     The second embodiment  300  can continue by forming sidewalls on gate stacks  416  (step  304 ). One approach to forming sidewalls is shown in FIGS. 4D and 4E. The illustrated approach can include depositing a layer of undoped silicon dioxide over gate stacks  416 . A semiconductor device following the deposition of undoped silicon dioxide  415  is set forth in FIG.  4 D. Portions of the silicon dioxide may be removed with an anisotropic etch to form sidewalls (or “spacers”). Gate stacks  416  with accompanying sidewalls  418  are illustrated in FIG.  4 E. Sidewalls  418  may provide insulation for the lateral edges of gate stacks  416 , and may thus allow contacts to be self-aligned with gate stacks  416 . 
     The second embodiment  300  can further include depositing a layer of BPSG over the gate stacks  416  (step  306 ). FIG. 4F illustrates a layer of BPSG  420  formed over gate stacks  416 . BPSG can be deposited according to conventional PECVD techniques. However, the invention should not be construed as being limited to such an insulating layer or deposition technique. As just one example, phosphosilicate glass (PSG) may be deposited over gate stacks  416 . Other examples have been discussed in conjunction with the first embodiment  100 . 
     A BPSG layer  420  may be planarized after it is deposited. Planarization can include a chemical-mechanical polishing (CMP) step, to name just one example. Alternatively, planarizing may be accomplished by an isotropic etching step, or by a relatively low temperature and/or short duration reflow step, but CMP is preferred. 
     Once a BPSG layer  420  is deposited, the second embodiment  300  can continue with a deposit USG step  308 . As shown in FIG. 4G, a layer of USG  422  can be deposited over a BPSG layer  420 . In the second embodiment  300 , a USG layer  422  can be deposited according to conventional PECVD techniques, and then planarized. However, different deposition methods could be employed. Some such methods have been discussed in conjunction with the first embodiment  100 . 
     As shown in FIG. 3, a second embodiment  300  can continue by depositing a layer of photoresist over a USG layer  422  (step  310 ). A semiconductor device following a step  310  is set forth in FIG. 4H. A photoresist layer  424  has been formed over a USG layer  422  that can include a bottom antireflective coating (BARC)  426  and a photoresist material  428 . 
     Once a photoresist layer  424  is deposited, such a layer can be patterned (step  312 ). A patterning step can include a conventional lithography step. Selected portions of photoresist material  428  can be exposed to radiation. Selective exposure can be accomplished with a photomask, or by way of beam writing technology. Such exposure can produce a contact hole pattern in a photoresist material  428 . Depending upon the type of photoresist material used (i.e., positive or negative), selected portions of a photoresist material  428  can be removed and the resulting pattern subjected to heat (“baked”). In this way, a contact hole pattern may be formed. 
     FIG. 4I shows a side cross-sectional view of a semiconductor device following the formation of a contact hole pattern  430 . A contact hole pattern  430  can include contact mask portions  432  and contact location openings  434 . Contact location openings  434  can be situated over the locations where a contact may be formed. In the particular arrangement of FIG. 4I, the patterning of a photoresist layer  424  can expose a BARC  426 . 
     The second embodiment  300  can further include a step that etches a USG layer  422  to form a hard etch mask (step  314 ). In one particular arrangement, a RIE step having a high degree of selectivity between USG and photoresist can transfer a photoresist contact hole pattern  430  to a USG layer  422 . A semiconductor device following such a step  314  is shown in FIG.  4 J. 
     In FIG. 4J, portions of a BARC  426  and USG layer  422  have been removed according to a photoresist contact hole pattern  430 . Consequently, a hard etch mask  436  has been formed that includes a USG layer  422 . In the particular arrangement of FIG. 4J, a hard etch mask  436  can include hard mask portions  438  and hard mask openings  440 . Hard mask openings  440  may be situated over a location where a contact hole may be formed, preferably a self-aligned contact hole. 
     It is noted that a photoresist contact hole pattern  430 , if used as a contact hole etch mask, would add significantly to an aspect ratio for a contact hole. Further, in the particular arrangement of FIG. 4J the thickness of the hard etch mask  436  may be less than a thickness of the photoresist contact hole pattern  430 . 
