Doubling of sensitivity and bandwidth in phonon cooled hot electron bolometer mixers

Doubling of sensitivity and bandwidth in phonon cooled hot

electron bolometer mixers

Jochem Baselmans

, MerlijnHajenius

, Jianrong Gao

, Piet de Korte

, Teun

Klapwijk

, Boris Voronov

, and Gregory Gol’tsman

1

_{Space Research Organisation of the Netherlands (SRON), Sorbonnelaan 2, 3584 CA}

Utrecht, The Netherlands

2

_{Kavli inst of Nanoscience and Faculty of Applied Sciences, Delft University of}

Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands.

3

_{Moscow State Pedagogical University, Moscow 119435, Russia}

Abstract - NbN hot electron bolometer (HEB) mixers are at this moment the best heterodyne

detectors for frequencies above 1 THz. However, the fabrication procedure of these devices is such that the quality of the interface between the NbN superconducting film and the contact structure is not under good control. This results in a contact resistance between the NbN bolometer and the contact pad. We compare identical bolometers, with different NbN - contact pad interfaces, coupled with a spiral antenna. We find that cleaning the NbN interface and adding a thin additional superconductor prior to the gold contact deposition improves the noise temperature and the bandwidth of the HEB mixers with more than a factor of 2. We obtain a DSB noise temperature of 950 K at 2.5 THz and a Gain bandwidth of 5-6 GHz. For use in real receiver systems we design small volume (0.15x1 µm2₎ HEB mixers with a twin slot antenna. We find that these mixers combine good sensitivity (900 K at 1.6 THz) with low LO power requirement, which is 160 - 240 nW at the Si lens of the mixer. This value is larger than expected from the isothermal technique and the known losses in the lens by a factor of 3-3.5.

1. INTRODUCTION

The development of new space based [1] and airborne [2,3] telescopes will create new opportunities for sub-mm astronomy, as ground-based observatories suffer from limited atmospheric transmission in this spectral range. The desire for these instruments to perform high-resolution spectroscopy with a sensitivity close to the quantum limit drives the development of low noise THz mixers.

In the frequency range from 100 GHz to 700 GHz SIS mixers incorporating Nb/Al-ALOx/Nb junctions with a niobium tuning circuit offer near quantum limited performance [4]. However, above that frequency, the Nb superconducting energy gap frequency, the sensitivity decreases drastically due to losses in the Nb strip line. Using either a low loss normal conductor or a superconductor with a larger energy gap for the tuning circuit is a possible way out and might extend the use of these devices to 1.2-1.4 THz, however, higher frequencies will be beyond their reach. Hence all practical mixers in the THz range have to use Schottky-diode mixers with rather poor sensitivity and high local oscillator requirement. This is a serious issue regarding the practical use of these devices.

As a result the development of THz receiver has focused in the past years on Hot Electron Bolometer (HEB) mixers, initiated from the early work of Gershenzon et al. [5]. The major advantage of these type of mixers is that their RF frequency bandwidth is not limited by the superconducting energy gap. The HEB mixer can in principle be operated from millimeter waves up to the far infrared without degradation in performance. Moreover, it is possible to use solid state multiplier chains as LO source for these mixers due to their limited LO power needs. Recently the practical usefulness of HEB mixers at THz frequencies was shown with a successful application of a NbN HEB based receiver in a radio telescope in Arizona [6].

Table 1: Device parameters. In all cases a 6 sec. ex situ Oxygen plasma etch is included prior to the contact pad deposition.

Device type

Contact pad-NbN interface Contact pad material R at 11K

I No additional cleaning 5 nm Ti + 65 nm Au 220 ± 20 Ω

II 15 sec Argon etch 10 nm NbTiN + 40 nm Au 55 ± 5Ω

In the past years the main focus in the THz frequency range has been on phonon-cooled HEB mixers [5]. These devices have benefited strongly from large advancements in NbN thin film technology and nowadays yield double sideband receiver noise temperatures of around 10 hf/k at THz frequencies. However, to combine a high sensitivity with a high IF bandwidth extremely thin (~3 nm) NbN films are used for phonon cooled bolometers. Only very few labs in the world are able to produce sufficiently high quality NbN films, with a thickness of about 3.5 nm and a critical temperature above 9 K. The state of the art films at this moment come from Moscow State Pedagogical University. Using these films on MgO substrate devices made in Chalmers have shown a DSB receiver noise temperatures of 1400 K at 2.5 THz on MgO substrates together with a 4.5 GHz gain bandwidth at the optimal bias point [7].

