Patent Application
20060065891
The invention relates to zener zap devices and, in particular, to a zener-zap device structure that is compatible with tungsten plug technology.
Trimming is a technique used to improve the accuracy and yield of precision integrated circuits. Trimming is usually performed after an integrated circuit has been fabricated and tested to modify or fine tune the performance of the integrated circuit. Zener zap diodes (or zener diodes) are devices that are often used for trimming of integrated circuits. In operation, the zener zap diodes are biased so that they behave as an open circuit as fabricated. When trimming is performed, the zener zap diode is zapped and the junction is short-circuited. Typically, the resistance across the diode reduces to about 10 Ω which is treated as equivalent to a “short circuit.” By shorting out selective zener zap diodes and thus the associated resistive elements, a desired change in resistance value is obtained.
In general, zener zap diodes are formed as a p-n junction of a heavily doped n+ diffusion and a moderately doped p-type diffusion. The doping level in the more lightly doped p-type diffusion usually determines the junction breakdown voltage. The higher the doping, the lower the breakdown voltage. For cost savings, zener zap diodes are usually constructed using existing layers and diffusions in the CMOS or bipolar fabrication process in which the diodes are to be incorporated. It is common to use the emitter-base junction of a standard NPN transistor device as the zener zap element.
To form the low resistive connection, zener zap diodes require the electromigration of a metal, usually aluminum, from anode to cathode of the zener zap diode, forming a metal filament. In mature technologies where the metallization contact structure allows the aluminum to make direct contact with the silicon, the metal filament created by the electromigration of aluminum in silicon is easily formed.
In technologies where aluminum spiking is a concern, a barrier metal consisting of some refractory metal, such as TiN or TiW, is used to prevent aluminum from directly contacting the silicon surface.
However, in deep submicron technologies (typically 0.5 um and below) aluminum cannot adequately cover the contact openings, and the industry has gone to the use of tungsten plugs. In the tungsten plug technology, the tungsten (W) completely fills the contact openings. Aluminum lines are formed on top of the contact openings to interconnect the contacts. Therefore, in the tungsten plug technology, the aluminum layer is formed far away from the surface of the silicon substrate. For instance, the height of a tungsten plug is typically a few thousand angstroms and thus the aluminum layer can be a few thousand angstroms away from the silicon surface.
The use of the tungsten plug technology in a fabrication process makes forming zener zap diodes almost impossible because zener zap diodes require the aluminum to be near the silicon surface so as to form a metal filament when zapped.
Another limiting feature of using tungsten plugs is that the tungsten plug process is optimized for a specific size for the contact opening so that all tungsten plug contacts formed on the wafer or the integrated circuit must have the same dimension. Specifically, the tungsten plug process is optimized so that the tungsten will consistently fill a contact cavity of a specific size. If the contact size is too small or too large, the contact cavity may not be filled adequately. Thus, in a tungsten plug process, the design rule requires all the metal contact to have a standard size or to be minimally sized and generally does not allow contact sizes to deviate from the standard size.
The typical tungsten plug process is as follows. The silicon wafer with the contact openings defined by a dielectric layer is subjected to a chemical vapor deposition process. The nucleation mechanics grow tungsten from the sides of the contact openings until the tungsten layer fills the cavity. A seam in the center area of the plug often results from the formation of the tungsten as the tungsten grows on the sides and merges to the center. The center seam sometimes can be observed in a scanning electronic microscope photograph of a cross-section of a W-plug contact. Such a center seam is not shown in the cross-sectional view of
It is desirable to form a zener zap diode in a fabrication process using tungsten plugs where the zener zap diode can programmed properly by the formation of a metal filament.
According to one embodiment of the present invention, a zener zap device is formed in a fabrication process using a tungsten plug process where the tungsten plug process dictates standard sized contact openings. The zener zap device includes a semiconductor layer, a first region of a first conductivity type formed in the semiconductor layer, a second region of a second conductivity type formed in the semiconductor layer, and a dielectric layer formed overlaying the top surface of the semiconductor layer. The dielectric layer has a first contact opening and a second contact opening positioned above and exposing portions of the first region and the second region respectively. The first contact opening is an enlarged contact opening having dimensions larger than the standard sized contact opening. The zener zap device further includes a first metal contact formed in the first enlarged contact opening where the first metal contact includes tungsten formed on the sidewall of the first enlarged contact opening and aluminum formed in electrical contact with the exposed surface of the first region.
