Method of forming vias in a semiconductor device having a low permittivity dielectric layer thereon

Abstract: In one embodiment, horizontal gaps between the patterned conductors 18 and 44 are substantially filled with an organic-containing dielectric material (Allied Signal 500 Series, for example) 22 and 54 . Inorganic dielectric layers 24 and 56 are formed over organic-containing dielectric layers 22 and 54 , respectively, from a material such as silicon dioxide. Vias are etched through the inorganic dielectric layers using an etch process such as fluorocarbons in a high density plasma which does not appreciably etch the organic-containing dielectric material.

Description: FIELD OF THE INVENTION This invention relates generally to the fabrication of semiconductor devices, and more particularly to a method as defined in the precharacterizing portion of claim 1. BACKGROUND OF THE INVENTION Semiconductors are widely used in integrated circuits for electronic devices such as computers and televisions. These integrated circuits typically combine many transistors on a single crystal silicon chip to perform complex functions and store data. Increasing the functionality or capacity of a given circuit generally requires a corresponding increase in the number of transistors which must be integrated on a single chip. This increase is often accomplished by decreasing the size of transistors and other devices contained on a chip. As the size and distance between individual devices on a chip diminish, the technical challenges related to design and production of the circuit multiply. One of the challenges in scaling VLSI (very large scale integration) circuit designs is coping with decreased mask alignment tolerance. Devices on a typical circuit are connected by two or more patterned layers of conductors, each separated from the others by insulating layers, or interlayer dielectrics. Vias are holes formed in an interlayer dielectric to allow electrical connections to the conductors or devices below the insulator. Masks are used to create a pattern for each layer of conductors and to create a pattern for the vias passing through the interlayer dielectric. If the masks are not aligned to each other, the vias may only partially expose the desired conductor or device. If the misalignment is severe, some connections will be shorted to other conductors or left open, and the circuit will not operate. Conventionally, this problem is solved by designing tolerance into the via connections, which requires more space on the chip, or by use of more elaborate mask alignment procedures and equipment A second challenge in VLSI design is the control of capacitance between adjacent conductors. In one aspect, capacitance is a measure of the ability of a signal travelling along one conductor to produce an undesirable signal (crosstalk) in a neighboring conductor. This may result in a limitation on the ultimate speed of the device, among other things. Generally, a capacitance effect exists between any two conducting regions separated by an insulator. The capacitance is directly related to the common area of the conducting regions, as well as to the dielectric constant of the insulator between them. The capacitance also increases as the distance between the two conducting regions is decreased. Conventional semiconductor fabrication uses silicon dioxide or similar insulating materials as both a gap filler between adjacent conductors at the same level and as an interlayer insulator. Silicon dioxide has a dielectric constant of about 3.9. This constant is based on a scale where 1.0 represents the dielectric constant of a vacuum. Various materials exhibit dielectric constants from very near 1.0 to values in the hundreds. As used herein, the term low-k will refer to a material with a dielectric constant less than 3.5. There exists a need for processes that can produce semiconductor devices with a satisfactory low-k material as an insulator between adjacent conductors. Unfortunately, many other factors besides dielectric constant have affected the maturation of such a process. For example, structural rigidity, method of application, temperature constraints, response to etchants, heat transfer, contamination of surrounding structure, and shrinkage are some of the factors which must also be considered when selecting new dielectric materials or processes. Several techniques have been attempted in an effort to solve this problem. One proposed technique uses a material containing hollow spheres, the air in the spheres serving to decrease the dielectric constant of the material. One of the major problems with this technique is the manufacture of such spheres at a size small enough to easily flow between conductors spaced at a fraction of a micron. Another proposed technique uses various porous materials, including porous oxides. The etching of such materials is very hard to control because of their inherent permeability. Furthermore, they tend to possess poor structural characteristics. Dielectrics formed from polymers or organics have several drawbacks, including low temperature constraints which can limit further processing steps, lack of structural rigidity, poor heat transfer, and etching problems. In particular, several candidates which appear promising have been affected by uncontrollability of the etching process. DE-A-33 45 040 discloses a method of fabricating a planarized inorganic isolating layer which involves application of