2. EXPERIMENTAL STUDY OF THE EFFECT OF THE CONTACT RESISTANCE

Because most groups nowadays obtain their NbN films from elsewhere, it is inevitable that the NbN film is exposed to ambient atmosphere for a prolonged period, i.e. the bolometer fabrication is ex situ [7-13]. In the fabrication procedure no cleaning is performed on the NbN film prior to the deposition of the contact pads and antenna structure. Hence a contact resistance is present between the NbN bridge

Fig 1. Unpumped and 3 pumped IV curves for 2 devices: G2 (left) which is a type II device, with a clean NbN-contact pad interface and a NbN-contact pad consisting of 10 nm NbTiN + 40 nm Au. Right device A3, type I, which did not receive any cleaning of the NbN film and a 65 nm Au contact pad. For the three pumped IV curves the noise temperatures are shown at different bias points.

0 2 4 6 8 10 0 50 100 150 200 250 300 350 400 450 500 2000 4000 6000 8000 10000 12000 I [ µ A]

Device G2 in-situ Ar cleaning + 10 nm NbTiN

V [mV] Absorbed LO Power 940 nW 730 nW 650 nW T n [K ] 0 2 4 6 8 10 0 50 100 150 200 250 300 350 400 450 500 2000 4000 6000 8000 10000 12000

Device A3 No aditional cleaning

I [ µ A] V [mV]

<

>

and the contact - antenna structure of the device. This contact resistance has important implications for the mixer performance [9,10,14]. In this article we study experimentally the effect of a possible contact resistance between the contact pad and the NbN film on the bolometer mixer performance. To do this we have made 4 different bolometer types which differ only in the way the NbN-contact pad interface is prepared. We discuss here the two most relevant cases, for a more complete description we refer to Ref [9,10,14]. All devices are made in one process run on one single wafer consisting of a 350 µm thick Si wafer with a 3.5 nm NbN film deposited in Moscow. The size of the bolometer is 0.4 x 4 µm and is coupled to a Spiral antenna. Table 1 shows the 2 different types discussed here denoted I and II. Type I is made using the conventional fabrication process. The NbN – contact pad interface has not been cleaned other than a short Oxygen etch necessary for a good lift-off process.

For Type II we perform a 15 second Argon etch prior to the in-situ deposition of 10 nm of NbTiN. We have calibrated the Argon etch time for a minimum in device resistance (maximum interface transparency) and minimum NbN film damage. Further details on the fabrication and DC characterization of these devices can be found in Ref. [9].

The bolometer chip is glued to a hyper-hemispherical Si lens without anti reflection coating and clamped to the mixer block that is thermally anchored to the 4.2 K plate of a L-He cryostat with 3 Zytex G104 heat filters, 1 at 77 K and 2 at 4.2 K and a 40 µm thick Mylar window. During the experiments the mixer block temperature is 4.35 K. As a local oscillator source we use a FIR laser at 2.5 THz. The LO power is coupled into the cryostat by means of a 6 µm Mylar beam splitter. A rotatable grid is used to control the laser power. The device output is connected through a bias T to a 1.2 - 1.8 GHz HEMT amplifier with a noise temperature of ~ 5K. The output is further amplified at room temperature and filtered through a 1.35 ± 0.04 GHz band pass filter and detected using a power meter. The DSB receiver noise temperature was determined using the standard Y-factor technique. We use a mirror hot/cold load consisting of Eccosorb at 295 K and 77K placed at 30 cm from the cryostat window. The losses in the optics (beam splitter, window and heat filters) are measured to be 3.3 dB, the estimated lens losses (reflection and absorption) and antenna coupling efficiency are estimated to be 2.2 dB, yielding a total loss of 5.5 dB in the RF optics. The air path between the hot/cold load gives an estimated additional loss of about 2 dB.

