1. Field of the Invention
The present invention relates to a method and apparatus for characterizing a specimen of semiconductor material and, more particularly, to a method and apparatus determining parameters such as the doping concentration profile of a specimen of semiconductor material.
2. Description of Prior Art
Various methods for measurements of semiconductor surface and semiconductor/insulator interface parameters have been described. They can be classified into destructive and non-destructive. Examples of destructive methods are the secondary ion mass spectrometry and the four-point sensor method.
Non-destructive methods can be further classified into those requiring post-processing and those that don't (post-processing is defined as additional processing steps beyond those that define properties of semiconductor surface or semiconductor/insulator interface that are the goal of characterization). An example of a method that requires post-processing are traditional Metal-Insulator-Semiconductor Volt-Farad (MIS CV) measurements. They require presence of an insulator over semiconductor surface and deposition of metal layer over the insulator. Additional steps are costly, time-consuming and they can change the properties that were the goal of measurements.
Methods that do not require post-processing can be subdivided into contact and non-contact methods. Contact methods are defined as those that involve bringing the specimen into physical contact with materials or chemicals other than the ambient environment or materials and equipment that are used to handle specimens (semiconductor wafers) during normal processing (cleaning solutions, wafer chucks, etc.). An example of such methods in the area of semiconductor surface/interface characterization is a Mercury Sensor. Since such methods require direct contact of a foreign material with the front side of the wafer there is a risk of contaminating the wafer, therefore they are not used with production wafers.
Finally, several non-contact methods for semiconductor surface/interface characterization are known in the field.
The apparatus described in U.S. Pat. No. 5,233,291 uses traditional CV methodology for measurement of the properties of semiconductor surface and semiconductor/insulator interface. The difference between this apparatus and MIS CV measurement systems is in the use of precisely controlled air gap as the insulating layer between the metal electrode and the semiconductor. Wafer proximity is determined by detecting the energy losses of the laser beam that undergoes total internal reflection on the surface of the transparent conductive electrode caused by interaction of the sample with the evanescent beam. The electrode is suspended at submicron distance from the sample, this distance as well as tip and tilt being monitored by three capacitive sensors and controlled by three piezo actuators in real time during the measurements.
In order to calculate semiconductor parameters using CV methods, it is necessary to know the width of the depletion layer in the semiconductor W.sub.d. In this method W.sub.d is calculated from the capacitance of the depletion layer, which in turn is calculated from the total capacitance between the metal electrode and the semiconductor substrate. EQU C.sub.tot.sup.-1 =C.sub.air.sup.-1 +C.sub.SiO2.sup.-1 +C.sub.si.sup.-1,
thus it is important to know C.sub.air at all times during the measurement, which means either measuring the air gap or holding it fixed with substantial precision during the measurements of electrical parameters of the sample, both tasks being technically complicated, considering the submicron width of the air gap.
Furthermore, this method requires knowledge of variations of C.sub.si which is obtained from measurements of C.sub.tot. The presence of two additive components in the equation above, C.sub.air.sup.-1 and C.sub.SiO2.sup.-1 means that the sensitivity of the capacitance meter has to be lower than desirable for measurements of variations in C.sub.si.sup.-1.
Additionally, the method described in U.S. Pat. No. 5,233,291 is not well suited for measurements of samples with poorly passivated semiconductor/insulator interface, e.g. non-passivated wafers of epitaxially grown films, because of the presence of slow surface states which may recharge during the measurements and thus affect both the measurements of C.sub.si.sup.-1 and the readings of the three capacitive gap sensors.
The surface photovoltage method described in U.S. Pat. No. 4,827,212 to E. Kamienicki, which patent is incorporated herein by reference makes use of modulated light whose wavelength corresponds to an energy greater than the band gap of the semiconductor and whose modulation frequency is greater than the reciprocal lifetime of minority carriers and whose intensity is low enough so that the modulation of the semiconductor surface potential is small compared to the surface potential. The photovoltage generated by the light is proportional to the width of the depletion region near the semiconductor surface. The photovoltage can be measured using capacitive coupling of a conductive electrode to the surface of the sample. The advantage of this method over regular CV methods is in the fact that it is not required to measure separate contributions of the insulating gap capacitance or the oxide capacitance which makes the method less complicated and less sensitive to errors in sensor positioning. However, this method is not a true non-contact method because it contacts the specimen with an insulating film.
In U.S. Pat. No. 5,453,703 to W. C. Goldfarb, which patent is incorporated herein by reference, a method and apparatus are disclosed for determining the minority carrier surface recombination lifetime constant (t.sub.s) of a specimen of semiconductor material. The specimen is positioned between a pair of electrodes, the specimen being disposed on one of the electrodes and being spaced from the other electrode. A signal is provided corresponding to the capacitance between the specimen and electrode spaced from the specimen. A region of the surface of the specimen is illuminated with a beam of light of predetermined wavelengths and which is intensity modulated at a predetermined frequency and varying in intensity over a predetermined range. A fixed bias voltage V.sub.g applied between the pair of electrodes, the fixed bias voltage being of a value such that the semiconductor surface is in a state of depletion or inversion. A signal is provided representing the ac photocurrent induced at the region of the specimen illuminated by the light beam. The intensity of the light beam and frequency of modulation of the light beam are selected such that the ac photocurrent is nearly proportional to the intensity of the light beam and reciprocally proportional to the frequency of modulation of the light beam. A signal is provided corresponding to the illumination intensity of the beam of light. The surface minority carrier recombination time constant (t.sub.s) is then determined using the ac photocurrent capacitance and illumination intensity information.
It is an object of this invention to provide a new and improved method and apparatus for characterizing a semiconductor.
It is another object of this invention to provide a new and improved method and apparatus for determining the doping concentration profile and average doping concentration of a specimen of semiconductor material.
It is another object of this invention to provide a non-contact method and apparatus for determining the doping concentration profile and average doping concentration of a specimen of semiconductor material.
It is still another object of this invention to provide a method and apparatus as described above in which photovoltage (or photocurrent) is measured using capacitive coupling of a sensor electrode to the surface of the specimen and which does not require precise control or direct monitoring of the gap between the sensor and the specimen.
It is a further object of this invention to provide a method and apparatus as described above which is particularly suitable for characterization of specimens with high density of slow surface states, e.g. non-passivated wafers.
It is still a further object of this invention to provide a new and improved sensor assembly for use in non-contact measuring of semiconductor surface photovoltage or photocurrent.
It is another object of this invention to provide a new and improved capacitive pickup type sensor assembly.
It is still another object of this invention to provide a non-contact method of measuring semiconductor surface properties.