Ion implant monitoring through measurement of modulated optical response

[0001] This application claims priority from prior provisional application Ser. Nos. 60/365,237, filed Mar. 18, 2002, and 60/378,140, filed May 14, 2002, both of which are incorporated herein by reference.

[0002] The subject invention relates to optical devices used to non-destructively evaluate semiconductor wafers. In particular, the present invention relates to systems for measuring dopant concentrations in semiconductor samples.

[0003] As geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. The basis for these techniques is the notion that a subject may be examined by analyzing the reflected energy that results when an optical beam is directed at a subject. This type of inspection and analysis is known as optical metrology and is performed using a range of different optical techniques.

[0004] One widely used type of optical metrology system, as shown in FIG. 1, includes a pump laser. The pump laser is switched on and off to create an intensity-modulated pump beam. The pump beam is projected against the surface of a subject causing localized heating of the subject. As the pump laser is modulated, the localized heating (and subsequent cooling) creates a train of thermal and plasma waves within the subject. These waves reflect and scatter off various features and interact with various regions within the sample in a way that alters the flow of heat and/or plasma from the pump beam spot.

[0005] The presence of the thermal and plasma waves has a direct effect on the surface reflectivity of the sample. Features and regions below the sample surface that alter the passage of the thermal and plasma waves will therefore alter the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be investigated.

[0006] To monitor the surface changes, a probe beam is directed at a portion of the subject that is illuminated by the pump laser. A photodetector records the intensity of the reflected probe beam. The output signal from the photodetector is filtered to isolate the changes that are synchronous with the pump beam modulation. For most implementations, this is performed using a heterodyne or lock-in detector (See U.S. Pat. No. 5,978,074 and in particular FIG. 2 for a discussion of such a lock-in amplifier/detector). Devices of this type typically generate separate “in-phase” (I) and “quadrature” (Q) outputs. These outputs are then used to calculate amplitude and phase of the modulated signal using the following equations:
Amplitude=√{square root over (I²+Q²)}  (1)
Phase=arctan(Q/I)  (2)

[0007] The amplitude and phase values are used to deduce physical characteristics of the sample. In most cases, this is done by measuring amplitude values (amplitude is used more commonly than phase) for one or more specially prepared calibration samples, each of which has known physical characteristics. The empirically derived values are used to associate known physical characteristics with corresponding amplitude values. Amplitude values obtained for test subjects can then be analyzed by comparison to the amplitude values obtained for the calibration samples.

[0008] Systems of the type shown in FIG. 1 (i.e., those using external means to induce thermal or plasma waves in the subject under study) are generally referred to as PMR (photomodulated reflectance) type systems. PMR-type systems are used to study a range of attributes, including material composition and layer thickness. PMR-type systems and their associated uses are described in more detail in U.S. Pat. Ser. Nos.: 4,634,290, 4,646,088, 4,679,946; 4,854,710, 5,854,719, 5,978,074, 5,074,669 and 6,452,685. Each of these patents is incorporate in this document by reference.

[0009] Another important use of PMR-type systems is measurement and analysis of the dopants added to semiconductor wafers. Dopants are ions that are implanted to semiconductors during a process known as ion implantation. The duration of the ion implantation process (i.e., total exposure of the semiconductor wafer) controls the resulting dopant concentration. The ion energy used during the implantation process controls the depth of implant. Both concentration and depth are critical factors that determine the overall effectiveness of the ion implantation process.

[0010] PMR-type systems are typically used to inspect wafers at the completion of the ion implantation process. The ion implantation damages the crystal lattice as incoming ions come to rest. This damage is typically proportional to the concentration and depth of ions within the crystal lattice. This makes measurement of damage an effective surrogate for direct measurement of dopant concentration and depth. PMR-type systems have proven to be adept at measuring damage and have been widely used for post implantation evaluation.