polyimide. The layer of polyimide, which is deposited on a layer of patterned conductors before the structure is covered by the inorganic layer, is only used for planarizing purposes. From EP-A-0 296 707 a method for forming composite dielectric layers in an integrated circuit to facilitate fabrication of a high density multi-level interconnect with external contacts is known. The method comprises forming a polymer layer on a first inorganic layer and on patterned conductors to form a planarized surface, depositing a second inorganic layer over the polymer layer, patterning the second inorganic layer and etching vias in the polymer layer to expose external contact surfaces of the conductors, wherein the first inorganic layer functions as an etch stop layer. SUMMARY OF THE INVENTION Several problems related to continued miniturization of integrated circuits, including decreased mask alignment tolerance, increased capacitance between adjacent conductors, and poor heat transfer and structural rigidity of low-k candidates, have been described above. The present invention is the first semiconductor fabrication procedure to combine solutions to many of these problems simultaneously: inorganic low-k dielectric materials are used as a gap filler between horizontally adjacent conductors, thereby decreasing line-to-line capacitance; an overlying organic-containing sublayer possesses etch-stopping properties which may be utilized to compensate for via misalignment problems; and the majority of the interlayer dielectric is constructed from oxide or other common dielectrics which possess good heat transfer and structural characteristics. The present invention is a method as mentioned in the beginning and comprising the features of the characterizing portion of claim 1. The organic-containing layer may have a thickness between 50% and 150% the thickness of the patterned conductors. In this process, the organic-containing dielectric layer acts as an etch stop, preventing overetching due to either misalignment between the vias and the patterned conductors, or uneven topography across the semiconductor device while reducing line-to-line capacitance as compared to a silicon dioxide dielectric. The method of this invention can be applied to any interconnection layer of patterned conductors on a semiconductor device. It can also be applied where the patterned conductors are part of the polysilicon layer which also forms the gate top plates of field effect transistors integrated into the semiconductor device. Preferably, the inorganic dielectric layer is composed of more than 95% silicon dioxide, silicon nitride, or combinations thereof. Preferably, the organic-containing layer is composed of a material containing 10% to 100% organic material by weight, and the organic material is a polymer, such as polyimide. BRIEF DESCRIPTION OF THE DRAWINGS This invention, including the features and advantages thereof, can be best understood by reference to the following drawings, wherein: FIG. 1A-1D are cross-sectional views showing the progressive steps in the fabrication of an interconnection layer of patterned conductors, organic-containing and inorganic dielectric layers, and a via to one of the conductors, where the organic-containing layer completely covers the patterned conductors; FIG. 2 is a cross-sectional view of another example, in which the organic-containing layer substantially fills the space between the patterned conductors, but does not cover them; FIG 3 is a cross-sectional view of an embodiment of the invention, which employs three dielectric layers, each overlying layer etchable by an etchant to which the underlying layer is selective; FIG 4 is a cross-sectional view of yet another example, which illustrates a thin passivation layer covering the patterned conductors and underlying structure, applied before the organic-containing dielectric layer; and FIG. 5 is a cross-sectional view of still another example, which illustrates a conformal organic-containing layer and a planarized inorganic dielectric layer applied over polysilicon and field oxide, with vias etched through the inorganic dielectric layer towards a poly gate and a source-drain region at a lower elevation. Figure 6 is a cross-sectional view of the example of Figure 5 , after the vias have been opened to the silicated contacts, filled with a conducting material, and electrically connected to a second layer of patterned conductors. Figure 7 is a cross-sectional view of an example containing conformal and planarized organic-containing layers on the same semiconductor device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for fabricating self-aligned vias between interconnection layers of patterned conductors on a semiconductor device, makes use of a layer of organic-containing material which fills the space between and completely covers the conductors. This method is illustrated in Figure 1 . With reference to Figure 1A, a conducting layer 12 is deposited on an insulating layer 10 . Conducting layer 12 may be connected through insulating layer 10 to an underlying structure (not shown). A thin layer of photoresist 14 is spun on over conducting layer 12 , exposed through a mask pattern and developed, such that the photoresist layer 14 contains gaps 16 where conducting layer 12 is to be removed. now to Figure 1B, conducting material has been removed using an etching process which removes material below gaps in the photoresist layer to create patterned