74
4 Scalable Interdigitated Photoconductive Emitters for the Electrical …
direction of the bias voltage in each gap, and therefore the orientation of the THz
polarisation that will be produced by that gap, is demonstrated by the arrows in
Fig. 4.1b. The pixels producing horizontally polarised radiation share a common set
of bias and ground electrodes, indicated in Fig. 4.1a by V H and G H respectively, as
do the vertically emitting pixels (V V and G V in Fig. 4.1a). Photoexciting the entire
active area of the device and varying the relative strengths of V H and V V will vary
the relative strengths of the horizontally and vertically polarised components of the
generated THz radiation; hence the orientation of the polarisation state of the far-field
THz pulse generated by the device can be defined by simply varying the relative bias
voltage on the horizontal and vertical contacts.
4.2.2 Device Fabrication
The multi-pixel interdigitated photoconductive emitter was fabricated by performing
UV photolithography, metal evaporation and lift-off processes on a 500 µm-thick
semi-insulating GaAs (SI-GaAs) substrate. The bottom layer of the device, shown
in Fig. 4.2b, consists of an interdigitated pattern of 5 µm-wide electrodes, with a
5 µm gap between adjacent electrodes. Initially a thin layer of AZ5214-E imagereversal photoresist was spin-coated onto the SI-GaAs substrate at 8000 rpm for 35 s,
which was then pre-baked for 2 min at 115
◦ C. The AZ5214-E resist was chosen as
it produces an ideal negative wall profile in the developed photoresist in order to
facilitate easier lift-off [15]. The electrode pattern in Fig. 4.1b was transferred to the
photoresist from a photomask via exposure to UV light with a fluence of 90 mJ/cm
2 .
The exposed resist was baked at 120
◦ C for 4 min, before a second exposure to UV
light with a fluence of 250 mJ/cm
2 without the photomask present. The photoresist
was then developed in MF-319 developer for 50 s, which removed part of the resist to
leave a negative image of the photomask pattern. Electron-beam metal evaporation
was then used to deposit an initial 5 nm-thick layer of Ti onto the substrate and
photoresist, before depositing a 300 nm-thick layer of Au. The Ti layer was used
in order to improve adhesion to the substrate. The bottom layer of the device was
finalised by removing the unwanted portions of gold via lift-off of the photoresist in
acetone.
The generation of THz radiation from the bare bottom layer of the device would
be subject to destructive interference in the far-field, as the bias voltage in adjacent
gaps has the opposing polarity, hence the THz pulses produced in adjacent gaps
will be π out of phase. To avoid this destructive interference, a 300 nm-thick Au
masking layer was deposited on top of the metal electrodes, shown in Fig. 4.2d, to
block the photoexcitation beam in every other gap. To avoid short circuiting, the
electrodes and masking layer were vertically separated by first depositing a 110 nmthick insulating layer of Al 2 O 3 over the bottom layer electrodes, shown in Fig. 4.2c,
with such a thickness chosen in order to maximize the transmission of the 800 nm
photoexcitation beam. Both the insulating and masking layers were produced by an
identical photolithography procedure as detailed for the bottom layer. The active
4 Scalable Interdigitated Photoconductive Emitters for the Electrical …
direction of the bias voltage in each gap, and therefore the orientation of the THz
polarisation that will be produced by that gap, is demonstrated by the arrows in
Fig. 4.1b. The pixels producing horizontally polarised radiation share a common set
of bias and ground electrodes, indicated in Fig. 4.1a by V H and G H respectively, as
do the vertically emitting pixels (V V and G V in Fig. 4.1a). Photoexciting the entire
active area of the device and varying the relative strengths of V H and V V will vary
the relative strengths of the horizontally and vertically polarised components of the
generated THz radiation; hence the orientation of the polarisation state of the far-field
THz pulse generated by the device can be defined by simply varying the relative bias
voltage on the horizontal and vertical contacts.
4.2.2 Device Fabrication
The multi-pixel interdigitated photoconductive emitter was fabricated by performing
UV photolithography, metal evaporation and lift-off processes on a 500 µm-thick
semi-insulating GaAs (SI-GaAs) substrate. The bottom layer of the device, shown
in Fig. 4.2b, consists of an interdigitated pattern of 5 µm-wide electrodes, with a
5 µm gap between adjacent electrodes. Initially a thin layer of AZ5214-E imagereversal photoresist was spin-coated onto the SI-GaAs substrate at 8000 rpm for 35 s,
which was then pre-baked for 2 min at 115
◦ C. The AZ5214-E resist was chosen as
it produces an ideal negative wall profile in the developed photoresist in order to
facilitate easier lift-off [15]. The electrode pattern in Fig. 4.1b was transferred to the
photoresist from a photomask via exposure to UV light with a fluence of 90 mJ/cm
2 .
The exposed resist was baked at 120
◦ C for 4 min, before a second exposure to UV
light with a fluence of 250 mJ/cm
2 without the photomask present. The photoresist
was then developed in MF-319 developer for 50 s, which removed part of the resist to
leave a negative image of the photomask pattern. Electron-beam metal evaporation
was then used to deposit an initial 5 nm-thick layer of Ti onto the substrate and
photoresist, before depositing a 300 nm-thick layer of Au. The Ti layer was used
in order to improve adhesion to the substrate. The bottom layer of the device was
finalised by removing the unwanted portions of gold via lift-off of the photoresist in
acetone.
The generation of THz radiation from the bare bottom layer of the device would
be subject to destructive interference in the far-field, as the bias voltage in adjacent
gaps has the opposing polarity, hence the THz pulses produced in adjacent gaps
will be π out of phase. To avoid this destructive interference, a 300 nm-thick Au
masking layer was deposited on top of the metal electrodes, shown in Fig. 4.2d, to
block the photoexcitation beam in every other gap. To avoid short circuiting, the
electrodes and masking layer were vertically separated by first depositing a 110 nmthick insulating layer of Al 2 O 3 over the bottom layer electrodes, shown in Fig. 4.2c,
with such a thickness chosen in order to maximize the transmission of the 800 nm
photoexcitation beam. Both the insulating and masking layers were produced by an
identical photolithography procedure as detailed for the bottom layer. The active