arrival of fs-laser pulse
2.4 Experimental notes and updates
The experimental setup used for laser-doping has been reviewed extensively else-where [55,72]. We briefly review it here. We also note, for the convenience of future students, some changes to “standard operating procedures” that may be convenient to know as one gets to know the history of the project.
2.4.1 Our experimental setup
In the experiments described in this thesis, we employ amplified femtosecond laser pulses of temporal duration τ ≤ 100 fs and center wavelength λ = 800 nm. The repetition rate of the pulse train (i.e. the number of pulses generated per second) is any integer division of 1000 (e.g., 1000 Hz, 500 Hz, 333 Hz, etc). The pulse has an approximately Gaussian
Chapter 2: Background 29
Figure 2.10: A schematic representation of the femtosecond laser-doping apparatus.
spatial profile, and an average pulse energy is 2.5 mJ. We can vary pulse energy continuously down to approximately 50 µJ using a half-wave plate to rotate the pulse polarization prior to temporal compression, a process that employs polarization sensitive optics. We direct the train of fs-laser pulses to the setup depicted schematically in Figure 2.10.
We prepare a silicon wafer for laser exposure using an RCA clean [104] followed by a dilute (5% HF) etch in hydrofluoric acid to remove the native oxide. If we are introducing Se or Te as dopants, we thermally evaporate a thin (75 nm) layer of that dopant onto the silicon wafer. We immediately load the wafer into the vacuum chamber, and evacuate the chamber to high vacuum (pressure ≤ 10−5 Pa). We then backfill with a gas: if we are introducing sulfur into silicon, we backfill with 6.7 × 104 Pa of SF6; if we are doping using a thin solid film of Se or Te, we fill the chamber with the same pressure of N2.2 The wafer
2In references [55] and [72], we find that the ambient gas has a tremendous impact on the hydrodynamics
Chapter 2: Background 30
is positioned in front of the focus of the fs-laser pulse, which is sent through a f = 350 mm plano-convex lens as it travels toward the vacuum chamber. A CCD camera, carefully positioned at an identical distance from a glass pick-off as the silicon wafer, is used to measure the spatial profile of the laser pulse.
The lens is positioned to create a spatial intensity profile at the wafer surface with a full-width at half-maximum w. The laser repetition rate is set to a frequency f ; the wafer is translated in the x-direction in front of the beam using stepper motors a distance typically on order of 1 cm, stepped vertically a distance ∆y, translated in the reverse direction, and the process is repeated. We assign a quantity, shots per area, to the exposure; it is defined as:
S/A = πw2
∆x∆y = f · πw2
v∆y , (2.2)
where ∆x = v/f , and all other quantities were defined above. The details of translation are selected in order to distribute laser pulses across the surface in a fashion that is particular to the experiment. Typically we fix ∆x= ∆y, a subject discussed further in section 2.4.2.
2.4.2 Updates to previous work
In this section, we emphasize differences between the fs-laser doped samples pro-duced for this thesis and prior work [55, 72]. Most of these changes were made for the sake of greater sample cleanliness or experimental accuracy.
Changes to the experimental setup
If comparing these results to previous work, note the following changes in procedures.
• We cleaned the interior of the vacuum chamber before most sample runs. Cleaning consisted of wiping down surfaces with methanol and baking out (Tpeak< 150◦C) for
of melting and ablation; thus, an inert atmosphere is used during fs-laser doping with a thin solid film
Chapter 2: Background 31
approximately 24 hours before most fabrication runs. A turbo pump was installed, with a dry scroll pump (rather than an oil pump) backing it. Typical base vacuum for the bake out was in the 5 × 10−8 torr range, with typical pre-doping pump down to approximately < 10−6 torr.
• We installed a new laser system, which we use for all experiments except those of Appendix A. Important parameters for both laser systems are outlined in Table 2.1 below.
• We clean all silicon wafer substrates with the RCA clean prior to laser-doping
Previous laser Current laser pulse duration (fs) > 100 < 70
pulse energy (mJ) 0.3 2.5
center wavelength (nm) 800 800 repitition rate (Hz) 1000 1000
Table 2.1: Changes to laser parameters relative to previous work
Corrections to optical data
Previous reports of optical data regarding fs-laser doped silicon included errors in the optical absorptance. Because laser-doped silicon has a rough surface that generates dif-fuse reflection, an integrating sphere is necessary measure reflectance. However, the coating used in the integrating sphere — intended to be a uniform reflector, with a reflectance of R = 1 over the entire spectral range of interest — has significant spectral features in the infrared. These features introduced spurious data at λ = 1400 nm and λ = 1900nm, as well as a general postive slope of the absorptance in the infrared. We have corrected these
Chapter 2: Background 32
Figure 2.11: Absorptance of a boron-doped (1 − 20 Ω·cm) silicon wafer irradiated with a homogenized 100 shots per area in a 500 torr SF6 environment. Uncorrected data is shown with dotted lines; corrected data is shown with solid lines.. The derivation of the correction can be found in [105].
errors using an unpublished technique that involves careful comparison to references [105].
Example data, both inclusive of and corrected for the error, are shown in Figure 2.11.
Also, in Figure 2.11 we show normalized absorptance ( ¯A = (1 − R − T )/(1 − R)) as opposed to absorptance A = 1 − R − T . Normalizing the absorptance in this fashion expresses the fraction of light absorbed that penetrates the initial air-silicon interface. This measurement accurately reflects the fact that silicon absorbs strongly in the visible (see, for example, Figure 2.8). Additionally, provided we had a non-roughened surface, ¯A would allow us to calculate the absorption coefficient for absorption through a uniform medium of thickness d, neglecting internal reflections:
A = I(d)/I¯ 0= exp(−αd). (2.3)
Chapter 2: Background 33
Homogenizing the doping process
Finally, we mention briefly a change to typical laser exposure conditions. Prior to this work, researchers employed a laser exposure scheme in which a the following parameters would be typical:
∆x = v/f = w/100 (2.4)
∆y = w/2, (2.5)
where ∆x,y is the spacing between incident laser pulses on the silicon surface, v is the velocity with which the silicon wafer is translated in x-direction, f is the repetition rate of the laser, and w is the full-width at half-maximum of the laser spatial-intensity profile.
Such an exposure recipe results in a highly non-uniform distribution of laser pulses across the silicon surface. This inhomogeneity has evident consequences that are visible as streaks and lines in laser-doped areas. The problem was exacerbated when we installed a higher power laser which enabled larger a w; an example is shown in Figure 2.12. To correct this problem, we homogenized the irradiation pattern such that ∆x= ∆y. A new laser exposure parameter was defined
S/A = πw2
∆x∆y
, (2.6)
where all parameters were defined above, except S/A which is the shots per area. The success of this method in reducing large-scale inhomogeneities is obvious in Figure 2.12.