Due to the high LO frequency the Rayleigh-Jeans limit is no longer applicable to calculate the noise temperature from a measurement of the Y-factor. We use the Callen and Welton law to calculate the effective temperatures of the hot and cold load [15].

1 10 -15 -10 -5 0 5 opt. Bias High Bias 6 GHz relat ive I F A m plit ude [ d B ] IF Frequency [GHz] 1 10 -15 -10 -5 0 5 opt. Bias High Bias 2 GHz 5 GHz relat ive I F A m plit ude [ d B ] IF Frequency [GHz]

Fig. 2. Left: Gain Bandwidth at optimal LO power and DC Bias and at a higher DC bias point with a twice lower sensitivity for a type II device with a clean interface and a NbTiN interlayer. Right: The same measurement, but now for a type I device.

Type I Type II

Noise Temperature

We measure the Double Sideband Receiver Noise Temperature (TN,DSB) for two identical devices of each type. We observe that identical devices give similar results. The unpumped and a few pumped I-V curves of one of the devices of type I and of type II are shown in Fig. 1 together with the receiver noise temperatures at different bias points. In all cases the region over which the noise temperature is minimal is reasonably broad, in agreement with other experiments [8]. We observe that, adding a thin layer of superconductor in combination with a cleaning of the interface, as for geometry II, strongly reduces the receiver noise temperature. The improvement is more than a factor 2 between geometry I and II.

Improving the optics of our system, by removing 2 heat filters, using an anti-reflection coated elliptical lens, a 3.5 µm beam splitter and using a direct hot/cold load 10 cm from the cryostat window, reduces losses in the air and the optics with about 3 dB. Using this improved setup enabled us to measure with a device with geometry II a DSB receiver noise temperature of 950 K at 2.5 THz [10,14]. The mixer block temperature is 4.4 K in this case; this somewhat higher temperature is caused by the reduced heat filtering. The reduction in noise temperature is consistent with a reduction of a factor 2 (3dB) in the optics losses.

Gain Bandwidth

The IF gain bandwidth is measured at 600 GHz using a BWO as LO source and a Carcinotron with a doubler as the RF signal. The output power is amplified using a 0.1–8 GHz Miteq® cryogenic amplifier at 4.2 K with a noise temperature of about 100 K and 30 dB gain. At room temperature the signal is further amplified and measured using a spectrum analyzer. The upper frequency limit of our IF chain is measured

to be 6 GHz using a calibrated noise source. We measure the bandwidth at two bias points: At the optimum bias point that yields the lowest TN,DSB and at the same LO power but a higher DC voltage.

HEB

2 1

Si Lens

Fig. 3: Measurement setup to measure the real LO power need of a HEB mixer. The inset shows the two relevant points where the LO power is evaluated: “2” refers to the position of the HEB mixer itself, within the antenna. This is the point of reference when the LO power can be estimated using the isothermal technique. “1” refers to the input of the receiver lens, usually the reference point for the integration of a complete receiver system.

This DC voltage is chosen such that TN,DSB is twice the value at optimum bias. The results are given in Fig.2. Two observations can be made here: First, the gain bandwidth at optimum bias increases from 2 to 6 GHz between types I and II. A gain bandwidth of 6 GHz is much larger than the best results obtained for a 3.5-nm-thick NbN HEB mixer on Si [7,8]. Second, the expected bias dependence of the gain bandwidth seems to be virtually absent for type II. The might be related to the upper frequency limit of our IF chain. This in contrast with type I, where we do measure the expected dc bias

dependence of the gain BW.

3: LOCAL OSCILLATOR POWER REQUIREMENT

The local oscillator (LO) power requirement is a crucial issue for the practical use of a THz receiver. For real (space based) applications the output power of solid state LO sources in the THz frequency range is limited to the order of a few µW. Optics losses and antenna coupling limitations reduce the LO power available for the mixer itself to less than 1 µW. In the past the LO power needed to pump a HEB mixer has always been evaluated using the isothermal technique. The LO power absorbed for device I is in agreement with the values reported in Ref. [7]. Geometry II needs 67 % more LO power, this might indicate that the type II has more electron-phonon cooling, consistent with the measured increase in IF bandwidth. Referring to Fig. 1 we see that, according to the isothermal technique, all the devices need less than 730 nW of LO power. This seems to be a low value for most applications. However, for a real application it is essential to verify whether the isothermal technique is correct. To address this issue, we have pumped a type II mixer HEB using a JPL solid state LO source operating at 1.524 THz. Schematically the experimental setup is shown in Fig.3. As a source we use a phase locked Gunn oscillator as input for a x16 JPL multiplier. This ensures low phase noise in the multiplied output signal. The LO output power at 1.524 THz is ~ 7 µW.