conductors 18 separated by gaps 20. The photoresist 14 of Figure 1A has also been stripped and does not appear in Figure 1B. Figure 1C shows additional layers added to the structure. An organic-containing dielectric layer 22 fills the gaps 20 of Figure 1B, as well as covering patterned conductors 18 to a thickness about 10% of their height This organic-containing dielectric layer may be made out of a spun-on glass (SOG) which contains a polymer (Allied Signal 500 Series, for example), which is annealed at a temperature of approximately 400 degrees C for 30 minutes An inorganic dielectric layer 24 , made of silicon dioxide for instance, is deposited over the organic-containing dielectric layer 22, and then planarized, e.g. using a chemical-mechanical polisher. A new layer of photoresist 26 is then deposited over the inorganic dielectric layer 24. A via 28 is shown after mask patterning and developing of photoresist layer 26, and after anisotropic etch of inorganic dielectric layer 24 . It is important that this step be completed with an etch process which will not appreciably etch the organic-containing dielectric layer 22, such as fluorocarbon etchants in a high density plasma for the organic-containing example Allied Signal 500 Series. Finally, referring to Figure 1D , a short anisotropic etch of the organic-containing dielectric layer 22 extends the via 28 down to the patterned conductor 18. The via 28 is purposely drawn misaligned slightly to the left of the conductor 18 to illustrate one of the advantages of this method Without the organic-containing dielectric layer 22 acting as an etch stop, the etch through the relatively thick inorganic dielectric layer 24 generally would have created a large void extending down the side of conductor 18. This void can not only cause mechanical problems, but also, if subsequently filled with metal, decrease the distance (and therefore increase the capacitance) between the conductor and its adjacent neighbor. The etch through the thick inorganic dielectric, which is stopped by the organic-containing dielectric, followed by a short, controllable etch of the organic-containing dielectric, results in a via which, even if misaligned, stops essentially at the top level of the conductor. This self-aligning feature, combined with the added advantage of a low-k material between adjacent conductors, results in lower capacitance between adjacent conductors and higher reliability for the overall metal system For several reasons, the dielectric constant of the inorganic dielectric layer is not as critical as that maintained between conductors on the same leveL First, the inorganic dielectric layer can be made relatively thick compared to the spacing between conductors on the same level. Secondly, two layers of conductors can generally be fabricated such that conductors which over or underly each other do not run parallel for large distances. This observation leads to another advantage; the inorganic dielectric layer, which can be essentially all of the interlayer dielectric, can be composed of a material which is not low-k (silicon dioxide, for example) to provide excellent heat transfer and structural rigidity while still maintaining adequate electrical isolation between the layers of conductors. Figure 2 shows an alternate method in which the process steps have been altered to produce a different structure. In this method, after the organic-containing layer 22 has been deposited and annealed, it is etched back such that the tops of the conductors 18 are exposed. The thickness of the organic-containing layer 22 preferably remains dose to the thickness of the conductors 18, perhaps 90%, to keep the capacitance between the conductors low. The inorganic dielectric layer 24 is deposited and etched as in the first method, except now the etch of the inorganic dielectric layer will be stopped by both the organic-containing layer 22 and the conductors 18 . The advantages of the first method are retained, but no anisotropic etching of the organic-containing layer is needed to complete the vias. Figure 3 illustrates an embodiment of the invention in which an inorganic low-k dielectric 32 (e.g. a porous silicide) is deposited between patterned conductors 18. An organic-containing cap layer 34, which could be relatively thin, is deposited over this structure. An inorganic dielectric layer 36 is deposited over the organic-containing cap layer 34 to complete the interlayer dielectric In this arrangement, the organic-containing cap layer 34 acts as an etch stop for the etch of inorganic dielectric layer 36. The inorganic dielectric 32 may also act as an etch stop for the cap layer 34 (although this may not be necessary if cap layer 34 is thin enough). The additional advantages of this embodiment may include an even lower intralayer dielectric constant and a relaxation on the etching properties of an inorganic low-k dielectric layer 32 , while maintaining other previously described advantages. Figure 4 shows still another method, wherein the conductors 18 and insulating layer 10 are covered by a relatively thin passivation layer 38. Deposition of remaining materials and formation of via 30 are completed as in the method of Figure 2. However, organic-containing layer 22 may span only a portion of the horizontal gap between conductors 18 , since passivation layer 38 is shown covering the sides of the conductors. Also in this method, after the etch of the inorganic dielectric layer 24 , a short etch of the passivation layer 38 may be required to dear the bottom of the vias and expose the conductors 18. Figure 5 shows another method which forms vias and contact holes to device features at the substrate/polysilicon level. Transistors are formed conventionally on a substrate 40 , which is shown with field oxide 42 , gate oxide 47 , a polysilicon gate 44 with sidewall oxide 46 , and with a self-aligned silicide 48 such as titanium silicide formed over the source/drain and gate regions of the device. Over this structure, a thin barrier layer 50 of a material such as TEOS (tetraethoxysilane) and a gettering layer 52 of a material such as BPSG (borophosphosilicate glass) are constructed. Finally, an organic-containing layer 54 is deposited conformally, preferably to a thickness which will adequately fill the space between adjacent polysilicon conductors. An inorganic dielectric layer 56 is deposited over the whole structure and planarized. A partially constructed source/drain contact hole 60 and polysilicon contact hole 58 are depicted to illustrate further advantages of this example. The gate contact would normally be made over the field oxide, although contacts to bipolar polysilicon emitters ar often made directly over the active device region. The etch stop property of organic-containing layer 54 enables the etch of the inorganic dielectric layer 56 to form contact holes of unequal length (as is typically the case due to the inherent device topography). For example, during etching the poly contact 58 will reach the organic-containing layer 54 well before source/drain contact hole 60 does, and yet the etching of poly contact 58 will stop instead of overetching deeply layer 54 (or even into the poly gate). A subsequent etch through substantially equal thicknesses of organic-containing layer 54 can then proceed to complete the contact holes. This example extends the method to illustrate another advantage, a reduction in capacitance between neighboring polysilicon conductors at the gate level. Figure 6 shows the structure of Figure 5, with via 58 and contact hole 60 etched through the organic-containing layer 54, the gettering layer 52, and the barrier layer 50 to expose the contact layer silicide 48. A conducting material (a Ti/TiN/AlCu alloy, for example) is deposited in the contact holes, electrically connecting the silicide contacts 48 to patterned conductors 18 which are formed overlaying inorganic dielectric layer 56. Figure 7 shows an example which illustrates how the various features and advantages described above may be employed on multiple layers of patterned conductors. On the left side of Figure 7 is a structure similar to that of Figure 6. On the right side, two additional polysilicon conductors 44 are shown on top of the field oxide 42, with a conformal organic-containing layer 54 substantially filling the space between the two conductors. The inorganic dielectric layer 56 is depicted with contacts of several different lengths passing through it. The patterned conductors 18 of an interconnection layer are shown overlaying the inorganic dielectric layer 56, with filled contacts connecting the patterned conductors 18 downwards to silicided contacts 48 and filled vias connecting conductors 18 upwards to a third layer of patterned conductors (not shown). The gaps between the patterned conductors 18 are substantially filled with an organic-containing dielectric layer 22, and capped with an inorganic interlayer dielectric 24. The following table provides an overview of some examples and the drawings. Drawing Element Preferred or Specific Examples Generic Term Other Alternate Examples 10 Silicon dioxide Interlayer dielectric Other oxides, P-glass, inorganic dielectric from previous application of invention 12,18 AlCu alloy with Ti and/or TiN underlayers Patterned conductors Al, Cu, Mo, W, Ti, and alloys of these Polysilicon, silicides, nitrides, carbides 14,26 Photoresist 22,34,54 Polymeric SOG (spun-on glass), polymers Organic-containing dielectric layer Allied Signal 500 Series, Teflon, parylene, polyimides 24,36,56 Silicon dioxide inorganic dielectric layer Other oxides, doped SiO 2 , P-glass, silicon nitride Particular examples should be paired with organic-containing examples to provide etchant selectivity 32 Porous oxide Low-k inorganic dielectric layer Porous silicides, other porous dielectrics 38 Silicon nitride Passivation layer Oxides, oxynitrides 40 Silicon Substrate Compound semiconductors (e.g. GaAs, InP, Si/Ge, SiC) Ceramic substrates 42 SiO 2 Field oxide Other oxides 44 Polycrystalline silicon Polysilicon Polycides, refractory metals 46 SiO 2 Gate side oxide Si 3 N 4 47 SiO 2 Gate oxide Oxynitrides 48 Titanium disilicide Contact layer Other silicides 50 TEOS Barrier layer oxides, nitrides, oxynitrides 52 BPSG Gettering layer PSG, BSG The invention is not to be construed as limited to the particular examples described herein, as these are to be regarded as illustrative, rather than restrictive. The invention is intended to cover all processes which do not depart from the scope of the invention. For example, it would be immediately obvious to one skilled in the art that the relative thicknesses and spacings depicted in the examples could be varied to enhance certain aspects of the invention, or that additional layers could be added by a similar process. Properties of some of the specific examples may be combined without deviating from the nature of the invention.

