1. Field of the Invention
This invention relates to the field of semiconductor processing and more particularly to a method of fabricating asymmetric transistors by patterning the transistor gate after the formation of the source/drain region.
2. Description of the Relevant Art
Fabrication of a MOSFET device is well known. Generally speaking, MOSFETs are manufactured by depositing polysilicon over a relatively thin gate oxide. The polysilicon material is then patterned to form a gate conductor with source/drain regions adjacent to and on opposite sides of the gate conductor. The gate conductor and source/drain regions are then implanted with an impurity dopant material. If the impurity dopant material used for forming the source/drain regions is n-type, then the resulting MOSFET is an NMOSFET ("n-channel") transistor device. Conversely, if the source/drain dopant material is p-tiype, then the resulting MOSFET is a PMOSFET ("p-channel") transistor device.
The gate conductor and adjacent source/drain regions are formed using well known photolithography techniques. Gate conductors and source/drain regions are located in openings formed through a thick layer of what is commonly referred to as field oxide. Those openings and the transistors formed therein are termed active regions. Conductive interconnects are routed over the field oxide to couple with the polysilicon gate conductor as well as with the source/drain regions to complete the formation of an overall circuit structure.
Integrated circuits utilize either n-channel devices exclusively, p-channel devices exclusively, or a combination of both on a monolithic substrate. While both types of devices can be formed, the devices are distinguishable based on the source/drain impurity dopant. The method by which n-type dopant is used to form an n-channel device and p-type dopant is used to form a p-channel device entails unique problems associated with each device. As layout densities increase, the problems are exacerbated. Device failure can occur unless adjustments are made to processing parameters and processing steps. N-channel processing must, in most instances, be dissimilar from p-channel processing due to the unique problems of each type of device. Problems inherent in n-channel fabrication will be discussed first followed by p-channel second.
N-channel devices are particularly sensitive to so-called short-channel effects ("SCE"). The distance between source and drain regions is often referred to as the physical channel length. However, after implantation and subsequent diffusion of the source and drains, distance between the source and drains regions become less than the physical channel length and is often referred to as the effective channel length ("Leff"). In VLSI designs, as the physical channel becomes small, so too must the Leff. SCE becomes a predominant problem whenever Leff drops below approximately 2.0 .mu.m.
Generally speaking, SCE impacts device operation by, inter alia, reducing device threshold voltages and increasing sub-threshold currents. As Leff becomes quite small, the depletion regions associated with the source and drain areas may extend toward one another and substantially occupy the channel area. Hence, some of the channel will be partially depleted without any influence of gate voltage. As a result, less gate charge is required to invert the channel of a transistor having a short Leff. Somewhat related to threshold voltage lowering is the concept of sub-threshold current flow. Even at times when the gate voltage is below the threshold amount, current between the source and drain nonetheless exist for transistors having a relatively short Leff.
Two of the primary causes of increased sub-threshold current are: (i) punch through and (ii) drain-induced barrier lowering ("DIBL"). Punch through results from the widening of the drain depletion region when a reverse-bias voltage is applied across the drain-well junction. The electric field of the drain may eventually penetrate to the source area, thereby reducing the potential energy barrier of the source-to-body junction. Punch through current is therefore associated within the substrate bulk material, well below the substrate surface. Contrary to punch through current, DIBL-induced current occurs mostly at the substrate surface. Application of a drain voltage can cause the surface potential to be lowered, resulting in a lowered potential energy barrier at the surface and causing the sub-threshold current in the channel near the silicon-silicon dioxide interface to be increased. One method in which to control SCE is to increase the dopant concentration within the body of the device. Unfortunately, increasing dopant within the body deleteriously increases potential gradients in the resulting device.
Increasing the potential gradients produces an additional effect known as hot-carrier effect ("HCI"). HCI is a phenomena by which the kinetic energy of the carriers (holes or electrons) is increased as they are accelerated through large potential gradients and subsequently become trapped within the gate oxide. The greatest potential gradient, often referred to as the maximum electric field ("Em") occurs near the drain during saturated operation. More specifically, the electric field is predominant at the lateral junction of the drain adjacent the channel.
Using the n-channel example, the electric field at the drain causes channel electrons to gain kinetic energy. Electron-electron scattering randomizes the kinetic energy and the electrons become "hot". Some of these hot electrons have enough energy to create electron-hole pairs through impact ionization of the silicon atoms. Electrons generated by impact ionization join the flow of channel electrons, while the holes flow into the bulk to produce a substrate current in the device. The substrate current is the first indication of the creation of hot carriers in a device. For p-channel devices, the fundamentals of the process are essentially the same except that the role of holes and electrons are reversed.