Surprisingly, with 7 µW of output power from the LO, the strongest pumping of the device we can reach is the optimal pumping level. Partly, this difference might be attributed to optics losses. Between the LO and the surface of the mixer lens there is 2 dB loss, due to Teflon the lens, 1 dB], window [-0.7 dB] and heat filter [-0.3 dB]. Hence at the mixer lens (denoted by “1” in Fig.3) we have 4.4 µW of power available. The power available at the input of the antenna in the HEB mixer is about 2.3 dB less than the power available at the front of the Si lens, due to lens reflection, lens absorption and antenna coupling efficiency. Hence at the mixer we estimate to have 2.5 µW of LO power available. The isothermal technique, which is an estimate of the LO power absorbed in the bridge, gives hence a 3.5 (or 5.4 dB) lower value than the actual power available at the mixer.

Ground plane

SAL Resist

Bolometer Antenna

RF choke

_{10 µm}

1 µm

Fig. 4: Scanning Electron Micrograph of a NbN hot electron bolometer coupled with a twin slot antenna. The device shown has a planar dimension of 0.3 x 2µm.

4: TWIN SLOT ANTENNA COUPLED HEB MIXERS WITH A SMALL VOLUME

The large volume Spiral antenna coupled HEB mixers presented in the previous Section are not suitable for real (space based) applications, despite their large IF bandwidth and good noise performance: They need too much LO power and the antenna is not polarization sensitive. Hence we have designed and fabricated small volume HEB mixers coupled to twin slot antenna’s. [16]

We start with a 3.5 nm NbN film, obtained from MSPU, Moscow, on a high purity Si wafer. The critical temperature of the film is 9.3-9.7 K. As a first step we define the contact pads using a standard PMMA double layer positive e-beam resist, e-beam lithography and a wet development. Subsequently, we use an in-situ 15 sec Argon etch, prior to the sputter deposition of the contact pad, consisting of 10 nm NbTiN and 40 nm Au. In the next step we deposit the antenna and ground plane structure using thermal evaporation of 5 nm Ti, 150 nm Au and 10 nm Ti. The bottom layer of Ti is used as an adhesion layer, the cap layer of Ti is used as a protection of the gold during the following step, which consists of the reactive ion etching of the NbN using CF4 / O2 to define the HEB bridge width. A Scanning Electron Micrograph of a finished device is given in Fig.4.

Noise Temperature

Two typical results of noise temperature measurements are shown in Fig.5. In the left panel we show a mid-size HEB (0.3 x 2 µm) mixer with a 1.8 THz antenna, evaluated at 1.89 THz. The lines corresponds to the unpumped and optimally pumped IV curves, the dots give the uncorrected values of TN,DSB at the optimally pumped curve. Using a coated lens and a direct Hot/Cold load 15 cm from the cryostat we obtain TN,DSB=900 K. The LO power is evaluated using the isothermal technique (see Section 3). The right panel shows the result for a small (0.15 x 1 µm) mixer using an uncoated Si lens. This results in a receiver noise temperature of TN,DSB = 1100 K at 1.6 THz. This corresponds to 950 K if we would have used an anti-reflection coated lens [17]. Note that in the smallest devices we also observe a direct detection effect, which is found to reduce the heterodyne sensitivity. This effect is at least of the same order as the reduction in the noise temperature observed from the mid-size HEB to the smallest ones. These results show that we have successfully transferred the new contact structure

0 1 2 3 4 5 0 10 20 30 40 50 60 70 80 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 unpumped 30 nW Device M6T_K2 1x0.15 um 1.6THz twin slot NbN HEBM