In one embodiment, the zener zap device includes a second metal contact that is formed as a standard tungsten plug contact. Thus, the second metal contact includes a tungsten plug formed in the second contact opening and aluminum formed on the top surface of the tungsten plug and in electrical contact with the tungsten plug.
In another embodiment, the second contact opening of the zener zap device is a second enlarged contact opening and a second metal contact formed in the second enlarged contact opening includes tungsten formed on the sidewall of the second enlarged contact opening and aluminum formed in electrical contact with the exposed surface of the second region.
According to another aspect of the present invention, a Schottky diode is formed in a fabrication process using a tungsten plug process where the tungsten plug process dictates standard sized contact openings. The Schottky diode includes a semiconductor layer, a first region of a first conductivity type formed in the semiconductor layer where the first region is lightly doped, and a second region of the first conductivity type formed in electrical contact with the first region and is heavily doped. The Schottky diode further includes a dielectric layer formed overlaying the top surface of the semiconductor layer. The dielectric layer has a first contact opening and a second contact opening positioned above and exposing a portion of the first region and a portion of the second region respectively. The first contact opening is an enlarged contact opening having dimensions larger than the standard sized contact opening. The Schottky diode includes a first metal contact formed in the first enlarged contact opening where the first metal contact includes tungsten formed on the sidewall of the first enlarged contact opening and aluminum formed in electrical contact with the exposed surface of the first region. Finally, the Schottky diode includes a second metal contact formed in the second contact opening where the second metal contact includes a tungsten plug formed in the second contact opening and aluminum formed on the top surface of the tungsten plug and in electrical contact with the tungsten plug.
In one embodiment, the first region is an N-well region and the second region is an N+region formed in the N-well.
The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.
In accordance with the principles of the present invention, a zener zap diode formed in a fabrication process using a tungsten plug technology uses an enlarged contact opening to form the anode or the cathode or both of the zener zap diode. The use of an enlarged contact opening results in the formation of tungsten spacers along the sidewall of the contact opening and allows the overlying aluminum to reach down into the bottom of the contact opening to be near the silicon surface. A zener zap diode thus formed can be programmed properly by the formation of an aluminum filament between the anode and the cathode as the aluminum layer is either directly contacting the silicon substrate surface or separated from the silicon substrate by only a thin barrier metal layer.
To make electrical connection to the anode and the cathode of zener zap diode 20, contact openings 36 and 38 are formed in dielectric layer 34. In
After the contact openings are formed, a barrier metal layer 40 consisting of a refractory metal, such as TiN or TiW, is deposited on all exposed surfaces of the silicon structure. Then, the tungsten plug process begins. In the present embodiment, a two-step tungsten plug process is used and the process is generally referred to as a Deposition/Etch back in-situ process. First, a layer of tungsten 42 is deposited on top of barrier metal 40, such as by chemical vapor deposition (CVD). The nucleation mechanics causes the tungsten to grow on the sidewalls of the contact openings and on the top surface of the dielectric layer, as shown in
After the deposition of the tungsten layer, a blanket etch back process is carried out. The etch back process is sometimes carried out in-situ—that is, in the same process chamber as the deposition without breaking vacuum. The etch back process is typically a plasma etch process and is anisotropic. Barrier metal layer 40 acts as the etch-stop for the etch back process.
Next, an aluminum layer 44 is deposited on the silicon structure and patterned to form metal contacts 52, 54 and 56. Metal contact 56 is a standard tungsten plug contact where the aluminum layer is formed above the tungsten-filled contact opening. On the other hand, metal contacts 52 and 54 are formed from enlarged contact openings so that the tungsten layer forms only sidewall portions along the contact openings and the aluminum layer fills the cavity of the contact openings. Metal contacts 52 and 54 thus have aluminum that reached to the bottom of the contact openings where the aluminum is separated from the silicon surface only by the thin barrier metal layer.
