Method for Forming a Ceramic Oxide Capacitor Having Barrier Layers

1. Field of the Invention

The present invention relates to integrated circuit memory elements, and more specifically, to fabrication of capacitors utilizing electrical ceramic oxides, such as ferroelectric materials, in such memory elements.

2. Discussion of the Prior Art

The use of ferroelectric materials to build memory cells is generally known. See U.S. Pat. No. 2,695,396 (Anderson). A ferroelectric material (FE) is "a crystalline substance that exhibits spontaneous electric polarization, electric hysteresis, and piezoelectricity." Webster's Seventh New Collegiate Dictionary, p.308 (1970).

In a typical ferroelectric cell application, capacitors are built having ferroelectric material sandwiched between a pair of electrodes. The ferroelectric polarization characteristic can be represented by a hysteresis loop (polarization vs. applied energy). Therefore, in a steady state condition (no applied energy) the capacitor will be polarized in one direction (representing, for example, a logical 0). The application of an electric field causes a small displacement in the ion orientation of the crystal structure, thus reversing the polarization of the crystal (logical 1).

Ferroelectric capacitors have been combined with integrated circuit technology to provide non-volatile memory devices. See U.S. Pat. No. 4,149,301 (Cook). For example, ferroelectric capacitors have been used in conjunction with dynamic random access memories (DRAM's) to store and refresh a volatile memory cell in a power interrupt mode, sometimes called a "shadow" RAM.

A variety of electrical ceramic oxides exist, such as those used as ferroelectric capacitors for integrated circuit memories (lead titanate, PbTiO.sub.3 ; lead zirconate titanate, "PZT" ; lanthanum doped PZT, "PLZT"; and barium titanate, BaTiO.sub.3). Electrical ceramic oxides are also used in electro-optical devices ("PLZT"; lithium niobate, LiNbO.sub.3 ; and bismuth titanate, Bi.sub.4 Ti.sub.3 O.sub.12) and high temperature superconductors (yttrium barium copper oxide, YBa.sub.2 Cu.sub.3 O.sub.7). The properties of these electrical ceramic oxides are typically optimized by heat treatments in oxidizing ambients at high temperatures (for example, 500.degree. C. to 1100.degree. C.). Many common materials are not suitable for use under such conditions. For example, aluminum melts or reacts with the electrical ceramic oxide material, while tungsten and molybdenum are destructively oxidized. Silicides and polysilicon either react with the electrical ceramic oxides at high temperature or are oxidized at the surface in contact with the electrical ceramic oxide. Silicon dioxide and silicon nitride may also react at these higher temperatures. For these reasons, it has been difficult to build rugged and reliable ferroelectric capacitors on a miniaturized scale appropriate for IC requirements. It has been recognized that homogeneity and uniformity of the ferroelectric region between capacitor plates is important to the ability of the capacitor to retain data (maintain polarization state) as the size of the device gets smaller.

Thus, it would be desirable if a reliable memory cell using a ferroelectric capacitor could be built in the course of integrated circuit fabrication, such that IC operation would be unaffected by the ferroelectric process.

The present invention provides a method for making an electrical ceramic oxide capacitor for use as a integrated circuit (IC) memory element. In its most general terms, the present invention describes a method for forming an electrical ceramic oxide capacitor over the top of an IC device (i.e., a transistor), wherein a protective diffusion barrier between the capacitor and the transistor prevents the process steps involved in the formation of the electrical ceramic oxide capacitor from altering the active regions of the already-formed transistor. The electrical ceramic oxide capacitor may be oriented either vertically or laterally.

After making the transistor and covering it with a diffusion barrier, metal contacts are formed through the barrier to contact the active regions of the transistor. The metal contacts have exposed sidewalls above the barrier which must also be protected from interaction with the electrical ceramic oxide material. Thus, a second barrier is formed to cover the exposed sidewalls. Electrical ceramic oxide material is then deposited and a final barrier is formed.

In the preferred embodiment, in which a vertical ferroelectric capacitor is formed over the IC, the reaction barrier comprises a barrier electrode formed on top of the metal plugs and spacers formed on the sidewalls of the metal plugs. The barrier electrodes above the first and second drain regions form the bottom electrodes of the capacitor. A layer of ferroelectric material is then deposited on top of the bottom electrodes and a top electrode is formed over the ferroelectric layer in the area above the bottom electrodes.