I [ µ A] V [mV] T n, D S B [K] 0 1 2 3 4 5 6 7 8 0 20 40 60 80 100 120 140 750 1000 1250 1500 1750 2000 V [mV]

M5 B4 2x0.3 um 1.8 THz twin slot NbN HEBM Anti reflection Coated lens

unpumped 125 nW TN, DS B [K ] Direct Hot/cold, 1190 K I [ µA] 0.5 1.0 1.5 2.0 0.0 0.2 0.4 0.6 0.8 1.0 D ir e ct r e sponse F [THz] Design Frequency: 1.6 THz 0.5 1.0 1.5 2.0 0.0 0.2 0.4 0.6 0.8 1.0 D ir e ct r e sponse F [THz] Design Frequency: 1.6 THz 0 1 2 3 4 5 0 10 20 30 40 50 60 70 80 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 unpumped 30 nW Device M6T_K2 1x0.15 um 1.6THz twin slot NbN HEBM

I [ µ A] V [mV] T n, D S B [K] 0 1 2 3 4 5 6 7 8 0 20 40 60 80 100 120 140 750 1000 1250 1500 1750 2000 V [mV]

M5 B4 2x0.3 um 1.8 THz twin slot NbN HEBM Anti reflection Coated lens

unpumped 125 nW TN, DS B [K ] Direct Hot/cold, 1190 K Direct Hot/cold, 1190 K I [ µA] 0.5 1.0 1.5 2.0 0.0 0.2 0.4 0.6 0.8 1.0 D ir e ct r e sponse F [THz] Design Frequency: 1.6 THz 0.5 1.0 1.5 2.0 0.0 0.2 0.4 0.6 0.8 1.0 D ir e ct r e sponse F [THz] Design Frequency: 1.6 THz

Fig. 5: Noise performance of 2 HEB mixers with twin slot antenna’s. To the left we give the result of a 2 x 0.3 µm HEB, identical to the one shown in Fig. 3 with an antenna design optimized for 1.8 Thz The lines give the pumped and unpumped IV curve, the dots TN,DSB evaluated at 1.89 THz using an uncoated lens and a manual Hot/Cold load 13 cm from the cryostat

window. The right panel shows TN,DSB and the pumped/unpumped IV curves for a 0.15 x 1 µm HEB with a 1.6 Thz twin slot

antenna design, evaluated at 1.63 THz. The inset shows the direct response, obtained using a Fourier Spectroscopy measurement.

technology from large spiral antenna coupled devices to much smaller twin slot antenna coupled HEB mixers. A great advantage of this technology, a part from the increase in sensitivity and bandwidth, is the good reproducibility of the mixer performance. This is illustrated in Table II where we give the results of similar measurements on a set of devices from 4 different fabrication runs. It is obvious that the receiver noise temperatures differ only by about 15% between all devices. Given the fact that the mixer volume, and with that the LO power requirements, differ largely between different mixers this is surprising. Past results [7,11] always have shown a rather strong dependence of the receiver noise temperature on mixer volume that is hardly present in our data.

LO Power requirement

The LO power need of these small devices is, according to the isothermal technique, a lot smaller than for the large volume HEB mixers discussed in the first Sections. This is indicated in the rightmost column of Table II. To evaluate the real LO power needed to pump these smaller mixers we have evaluated a set of experiments with calibrated LO sources at 1.524 and 0.673 THz. For device M5 B4, presented already in the left panel of Fig. 5 we find, at an LO output power of 1.1 µW at 1.524 THz (using the same JPL LO source), a power at the mixer lens (“1” in Fig. 3) of 700 nW, and 400 nW at the HEB itself (“2” in Fig. 3). The isothermal technique gives in this case 125 nW, hence there is a factor 3.3 between the real LO power need and the isothermal technique. For device M6T_ K2 (presented in the right panel of Fig. 5) we perform an identical experiment using a phased locked Gunn oscillator with a x6 multiplier chain at 673 GHz. The antenna response at this frequency is reduced by a factor 2 when compared to the center frequency of 1.6 THz. This is shown at the insert of Fig.5. The factor of 2 due to the reduced antenna response is included in the LO power quoted below. We measure the LO power need for an slightly overpumped IV curve that requires 45 nW according to the isothermal technique. At the optimal operating point 30 nW is required (see Fig.5). We find a real LO power available at the mixer lens of 240 nW and 140 nW at the mixer. The difference with the isothermal technique, evaluated at the mixer, is a factor 3.1. At the optimal pumping we estimate therefore that roughly 2/3 of 240 nW is needed, i.e. 160 nW at the mixer lens.