[0011] As shown in FIG. 2, the relationship between dopant concentration and amplitude measurements (i.e., as defined by Equation (1)) is monotonic for low dopant concentrations. As dopant concentrations increase (e.g., greater than 1E14 for As⁺ or P⁺ ions or greater than 1E15 for B⁺ ions) the monotonic relationship breaks down. In fact, at high concentrations, the amplitude measurements are no longer well behaved and as a result cannot be used to accurately derive corresponding dopant concentrations. In FIG. 1, this is illustrated by the points A, B and C all having the sane the same amplitude measurement, even though each point represents a different dopant concentration. The same sort of breakdown occurs as the type of implanted ions becomes heavier (e.g., As⁺ or P⁺ ions). In both cases, this is attributable to the appearance of a Si amorphous layer resulting in optical interference effects. Although not shown in FIG. 2, phase information becomes flat or insensitive to changes in concentration at high dopant concentrations or where heavy ions are implanted.

[0012] One approach for dealing with the problem of monitoring samples with high dopant concentrations is to measure the DC reflectivity of both the pump and probe beams in addition to the modulated optical reflectivity signal carried on the probe beam. Using the DC reflectivity data at two wavelengths, some ambiguities in the measurement can often be resolved. The details of this approach are described in U.S. Pat. No. 5,074,669 (incorporated in this document by reference).

[0013] In general, PMR-type systems of the type described above have proven to be effective methods for testing and characterizing semiconductor devices. Their ability to function in a non-contact, non-destructive fashion, combined with their high-accuracy and repeatability have ensured their widespread use as part of semiconductor manufacturing. Still, there is an obvious need for methods to provide this type of measurement capability for high dopant concentrations and ion implantation of relatively heavy ions.

[0014] The present invention provides a method of simultaneously monitoring ion implantation dose, damage and/or dopant depth profiles in ion-implanted semiconductors. For this method, a PMR-type optical metrology tool is used to record both quadrature (Q) and in-phase (I) values for a series of specially prepared calibration subjects. Each calibration subject is fabricated at the same implantation energy. As a result, variations recorded by the PMR-type system are largely attributable to variations in dopant concentration.

[0015] The measurement method performs a linear fit using the recorded points to define a calibration line within an I-Q plane. The slope of the calibration line is defined by the implantation energy used to create the calibration subject. Points along the calibration line correspond to different dopant concentrations. The calibration line is used to define a calibration region within the I-Q plane. The calibration region includes all points within a specified distance (often defined in terms of a percentage) of the calibration line. Typically, this is done by defining an upper boundary line that has a slightly greater slope than the calibration line and a lower boundary line that has a slightly smaller slope than the calibration line. The calibration region is the area between the upper and lower boundary lines.

[0016] After creating the calibration region, the PMR-type optical system may be used to inspect and analyze semiconductor wafers. For each subject wafer, the PMR-type system makes one or more measurements. Measurements that fall within the calibration region are known to share the damage profile of the calibration subject. Measurements that do not fall within this region are assumed to deviate from the known damage profile of the calibration subject. This test provides an effective method of accepting or rejecting wafers that provide acceptable accuracy even when dopant concentrations are high or where heavy ions have been implanted.

[0025] The present invention provides a method for simultaneously measuring ion implantation dose, damage and/or dopant depth profiles in ion-implanted semiconductors. The measurement method is logically divided into two steps: a calibration and a measurement step. During the calibration step, the photo-modulated reflectance of a known damage profile is characterized. Typically, this involves identifying one or more areas within I-Q space that correspond to the photo-modulated reflectance of the known damage profile. All other areas within I-Q space are then assumed to be dissimilar to the known damage profile. In the measurement step, I-Q measurements for a test subject are obtained empirically. The empirically obtained I-Q measurements are then compared to determine if they fall within an identified region of I-Q space. This comparison indicates whether the test subject has a damage profile that is similar to the known damage profile. The following sections describe several possible implementations for both the calibration and measurement steps.

[0026] For a first implementation of the calibration step, a PMR-type optical metrology tool is used to record both quadrature (Q) and in-phase (I) values for a series of specially prepared calibration subjects. Each calibration subject is fabricated at the same implantation energy. As a result, variations recorded by the PMR-type system arc largely attributable to variations in dopant concentration. Each measured value is treated as a point within an I-Q plane. FIG. 3 shows a representative series of measured values plotted as points within an I-Q plane. The calibration step uses a linear fitting algorithm (such as least squares) to define a calibration line that best fits the points within the I-Q plane. FIG. 4 shows a calibration line that corresponds to the representative points of FIG. 3. The slope of the calibration line is defined by the implantation energy used to create the calibration subjects. Points along the calibration line correspond to different dopant concentrations.