In the lateral ferroelectric capacitor embodiment, an additional metal plug is formed over the field oxide regions. As in the preferred embodiment, the reaction barrier comprises a barrier electrode formed on top of the metal plugs, and spacers formed on the sides of the metal plugs. However, in the lateral embodiment, the spacers form the capacitor electrodes. A layer of ferroelectric material is deposited on top of the barrier electrodes and between the spacers, and a top barrier is formed over the ferroelectric layer.

Other features and advantages of the present invention will be more fully understood and appreciated upon consideration of the following detailed description of the invention and accompanying drawings which set forth an illustrative embodiment in which the concepts of the invention are utilized.

FIGS. 1a and 1b are flow charts illustrating the process steps required to carry out the present invention.

FIGS. 2-4a are side plan views of the preferred embodiment of a vertical ferroelectric capacitor according to the present invention.

FIG. 4b is a top plan view of a particularly preferred construction of a vertical ferroelectric capacitor.

FIG. 4c is an electrical schematic corresponding to FIG. 4a.

FIG. 5 is a side plan view of an alternative embodiment of a vertical ferroelectric capacitor according to the present invention.

FIGS. 6-8 are side plan views of a third embodiment of a vertical ferroelectric capacitor according to the present invention.

FIGS. 9 and 10a are side plan views of a lateral ferroelectric capacitor according to the present invention.

FIG. 10b is a top plan view of FIG. 10a.

FIG. 10c is an electrical schematic of FIG. 10a.

FIGS. 1a and 1b show a standard process flow into which various process modules can be inserted to build ferroelectric capacitors in accordance with the present invention. It should be recognized, however, that ferroelectric material is merely one example from the class of materials which are termed electrical ceraminc oxides, and the present invention contemplates the formation of barriers for reactive electrical ceramic oxides in general, rather than being limited only to ferroelectrics. Thus, for all embodiments of the invention described below, steps 10-16 are performed. Then, the steps in either module A, module B, or module C may be performed to obtain one of desired ferroelectric capacitor structures. Finally, steps 26-46 are performed to complete the process.

The process indicated in steps 10-16 is described with reference to FIG. 2, where a portion of a typical MOS memory device 110 showing two n-channel transistors sharing a common drain and formed between field oxide regions 112 is illustrated. A p-type semiconductor substrate 108 has a first n-type drain region 114, a common n-type source region 116, and a second n-type drain region 118 formed therein by conventional techniques. The substrate is then covered by a thin gate oxide layer 120. A pair of polysilicon gates 124, 126 are formed by depositing polysilicon over the gate oxide 120, patterning the polysilicon, and covering the polysilicon with a dielectric layer 122, such as silicon dioxide.

Since the steps required to obtain the MOS structure 110 of FIG. 2 are conventional, they need not be discussed in detail herein. It is the remainder of the process steps that define the present invention. It is to be recognized that the present invention is not limited to MOS architectures, but has applicability to integrated circuits in general. It should be noted, however, that gallium arsenide constructions would be more difficult to obtain because of the high temperature steps required to fabricate a ferroelectric capacitor.

With continuing reference to FIG. 2, according to the present invention, after formation of the silicon dioxide layer 122, titanium dioxide barrier regions 128 are formed by depositing approximately 1000 .ANG. to 2000 .ANG. of titanium over the dielectric layer 122 at a low temperature (less than 400.degree. C.) and oxidizing the titanium at 450.degree. C. to 850.degree. C. for thirty minutes. As will be described in greater detail below, the barrier regions 128 protect the underlying MOS structure 110 from being altered by subsequent process steps. It may be necessary to deposit a layer of silicon nitride 128a in the range of 500 .ANG. to 2000 .ANG. before the titanium to ensure that barrier regions 128 are truly impervious to the subsequent process steps. The titanium/silicon nitride is then masked and etched to define contact windows 130, 132, 134 to the first drain 114, common source 116, and second drain 118, respectively. A layer 139 of refractory metal is then deposited by known methods in order to fill the gaps between the contacts. The refractory metal must be capable of surviving the subsequent high temperature steps intact. Tungsten is preferred. It may be necessary to make the metal layer 139 thicker than would be required for a standard process without the ferroelectric process to allow for additional planarization of the metal.