From these measurements we can conclude that the smallest volume HEB mixers need about 160-240

Table II: TN,DSB for several twin slot coupled HEB mixers with different volumes. The device ID is indicated as

“batch-device”, showing that results from 4 batches (M5,M6,M8,M9) are given. The receiver noise temperature is evaluated at 1.63 Thz fore the 1.6 Thz antenna and 1.89 Thz for the 1.8 Thz antenna. Table 3: The real LO power given in the leftmost columns differs strongly from a measurement of the LO power need using the isothermal technique.

Device ID Twin Slot center frequency [THz] Planer Dimension [µm x µm] Measurement Frequency [Thz] TN,DSB with coated lens [K] LO power at mixer (isothermal/ technique) [nW] M6-11K 1.8 1x0.15 1.89 940* 30 M6-2K 1.6 1x0.15 1.62 960* 30, 160-240nW at mixer lens $# M6-3K 1.6 1x0.15 1.62 900* 45 M9-C3 1.6 1x0.2 1.62 880* 75 M8-H1 1.8 1x0.15 1.89 1000* 65 M5-4B 1.8 2 x 0.3 1.89 900 125, 700nW at mixer lens# M6-3D 1.6 1.5x0.25 1.62 860* 170 M6-5A 1.6 2x0.3 1.62 800* 330

# See section 4. This value is measured using a calibrated LO source.

$ obtained for a slightly overpumped IV that yields 45 nW according to the isothermal technique. * Obtained from a measurement using an uncoated lens.

nW of LO power. Hence they are suitable for Space based applications or remote systems where Solid State LO sources are preferred. It is also clear that the isothermal technique gives a large underestimate of the real LO power needed to pump the mixer. At the mixer level the difference is a factor 3-3.5. If the isothermal technique is used to estimate the LO power at the mixer lens needed to pump the mixers the difference is a factor of 5-5.5. Moreover the difference between the real LO power needs and isothermal technique found in our devices seems to be roughly constant. Again, this might be related to the better reproducibility of the contact structure.

5. CONCLUSIONS

We have studied the effect of the NbN-contact pad on the noise temperature of a NbN based phonon cooled HEB mixer. Cleaning the NbN - contact pad interface using Argon etching can improve the mixer noise temperature by a factor of 2 when compared to an identical device in which no interface cleaning was performed. However, to achieve this improvement an additional superconductor needs to be added between the NbN film and the contact pad to compensate the reduction of TC associated with a stronger superconducting proximity effect. Using this strategy to fabricate a HEB mixer based on a 3.5 nm NbN film on a Si substrate we have achieved a DSB receiver noise temperature of 950 K at 2.5 THz at a mixer block temperature of 4.4 K. The LO power needed to pump a type II mixer is 730 nW using the isothermal technique. However, using a calibrated solid state LO source we find that 4.4 µW is needed to pump the mixer, evaluated at the mixer lens, and 2.5 µW at the HEB itself. This is a difference of a factor of 3.5 with the isothermal technique. Small HEB mixers with a twin slot antenna show a DSB receiver noise temperature of 900 K at 1.6THz or 1.89 THz. These mixers require about 160-240 nW of LO power at the mixer lens which make them suitable for most applications.

Acknowledgement: We would like to thank Willem Jan Vreeling for assistance, Pavel Yagoubov for assistance and the spiral antenna design and S. Cherednichenko, P. Khosropanah and H. Merkel for discussions at Chalmers. We also wish to thank John Pearson for making the JPL LO source available for our experiments.

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[17]: The optics losses are identical at 1.63 and 1.89 Thz. Using a coated lens reduces the loss with 1 dB, using a mirror hot cold load in stead of a direct hand held Hot/Cold increases the loss with ~0.3 dB due to the longer air path.