HCI occurs when some of the hot carriers are injected into the gate oxide near the drain junction, where they induce damage and become trapped. Traps within the gate oxide generally become electron traps, even if they are initially filled with holes. As a result, there is a negative charge density in the gate oxide. The trapped charge accumulates with time, resulting in positive threshold shifts in both n-channel and p-channel devices. It is know that since hot electrons are more mobile than hot holes. HCI causes a greater threshold skew in n-channel devices than p-channel devices.
Unless modifications are made to the transistor structure, problems of sub-threshold current and threshold shift resulting from SCE and HCI will remain. To overcome these problems, alternative drain structures such as double-diffused drains (DDDs) and lightly doped drains (LDDs) must be used. The purpose of both types of structures is the same: to absorb some of the potential into the drain and thus reduce Em. The popularity of DDD structures has given way to LDD structures since DDD causes unacceptably deep junctions and deleterious junction capacitance.
A conventional LDD structure is one whereby a light concentration of dopant is self-aligned to the gate electrode followed by a heavier dopant self-aligned to the gate electrode on which two sidewall spacers have been formed. The purpose of the first implant dose is to produce a lightly doped section of both the source and drain areas at the gate edge near the channel. The second implant dose is spaced from the channel a distance dictated by the thickness of the sidewall spacer and results in a dopant gradient occurs at the junction between the source and channel as well as the junction between the drain and channel.
A properly defined LDD structure must be one which minimizes HCI but not at the expense of excessive source/drain resistance. The addition of an LDD implant adjacent the channel unfortunately adds resistance to the source/drain path. This added resistance, generally known as parasitic resistance, has deleterious effects including. First, parasitic resistance can decrease the saturation current (i.e., current above threshold). Second, parasitic resistance can decrease the overall speed of the transistor.
The deleterious effects of decreasing saturation current and transistor speed is best explained in reference to a transistor having a source resistance and a drain resistance. The source and drain parasitic resistances are compounded by the presence of the conventional source and drain LDDs. Using a n-channel example, the drain resistance R.sub.D causes the gate edge near the drain to "see" a voltage less than VDD, to which the drain is typically connected. Similarly, the source resistance R.sub.S causes the gate edge near the source to see some voltage more than ground. As far as the transistor is concerned, its drive current along the source-drain path depends mostly on the voltage applied between the gate and source, i.e., V.sub.GS. If V.sub.GS exceeds the threshold amount, the transistor will go into saturation according to the following relation: EQU I.sub.DSAT =K/2*(V.sub.GS -V.sub.T).sup.2
where I.sub.DSAT is saturation current, K is a value derived as a function of the process parameters used in producing the transistor, and V.sub.T is the threshold voltage. Reducing or eliminating R.sub.S would therefore draw the source voltage closer to ground, and thereby increasing the effective V.sub.GS. From the above equation, it can be seen that increasing V.sub.GS directly increases I.sub.DSAT. While it would seem beneficial to decrease R.sub.D as well, R.sub.D is nonetheless needed to maintain HCI control. Accordingly, substantial LDD is required in the drain area. It would therefore seem beneficial to decrease R.sub.S rather than R.sub.D. This implies the need for a process for decreasing R.sub.S (source-side LDD area) while maintaining R.sub.D (drain-side LDD area).
Proper LDD design must take into account the need for minimizing parasitic resistance R.sub.S at the source side while at the same time attenuating Em at the drain-side of the channel. Further, proper LDD design requires that the injection position associated with the maximum electric field Em be located under the gate conductor edge, preferably well below the silicon surface. It is therefore desirable to derive an LDD design which can achieve the aforesaid benefits while still properly placing and diffusing Em. This mandates that the channel-side lateral edge of the LDD area be well below the edge of the gate. Regardless of the LDD structure chosen, the ensuing transistor must be one which is not prone to excessive sub-threshold currents, even when the Leff is less than, e.g., 0.5 .mu.m.
A properly designed LDD-embodied transistor which overcomes the above problems must therefore be applicable to either an n-channel transistor or a p-channel transistor. That transistor must be one which is readily fabricated within existing process technologies. In accordance with many modern fabrication techniques, it would be desirable that the improved transistor be formed having a net impurity concentration within the polysilicon gate of the same type as the LDD implant area and/or source/drain area.