     Unlike a conventional approach to forming self-aligned contacts, a second embodiment  300  can remove a photoresist layer  424  prior to forming a contact hole (step  316 ). Such a step can include “ashing” a photoresist layer  424  with a plasma etch, or the like. Of course, other conventional photoresist removal techniques can be utilized, according to the particular type of photoresist layer  424 . 
     FIG. 4K illustrates a semiconductor device following the removal of a photoresist layer  424 . A hard etch mask  436  that includes USG may be formed over a BPSG layer  420 . It is noted that removal of a photoresist layer  424  can result in a hard self-aligned contact etch mask that produces a smaller aspect ratio for contact holes than other conventional approaches, such as those that use photoresist as an etch mask. 
     Once a hard etch mask  436  is formed, a self-aligned contact (“SAC”) etch may be performed (step  318 ). A SAC etch may be a substantially anisotropic reactive ion etch (RIE). Such a RIE SAC etch may be more selective to a BPSG layer  420  than a USG hard etch mask  436 . 
     A portion of an integrated circuit following a step  318  is set forth in FIG.  4 L. Contact holes  442  may be formed below hard mask openings  440 . A contact hole  442  (including its hard etch mask  436 ) may have a smaller aspect ratio than conventional approaches. Consequently, smaller contact geometries may be achievable. As just a few examples, aspect ratios of less than or equal to 7:1, or preferably less than or equal to 5:1, may be formed with a hard etch mask  436 . Further, contact geometries of 0.25 μm or smaller, preferably 0.20 μm, more preferably 0.18 μm or smaller, may be formed. 
     While contact holes formed according to the second embodiment  200  are self-aligned, the teachings of the second embodiment  300  may be used to form contacts that are not self-aligned, and/or vias. 
     It is understood that the invention should not be construed as being limited to BPSG  420  as a lower insulating layer and USG  422  as a hard etch mask layer. Other materials may be used with a contact etch having a high degree of selectivity between a hard etch mask layer and a lower insulating layer through which contacts can be formed. 
     It is further noted that a USG hard etch mask can have the advantages described above. Namely, a low dielectric constant relative to alternate hard etch mask materials, and an ability to function as a cap layer for BPSG and/or PSG. 
     A second embodiment  300  can also include forming a contact (step  320 ). A contact may be formed by depositing one or more conductive materials in a contact hole, as just one example. A subsequent planarization step may then be applied to provide a substantially planar surface. A portion of an integrated circuit following a step  316  is set forth in FIG.  4 M. 
     FIG. 4M illustrates contacts  444  formed within contact holes  442 . A contact  444  may provide an essentially vertical conductive path through a BPSG layer  420  to a substrate  400 . 
     While the particular arrangement of FIG. 4M illustrates a contact structure in which a hard etch mask  436  has been retained, it is understood that a hard etch mask  436  could be removed prior to the formation of a contact. 
     A number of embodiments have been described setting forth approaches to forming a contact hole and/or a self-aligned contact hole with a hard etch mask. Such approaches may be implemented in existing fabrication processes to take advantage of the benefits of the various methods and structures described. As just one example, a manufacturer may alter its current fabrication process, which might form contacts with an etch mask of photoresist, or the like, to a new process flow that can form contacts with a hard contact etch mask having a smaller thickness than a photoresist etch mask. In addition, or alternatively, a hard etch mask can be retained (i.e., not removed) and have a relatively low dielectric constant. 
     Further, by utilizing the advantageous lower aspect ratio contact holes illustrated by the disclosed embodiments, a manufacturer may be capable of re-tooling a fabrication process to accommodate smaller geometry devices, as smaller contact hole aspect ratios may produce smaller geometry contacts. 
     A structure formed according to an embodiment may include contact holes formed through two insulating layers. A second insulating layer can be formed over a first insulating layer and function as a hard etch mask for a first insulating layer. Side cross sectional views of a hard etch mask may show more rounded corners than a conventional approach that utilizes a photoresist etch mask to etch through a first insulating layer and a second insulating layer. Of course, other etch approaches may not produce such a corner rounding effect. 
     It is understood that while the various particular embodiments have been set forth herein, methods and structures according to the present invention could be subject to various changes, substitutions, and alterations without departing from the spirit and scope of the invention. Accordingly, the present invention is intended to be limited only as defined by the appended claims.