In accordance with the preferred embodiment of the present invention, a vertical ferroelectric capacitor is made over the above-described structure by following the steps outlined in module B of FIG. 1a.

Referring to FIG. 1a and to FIG. 3, to prevent the refractory metal from interacting with the ferroelectric material which is deposited in a subsequent step, a reaction barrier 138 is formed by sputtering ruthenium over the refractory metal layer 139. A layer of titanium may be used as a hard mask to pattern the ruthenium. The titanium is masked and etched. Then the reaction barrier 138 is sputter etched to define the ruthenium barrier electrodes 138a, 138b, 138c, 138d. The tungsten is then etched through a hard mask, and the barrier regions 128 serve as an etch stop, such that contact plugs 136a, 136b, 136c, 136d are formed having barrier electrodes 138a, 138b, 138c, 138d on top of metal contacts 139a, 139b, 139c, 139d. The titanium hard mask is stripped before the ferroelectric is deposited. The barrier electrodes 138a, 138c above drain regions 114, 118 comprise the bottom electrodes of two vertical ferroelectric capacitors. It should be noted that contact plug 136 d corresponds to contact plug 136a in that symmetry of construction allows two series capacitors to be conveniently built above each field oxide region.

It is necessary to protect the exposed sides of contact plugs 136a, 136b, 136c, 136d to prevent the exposed refractory metal from reacting with the ferroelectric material. Thus, spacers 146, 148, 150 are formed by depositing titanium over the plugs 136a, 136b, 136c. Alternatively, it may be possible to reactively sputter titanium dioxide to form the spacers 146, 148, 150. The titanium is then etched anisotropically to remove it from the top of the plugs 136a, 136b, 136c and from the top of the barrier regions 128 leaving spacers 146, 148, 150 only on the sides of the plugs 136a, 136b, 136c to protect the refractory metal. This is a critical step, since any residue from the spacer etch may affect the interface between the bottom electrodes 138a, 138c and the ferroelectric material.

The process flow now leaves module B of FIG. 1a and returns to the standard process to perform steps 26 through 46, as shown in FIG. 1b. As shown in FIG. 4a, after spacers 146, 148, 150 have been formed, a ferroelectric material 152, such as PZT, is deposited via a spin-on process or by chemical vapor deposition. The device is then sintered by furnace or rapid thermal processing at temperatures of 450.degree. C.-850.degree. C. During the sinter step, the ruthenium and the titanium are oxidized, but the tungsten is not affected. Thus, the spacers 146, 148, 150 are converted to titanium dioxide. The top electrodes 154a, 154b are formed by depositing a layer of ruthenium over the contact plugs 139a, 139c, 139d and patterned by using a titanium hard mask.

The top electrodes 154a, 154b are then masked and etched. The exposed ferroelectric material 152 must also be etched. The target dimension for the ferroelectric etch is for 1 micron holes with a 0.3 micron undercut.

In order to prevent an oxidation reaction between the ruthenium and the aluminum contact 156, second top electrodes 158a, 158b are formed over the top of the first electrodes 154a, 154b. The second top electrodes 158a, 158b may be a single layer of ruthenium, or aluminum over ruthenium, or titanium over aluminum over ruthenium. The thickness depends on whether or not this layer is to be used by the designer as an extra interconnect layer to carry large currents. For large currents, the aluminum would be used, making later planarization more difficult. If the extra interconnect layer is not needed, only a thin layer of ruthenium is needed, which will make it easier to pattern the layer and cover it later with dielectric. The second top electrodes 154a, 154b must be etched to clear out the valleys between plugs 139a, 139b, 139c.

A second dielectric layer 160 is deposited and planarized. Only a small class of oxides are effective over ferroelectric materials, such as APCVD or low temperature vacuum processes like ECR deposition. Generally, the profile of the first dielectric layer 122 is reproduced, but additional deposition and planarization may be required. The second dielectric layer 122 is masked and etched to create a tapered via for the deposit of metal contact 156, typically aluminum.

To complete the device, a passivation layer is deposited over the entire device, then masked and etched to provide for external connection to the device, in accordance with conventional procedures therefor.