Zener zap diode 20 thus formed, including enlarged metal contacts 52 and 54, is well suited for zapping as the aluminum layer is formed close to the silicon surface to allow the necessary metal filament to be formed when the zener zap diode is programmed by the application of the appropriate programming voltage and current.
In some fabrication process, a second barrier metal may be deposited prior to the aluminum deposition. Then, there may be two layers of barrier metal on the bottom of the enlarged contact openings before the aluminum is deposited. Such a zener zap diode can still be zapped to form a metal filament as the metal barrier layers can still breakdown under normal zapping conditions. Of course, the barrier metal is optional and the zener zap diode of the present invention can be formed using fabrication processes that do not employ any barrier metal layer at all.
In the embodiment shown in
Furthermore, in the embodiments shown in
In the above embodiments, the zener zap diode is described as being fabricated in a bipolar or BiCMOS process and the zener zap diode is formed in a P-base diffusion region. The zener zap diode of the present invention can be formed in other fabrication processes employing a tungsten-plug technology.
Furthermore, in the above-described embodiments, the silicon structure in which the zener zap diodes are formed includes an epitaxial layer. The use of epitaxial layer is illustrative only and may be omitted in fabrication processes not using an epitaxial layer. In that case, the zener zap diode will be formed in the substrate as with the other devices formed on the substrate.
In yet another alternate embodiment of the present invention, a degenerate zener zap diode can be formed by using P+anode region and N+cathode region that are contiguous or merged, as shown in
In the above descriptions, the tungsten plug process is described as being a deposition/etch-back process. The zener zap diode of the present invention can also be formed in a tungsten plug process that uses chemical mechanical polishing (CMP) to remove the tungsten instead of the etch-back process. When the tungsten process involves CMP, the tungsten in the enlarged contact openings can be removed by using a selective etch process after the CMP step is applied to remove the tungsten formed on the top surface of the dielectric layer. That is, the standard sized tungsten plugs can be masked by using a photoresist and the exposed enlarged sized contacts can then be subject to an anisotropic etch process to remove tungsten from the enlarged openings. Other process step variations are possible to realize the formation of aluminum contacts in a tungsten plug process for forming zener zap diodes in accordance with the present invention.
According to another aspect of the present invention, the enlarged sized contacts can be used advantageously in a tungsten-plug process for forming a Schottky barrier diode. As is well understood in the art, a Schottky barrier diode (or a “Schottky diode”) is formed by a metal-semiconductor junction. Typically, aluminum is used as the metal for the Schottky diode. When the fabrication uses a tungsten-plug process, a Schottky diode will have to be formed using tungsten as the metal.
When a fabrication process employs a tungsten plug technology, the formation of a Schottky diode in such a process faces the same challenge as the formation of a zener zap diode. That is, when tungsten plugs are used, the aluminum is no longer in close proximity to the silicon surface but rather is separated from the silicon surface by the height of the tungsten plug. Also, the contact size in a tungsten plug technology is fixed and minimally sized contacts must be used to ensure proper tungsten plug formation. The interface between tungsten and silicon does not form a satisfactory Schottky diode for various reasons. For instance, tungsten has higher resistance than aluminum and tungsten plugs require minimally sized contact opening. Thus, the overall resistance of a Schottky diode formed using tungsten plugs or an array of tungsten plugs can be very high. Therefore, it is often desirable to form an aluminum-silicon Schottky diode in a fabrication process that uses tungsten plug technology.
In accordance with the present invention, a Schottky diode formed in a fabrication process employing a tungsten plug technology uses an enlarged contact opening to form the anode of the diode so that an aluminum-semiconductor interface is realized. The use of an enlarged contact opening allows the aluminum overlying the tungsten to reach down into the bottom of the contact opening to be near the silicon surface. A Schottky diode thus formed provides the desired Schottky diode characteristics as the aluminum layer is either directly contacting the silicon substrate surface or separated from the silicon substrate by only a thin barrier metal layer.
In
The aluminum-silicon Schottky diode as formed in
The above detailed descriptions are provided to illustrate specific embodiments of the present invention and are not intended to be limiting. Numerous modifications and variations within the scope of the present invention are possible. The present invention is defined by the appended claims.