[0027] The calibration line is used to define a calibration region within the I-Q plane. The calibration region includes all points within a specified distance (often defined in terms of a percentage) of the calibration line. As shown in FIG. 5, this is typically accomplished by defining an upper boundary line and a lower boundary line. The upper boundary line has a greater slope and the same Q-intercept as the calibration line. The lower boundary line has a smaller slope and the same Q-intercept as the calibration line. The calibration region is the area within the I and Q space that is bounded by the upper and lower boundary lines.

[0028] For the associated measurement step, the PMR-type optical system is typically used to inspect and analyze a series of semiconductor wafers. For each subject wafer, the PMR-type system makes one or more measurements. Each measurement includes both I and Q values and defines a point within the I-Q plane. For the measurement method, the proximity of each point to the calibration line measures the similarity of that point to the damage profile of the calibration subject. Points that are close to the calibration line represent minor departures from the dopant depth and concentrations of the calibration subjects. Points that are further away represent larger departures. Points that fall outside of the calibration region (shown as black dots in FIG. 6) represent even larger departures from the calibration subject. These points are assumed to represent large deviations from the known damage profile of the calibration subject resulting from channeling effects, wafer/beam nonuniformities, etc. It should be noted that points that inside of the calibration region or even on the calibration line (such as point X) can be very similar in amplitude to points that fall outside of the calibration regions (such as point X¹). In a prior art system, that examines only amplitude, the difference between these two points would be undetectable. Similar ambiguities can also arise where points that have different amplitudes correspond, in fact, to the same damage profile. Prior art techniques would be unable to detect the similarity of such points.

[0029] For a second implementation of the calibration step, a PMR-type optical metrology tool is again used to record both quadrature (Q) and in-phase (I) values for a series of specially prepared calibration subjects. A representative series of points of this type are shown in FIG. 7. As shown in FIG. 8, the calibration step uses these points to create a series of calibration lines. Each line is localized to fit a subset of the points measured by the PMR-type optical metrology tool. In this example, three calibration lines have been defined. The first is localized to fit the points that are local to point A. The second and third are localized to fit the points near point B and C, respectively The slope of each calibration line reflects its associated implantation energy. Points along each line reflect different dopant concentrations.

[0030] For the associated measurement step, each point measured by the PMR-type optical metrology tool is compared to see if it lies on or near any of the calibration lines. Points on or nearby calibration lines share the damage profile of the associated calibration line. Points that are not near (or on) calibration lines represent major departures from the dopant depth and concentration of the calibration subjects due to channeling effects, wafer/beam nonuniformities, etc. It should be noted that points on different calibration lines (such as points A, B and C) can have identical amplitudes. Using prior art techniques that examine only amplitude, the difference between these points would be undetectable. Similar ambiguities can also arise where points that have different amplitudes correspond, in fact, to the same damage profile. Prior art techniques are unable to detect the similarity of such points.

[0031] The preceding description has focused on the use of in-phase (I) and quadrature (Q) signals. It is important to realize that there may be implementations that use linear combinations of these signals, in place of the I and Q values. This description and the following claims are specifically intended to cover all useful linear combinations of this type, without limitation.

[0032] It should be noted that this approach is useful in systems that measure the modulated reflectivity of the probe as well as systems that monitor other periodic surface variations such as in interferometry systems or periodic angular variations (“pump” type systems). To the extent these experiments are performed on semiconductor samples, it should also be understood that a portion of the signal would be the result of the modulated electron hole plasma as opposed to being a purely thermal signal. The relative contributions of the plasma and thermal effects on the signals depends on the dosage level and experimental conditions such as pump and probe beam wavelengths, beam spot size and pump modulation frequency.

[0033] It should also be noted that the measurement method is useful both as described, and as part of a more complex analysis. This means, for example that there may be cases where the measurement method will be used in combination with related measurements that analyze either or both of amplitude and phase information.