Claims: A method of fabricating vias for electrical connections between layers on semiconductor devices, said method comprising: forming a layer of patterned conductors (18) on a substrate (10); forming an organic-containing dielectric layer (32, 34) between at least two of said patterned conductors (18); depositing an inorganic dielectric layer (36) covering said organic-containing dielectric layer (32, 34) and any exposed portions of said conductors (18); etching a via (30) through said inorganic dielectric (36); and further etching said via (30) through said organic-containing dielectric (32, 34); said method being characterized in that said organic-containing layer (32, 34) is comprised of at least two sublayers, including an organic-containing sublayer (34) over an inorganic sublayer (32), said organic-containing layer (32, 34) having a dielectric constant of less than 3.0; said via (30) is etched through said inorganic dielectric (36) with an etch which is selective to said organic-containing sublayer (34); and that said step of further etching said via (30) through said organic-containing dielectric (32, 34) comprises etching said via (30) through said organic-containing sublayer (34). The method of claim 1, wherein said via (30) is etched through said organic-containing sublayer (34) with an etch which is selective to said inorganic-containing sublayer (32). The method of claim 1 or claim 2, further comprising providing said organic-containing dielectric layer (32, 34) having a thickness between 50 % and 150 % the thickness of said patterned conductors (18), as measured in the gap between said conductors (18), whereby said organic-containing sublayer (34) acts as an etch stop and prevents overetching due to mask misalignment or uneven device topography, and whereby the capacitance between adjacent pairs of said conductors (18) is reduced as compared to a silicon dioxide dielectric. The method of any preceding claim, further comprising providing said layer of patterned conductors (18) as an interconnection layer. The method of claim 4, wherein said interconnection layer is composed of materials selected from the group consisting of: aluminum, copper, titanium, platinum, gold, tungsten, polysilicon, tantalum, TiN, TiSi 2 , and combinations thereof. The method of any preceding claim, wherein said step of forming an organic-containing dielectric layer (32, 34) comprises depositing a first layer onto said device in a manner such that the final thickness of said first layer as measured in the gap between conductors (18) is between 105 % and 150 % the thickness of said patterned conductors (18). The method of any preceding claim, wherein said step of depositing said organic-containing sublayer (34) comprises spinning on a SOG containing 10 % to 100 % polymer by weight and annealing at a temperature of greater than 300 degrees C. The method of any preceding claim, further comprising providing said organic-containing sublayer (34) composing a material containing 10 % to 100 % polymer by weight. The method of claim 8, further comprising providing said inorganic dielectric layer (36) composing more than 95 % silicon dioxide, silicon nitride, or combinations thereof. The method of any preceding claim, further comprising forming one or more relatively thin passivation layers over said patterned conductors (18), prior to formation of said organic-containing dielectric layer (32, 34). The method of any preceding claim, further comprising forming said layer of patterned conductors as a polysilicon layer, said polysilicon layer also forming the top plate of gates on field effect transistors integrated into said semiconductor device. The method of claim 11, further comprising conformally depositing said organic-containing dielectric layer over said patterned conductors and said substrate. The method of claim 11 or 12, further comprising forming conductors in said vias to provide electrical contacts to said polysilicon layer.

Inventors: HAVEMANN ROBERT H, HAVEMANN, ROBERT H.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1995
Main classification: H01L21
IPC codes: H01L23, H01L21