1. Field of the Invention
The present invention relates to the field of fabrication of semiconductor devices. More specifically, the invention relates to the fabrication of silicon-germanium semiconductor devices.
2. Related Art
In a heterojunction bipolar transistor (xe2x80x9cHBTxe2x80x9d), a thin silicon-germanium layer is grown as the base of a bipolar transistor on a silicon wafer. The silicon-germanium HBT has significant advantages in speed, frequency response, and gain when compared to a conventional silicon bipolar transistor. Speed and frequency response can be compared by the cutoff frequency which, simply stated, is the frequency where the gain of a transistor is drastically reduced. Cutoff frequencies in excess of 100 GHz have been achieved for the HBT, which are comparable to the more expensive GaAs. Previously, silicon-only devices have not been competitive for use where very high speed and frequency response are required.
The higher gain, speeds, and frequency response of the HBT have been achieved as a result of certain advantages of silicon-germanium not available with pure silicon, for example, narrower band gap, and reduced resistivity. In addition, silicon-germanium may be epitaxially grown on silicon wafers using conventional silicon processing and tools, and allows one to engineer device properties such as the band gap, energy band structure, and mobilities. For example, it is known in the art that grading the concentration of germanium in the silicon-germanium base builds into the HBT device an electric field, which accelerates the carriers across the base, thereby increasing the speed of the HBT device compared to a silicon-only device. One method for fabricating silicon and silicon-germanium devices is by chemical vapor deposition (xe2x80x9cCVDxe2x80x9d). A reduced pressure chemical vapor deposition technique, or RPCVD, used to fabricate the HBT device allows for a controlled grading of germanium concentration across the base layer.
Because the benefits of a high gain and high speed silicon-germanium HBT device can be either partially or completely negated by high base contact resistance, it is important that the resistance of the base contact be kept low. In addition to the contact resistance, the geometry of the base regions may also affect the base resistance. The geometry of the base region may necessitate providing a low resistance electrical pathway through a portion of the base itself between the base contact and the base-emitter junction, referred to as the extrinsic base region. The extrinsic base region is heavily doped by implantation (also called extrinsic doping) in order to provide reduced resistance from the base contact to the base-emitter junction.
During the manufacture of an integrated circuit chip there are many processing steps which involve heating the wafer in which the integrated circuit chip is included. It is normal for dopants to diffuse out from where they have been implanted into surrounding areas of the chip during these heating processes. Typically, the out diffusion of dopants is accounted for in the design of a circuit device such as the HBT. Unwanted out diffusion can have disadvantageous effects, however, especially under certain circumstances. For example, there is drive in current technology to operate the HBT at lower voltages and comparatively higher collector currents for those low voltages. When the HBT is operated in a range of low voltage and high collector current, the effects of an energy barrier at the metallurgical transition from silicon-germanium to silicon near the base-collector junction can become more pronounced. Such an operating range can be characterized, for example, by a collector-emitter voltage in the range of approximately 1.0 to 4.0 volts and a collector current in the range of approximately 0 to 3.0 milliamperes (xe2x80x9cmAxe2x80x9d). Under these operating conditions, an energy barrier at the metallurgical transition from silicon-germanium to silicon near the base-collector junction has an effect of limiting current flow through the collector.
Out diffusion of dopants from the heavily doped extrinsic base region acts to further restrict collector current flow under these conditions and there are other deleterious effects on the operating characteristics and device parameters of the HBT device. For example, the operating range over which the HBT can operate linearly as class A amplifier is reduced. Briefly stated, a device operates as a class A amplifier if output current flows for all values of the input, as opposed to, for example, class B operation in which output current flows for one-half the cycle of the input waveform. Linear operation, simply stated, is the amplification of an input signal without distortion. A wider operating range for linear class A operation, i.e. one in which the maximum and minimum voltages and currents of the device are spaced further apart, is desirable because design flexibility and reliability are increased. As another example, power output in class A operation can be reduced because the reduced collector current directly reduces power which, simply stated is the product of current times voltage.
As a further example, out diffusion of dopants from the heavily doped extrinsic base region may increase a parasitic capacitance between the base and collector. Briefly, capacitance in an electric circuit relates to an effective flow of current due to the storage of electric charge between two otherwise electrically separated conductors. Parasitic capacitance between the base and collector effectively represents a near-short circuit in the HBT for a high frequency signal being amplified and is, thus, undesirable.
Also, for example, out diffusion of dopants from the heavily doped extrinsic base region can reduce a breakdown voltage of the HBT device. Briefly, the presence of a voltage, greater than the breakdown voltage, between the base region and a conductive region below the collector can cause the intervening material, which physically and electrically separates the two, to start to conduct electricity, known as xe2x80x9cbreakdownxe2x80x9d of the intervening material. When breakdown occurs the HBT device no longer functions as intended, and can be permanently damaged. Thus, it is undesirable for breakdown voltage to be reduced.
Moreover, the effects of out diffusion on a device can limit the scalability of the device. Scalability, simply stated, refers to preserving the relative proportions of the various features of a device in such a way that the device still functions when the overall size of the entire device is reduced. As feature sizes of bipolar devices are reduced, it is important to achieve accurate control over the size of the various features in order to keep feature sizes in proportion. So for example, if out diffusion is not properly controlled the relative size of the out diffusion regions increases as the size of the entire bipolar device is reduced. An increase in the relative size of the out diffusion regions exacerbates the problems and disadvantages described above. Thus, the effects of out diffusion on the device can limit the scalability of the bipolar device. Furthermore, as feature size of CMOS devices is reduced it is important to achieve a concomitant reduction of feature size in bipolar devices on the same chip as CMOS devices.
One approach to the problem has been to use carbon in conjunction with the implant doping of the extrinsic base regions as a xe2x80x9csuppressantxe2x80x9d to control the amount of subsequent out diffusion of dopants from the extrinsic base regions. In general, the use of carbon for control of diffusion is complicated to implement from a technological viewpoint, is not generally available in the industry, and can require expensive tooling or retooling of the fabrication facility.
Thus, there is a need in the art to control out diffusion of dopants in bipolar devices. There is also need in the art for technologically simple, relatively low cost, readily available control of out diffusion of dopants in bipolar devices. There is a further need in the art for fabrication of bipolar devices which is scalable as the size of MOS and CMOS devices decreases.
The present invention is directed to a high performance bipolar transistor. The invention is used to control out diffusion of dopants in bipolar devices. The invention overcomes the need in the art for technologically simple, relatively low cost, readily available control of out diffusion of dopants in bipolar devices. The invention also provides fabrication of bipolar devices which is scalable to the size of MOS and CMOS devices as the size of MOS and CMOS devices decreases.
In one aspect of the invention a collector is deposited and a base is grown on the collector. For example, the base can be grown by epitaxially depositing either silicon or silicon-germanium on the collector. An emitter is then fabricated on the base followed by implant doping an extrinsic base region outside the emitter. For example, the extrinsic base region can be implant doped using boron with an implant dose of approximately 1015 atoms per square centimeter. The extrinsic base region doping diffuses out during subsequent thermal processing steps in chip fabrication, creating an out diffusion region in the device. The out diffusion region can adversely affect various operating characteristics of the device, such as parasitic capacitance and linearity. The out diffusion is controlled by counter doping the out diffusion region. For example, the counter doped region can be implant doped using arsenic or phosphorous with an implant dose of approximately 1013 atoms per square centimeter. Also, for example, the counter doped region can be formed using tilt implanting or, alternatively, by implant doping the counter doped region and then forming a spacer on the base prior to implanting the extrinsic base region.