1. Field of the Invention
The present invention relates generally to semiconductor circuits and, more particularly, to semiconductor structures for protecting transistor gate oxides during plasma etch fabrication operations.
2. Description of the Related Art
As is well known in the art, semiconductor devices are fabricated over a semiconductor substrate that is subjected to numerous processing operations. By way of example, a semiconductor device is typically subjected to several plasma etching operations, which are designed to pattern the various substrate, oxide and metallization layers and construct the desired circuit layout. Although plasma etching has become the etching process of choice, the intense energies used to create the etching plasma has had a degrading effect on thin sensitive gate oxides that lie under gate electrodes of a given circuit design. Specifically, the plasma that is generated in etching chambers is designed to bombard a layer being etched with a high concentration of electrons and positively charged ions. Unfortunately, these electrons and positively charged ions are known to induce intense currents through the gate oxides, which necessarily produce oxide degrading traps in the gate oxides.
With this in mind, FIG. 1A shows a cross-sectional view of a semiconductor substrate 100 during a plasma etch 102 operation. In this example, the semiconductor substrate 100 is patterned with a photoresist mask 108, which enables patterning of a polysilicon gate 106 that lies over a gate oxide 104. As mentioned above, when the plasma etch operation is performed, a large amount of positively charged ions and negatively charged electrons are caused to come into contact with exposed surface areas of the polysilicon gate 106. When this happens, strong electrical currents are caused to flow between the polysilicon gate 106, through the gate oxide 104, and into the substrate 100.
Unfortunately, during normal plasma etch operations, these electrical currents can become quite substantial, and therefore may cause what are known as "traps" inside the gate oxide 104. These traps that are formed inside the chemical bonds of the gate oxide 104 therefore detrimentally degrade the gate oxide 104, which can lead to gate current leakage. For example, properly functioning transistor devices require gate oxides 104 that adequately isolate the polysilicon gate 106 from the substrate 100. However, when the gate oxide 104 accumulates a large amount of trap charging, the degraded gate oxide 104 may no longer insulate the polysilicon gate 106 from the semiconductor substrate 100 and leakage currents will occur through the gate oxide 104.
FIG. 1B shows a more detailed diagram of the polysilicon gate 106 that includes a silicided metallization layer 106a. The polysilicon gate 106 is now shown in contact with a degraded gate oxide 104'. On either side of the polysilicon gate 106 and the degraded gate oxide 104', are oxide spacers 109. The oxide spacers 109 sit partially over the diffusion regions 112. In a properly functioning transistor gate, the gate oxide is supposed to electrically isolate the polysilicon gate 106 from the substrate 100. However, when the degraded gate oxide 104' builds up a large amount of chemical bond altering traps due to the repeated electrical exposure to plasma etch operations, the degraded gate oxide 104' will no longer perform its isolation function.
As shown, a channel region 110 is formed between the diffusion regions 112 in the semiconductor substrate 100. In a properly functioning device, the transistor having the polysilicon gate 106 can control the channel region 110 to be in an ON state or OFF state depending on the voltage applied to the polysilicon gate 106. However, when the degraded gate oxide 104' no longer isolates the polysilicon gate 106 from the substrate 100, the polysilicon gate loses control over the channel region 110. As can be appreciated, when this happens, the semiconductor device will fail to operate in its designed manner. As a result, not only will a single transistor fail to operate for its intended purpose, but an entire semiconductor chip may fail to operate properly and perform its desired functional operations.
It should also be appreciated that the gate oxide 104 is susceptible to degradation during each stage that a plasma etching operation is performed, and electrical conduction between the plasma etching and the gate oxide 104 exists. By way of example, the oxide spacers 109 are generally formed by depositing an oxide layer over the entire surface of a wafer, and then subsequently performing a plasma etch until this oxide spacers 109 remain. However, when such oxide spacer etching is performed, the polysilicon gate 106 will come into electrical contact with the plasma etching that is used to perform the oxide spacer formation.
Thus, additional plasma induced currents "I" will unfortunately cause further trap formation in the degraded gate oxide 104'. Additionally, when subsequent via hole etching operations and metallization interconnect patterning operations are performed, that plasma etching will also come into electrical contact with the polysilicon gate 106, which will then conduct additional current through the degraded gate oxide 104', thereby producing additional traps in the degraded gate oxide 104'.
As shown in FIG. 1C, when a via hole is plasma etched in an intermetal dielectric 116, the plasma ions and electrons will also come into contact with the silicided metallization layer 106a that lies over the polysilicon gate 106. As a result, oxide destructive currents will also be caused to occur through the degraded gate oxide 104'. Additionally, when a patterned metallization line 118 is formed over and in contact with an electrical tungsten plug 117, the ions and electrons produced by the plasma patterning will again cause currents to flow through the degraded gate oxide 104'.
In prior art attempts to divert plasma induced currents away from the gate oxide 104, diode structures have been fabricated beside sensitive transistor gate structures. As shown in FIG. 1D, the diode structure includes an N+ well 150 which interfaces with a P-type substrate to form a solid state diode structure. The diode structure is a leaky diode structure which allows a current I.sub.D to flow away from the gate oxide 104 and down a via 117a during plasma etching.
Unfortunately, fabricating these diode structures beside sensitive transistor devices has the downside of requiring a substantial amount of additional chip space. This is because such diode designs are typically only fabricated to suppress large currents produced when transistor gates have large contact surfaces 106b as shown in FIG. 1E. These large contact surfaces 106b are known to cause "antenna effects," which also detrimentally produce very large oxide destructive currents "I".
Furthermore, the diode structure of FIG. 1D will not assist in protecting the gate oxide 104 until interconnect layers, such as the patterned metallization layer 118' is formed. That is, the diode structure will not protect the gate oxide 104' during the polysilicon gate 106 patterning, during the spacer 109 formation, and when a via hole is etched through the intermetal oxide 116 in order to form a conductive via 117. Accordingly, by the time a protection diode structure is formed beside a sensitive transistor device, the gate oxide 104 will already have been exposed to a substantial amount of destructive current induced by the plasma patterning operations.
In view of the foregoing, there is a need for compact protection device structures which assist in protecting gate oxides from the formation of traps and subsequent oxide degradation. There is also a need for protecting the gate oxides in order to prevent leakage currents and malfunctioning transistor structures.