In a particularly preferred construction, shown in the top plan view of FIG. 4b, two parallel circuit features are formed which represent, for example, bit 0 and bit 1 lines in a typical memory configuration. The corresponding electrical schematic for one of the bit lines is shown in FIG. 4c. Contact to the bit lines is provided at points 156a, 156b. Due to symmetry surrounding the bit lines, two capacitors 160a and 160b share a common bit line, bit 0, and two capacitors 161a, 161b share a common bit line, bit 1. Contact to the top electrode of any of the capacitors is at point 163. Contact to the bottom electrode of each capacitor 160a, 160b, 161a, 161b is made at points 165, 167, 169, 171 respectively. It can be seen that additional memory elements may be constructed in similar fashion along both the x- and z-axes.

An alternative embodiment of a vertical ferroelectric capacitor can be made according to the steps outlined in process module A of FIG. 1a.

Referring to FIG. 5, in this embodiment, process steps 10-16 are as described above. However, the metal layer is masked and etched to define the contact plugs 139a, 139b, 139c before the reaction barrier 138 is deposited. The reaction barrier 138 is formed by depositing ruthenium, then masking and etching to create a 0.1 micron overlap of the metal region, thus adding 0.3 microns in the bit line direction, and 0.2 microns along the drive line for alignment tolerances. The remainder of the process steps are as described above with reference to the first embodiment of the invention.

The advantage of the second embodiment is that no spacers are used, thus all processing steps are known and conventional. The hazard associated with forming the device according to the second embodiment is that thinning will occur at the edges of the metal deposit step. A spacer could be incorporated to help avoid thinning, and in this geometry, a spacer would not be as critical for use as a barrier as in the first embodiment. The use of a spacer would, however, take up additional space on the chip.

A third embodiment of a vertical ferroelectric capacitor can be made according to the process steps in module C of FIG. 1a. Again, process steps 10-16 are as described above.

Referring to FIGS. 6-8, the deposited tungsten is masked and etched to create metal contacts 139a, 139b, 139c, 139d as in the second embodiment. Next, an oxide layer 162 is deposited over the tungsten and planarized using well known techniques. The resulting surface is smoothed. An anisotropic oxide etch is then used to reduce the oxide layer 162 below the surface of the tungsten.

A barrier layer 164 is then formed. Preferably, titanium dioxide is reactively sputtered onto the device, but other oxides may be used if they provide adequate electrical isolation. The metal contacts 139a, 139b, 139c, 139d are again planarized and polished to produce a smooth surface with exposed refractory metal on top of metal contacts 139a, 139b, 139c, 139d. The barrier or bottom electrodes 166a, 166b, 166c are then formed by depositing ruthenium and etching such that a 0.1 micron overhang of the metal contacts 139a, 139b, 139c, 139d is created. The remaining process steps are as previously described.

The advantage of the third embodiment is that no spacer is required. The primary disadvantage of the third embodiment is that additional non-uniformities may be introduced to the process as a result of several planarization and etchback steps, thus increasing the cost while decreasing the yield.

According to the present invention, a lateral ferroelectric capacitor may also be constructed by using different masks in the process flow of module B.

Through step 18, the present embodiment is identical to that of the preferred embodiment. However, with reference to FIG. 9, the metal layer 139 and the reaction barrier 138 are etched to form contact plugs 168a, 168b, 168c, which correspond to plugs 139a, 139b, 139c in the previous vertical embodiments, and an additional contact plug 168d. Note that contact plug 168d is oriented above the field oxide region 112. The process follows module B except that the spacers 172b, 172e, 172d must be made of an electrode material such as ruthenium. A ferroelectric material is then deposited by a known sol-gel technique in order to fill the gaps between the electrode cladded metal plugs 168a, 168c, 168d and leave a thin layer on top of the plugs. The thin layer of ferroelectric material is more easily removed in this structure than in the vertical structures. An additional barrier layer 170 is not necessary, but can be patterned with the same mask as the ferroelectric material and would provide additional isolation of the ferroelectric capacitors. The process continues from here with step 40.

The resulting structure is shown in FIG. 10a, and a particularly preferred two-bit line construction is shown in top plan view in 10b. The corresponding electrical schematic for one of the bit lines is shown in FIG. 10c. As in the vertical configuration, polysilicon gates 124, 126 are the word lines. The aluminum 156 contacts the common source 116, which acts as one of the bit lines.

It should be understood that the present invention is not intended to be limited by the specifics of the above-described embodiments. The scope of the invention is defined by the accompanying claims.