Dark current is considered to be a critical parameter when one is looking to acquire a scientific imaging camera, specifically in the short-wave infrared (SWIR) region. Extremely careful attention must be paid to the cooling method used to optimize these parameters.
Multiple cooling technologies are available, each having specific drawbacks and benefits. In its new ZephIR line of SWIR cameras, Photon etc. makes use of a four stage thermoelectric (TE4) air-cooled system in order to improve the sensitivity of its imaging sensors.
The ZephIR 1.7 is an InGaAs camera sensitive in the 800 nm to 1700 nm range. The ZepIR 2.5 and 2.9 are HgCdTe cameras sensitive in the 0.85 µm to 2.5 µm and 0.85 µm to 2.9 µm ranges, respectively. With their combined TE4 air-cooled systems, these cameras are capable of reaching an operating temperature of -80 °C and have dark currents of 300 e-/p/s s (ZephIR 1.7), 30 Me-p/s (ZephIR 2.5) and 340 Me-/p/s (ZephIR 2.9).
This article presents a short introduction to thermoelectric (TE) cooling along with a comparison with other available cooling methods.
Overview of Cooling Methods
Thermoelectric (TE) stages are solid-state devices composed of two different faces. These stages use Peltier effect to produce a temperature difference between the two faces. Semiconductors with varied electron densities, n-type and p-type (Figure 1), are placed in series and connected with a conducting material on each side. The passage of an electrical current via the junction induces a heat flow from one face to the other, producing a hot and cold side. The cold face absorbs heat which is sent to the other side where the heat sink is located. TE stages are typically connected side by side and inserted between two insulators. Water or air cooling is usually used to dissipate the heat accumulated in this process.
The temperature that can be reached by TE coolers is related to the number of stages being used. Hence, it is possible to stack several stages for more effective cooling. This is the case of Photon etc’s SWIR sensors, where four thermoelectric stages are cascaded together in order to lower the temperature. A ΔT* of 120 °C can be reached with four stages. This results in a detector operating temperature of -80 °C (193 K) with proper heat extraction, at 25 °C ambient temperature.
Figure 1. Schematic of a thermoelectric device where the Peltier effect is used to generate heat flow between two materials.
Stirling cooler based devices function on a closed Stirling cycle where a nearly ideal gas (generally helium) is being constantly compressed and expanded. Figure 2 shows a schematic of an ideal Stirling cycle and a stirling cooler. Two pistons are needed for obtaining the change in pressure and temperature of the gas. These include a displacer which puts alternatively the gas in contact with a cold and hot reservoir and a working piston which is shifted by the expansion and compression of the gas. A regenerator is also needed and acts as an internal heat exchanger.
Following the Ideal Gas Law, heat from the surroundings is absorbed by the expanded gas during the expansion which makes it colder. When the gas is being compressed, heat is emitted from the gas to the atmosphere.
Four steps are required in an ideal cycle, see Figure 2 (A):
- Isothermal compression: heat ejected.
- Isochoric process: the system is kept a constant volume. Heat is rejected to the regenerator.
- Isothermal expansion: heat is absorbed by the gas.
- Isochoric process: the system is kept a constant volume. Heat is absorbed from the regenerator.
Stirling cooled detectors can reach -210 °C (63 K).
Figure 2. (A) Pressure-Volume diagram or the Stirling cycle (B) schematic of a stirling cooler.
Liquid Nitrogen Cooling
Detectors can also be cooled with liquid nitrogen to reach -196oC (77 K). In a liquid nitrogen cooled system, the detector is placed in a cryostat that holds a dewar where the liquid nitrogen is stored. Different types of detector chambers are available. It is possible to connect the detector to a copper cold finger inserted in the dewar. The finger carry the heat from the detector to the liquid nitrogen tank.
Why a TE4 Air-Cooled System?
- Highly reliable
- No moving parts
- Long lifetime
- Low dark current
- Low readout noise
- No maintenance
Each cooling method has its own benefits and downsides, the application will dictate the appropriate approach. Liquid nitrogen is used, for instance, with MCT sensors working in the long wavelength infrared (LWIR - 8-15 µm) range in order to reduce thermal noise. It is also used for applications needing high cooling capacity and stability. Liquid nitrogen cooled sensor also comprises of relatively low initial cost and long lifetime. The key disadvantage is the steady need for liquid nitrogen supply, the limited autonomy and the time needed to stabilize the temperature.
Stirling cooling also provides extremely low temperature and offers a good solution for applications needing long acquisition time with low power consumption or low dark current. However, Stirling cooling is compact and efficient, and prompts vibration, it has a limited lifespan and high initial cost to which rework costs need to be added later on.
Thermoelectric cooling is considered to be the best suited option in industries where easy maintenance and long life time are vital. It is also user friendly and vibration-less when compared to Stirling and LN2 cooling respectively, two advantages that are also frequently mandatory in advanced scientific imaging. This is the reason why Photon etc. decided to go in this direction for its ZephIR line of cameras. This article presents an overview of the main advantages of TE cooling.
Unlike Stirling coolers, TE stages do not comprise of moving parts, which is a vital advantage for the total durability and maintenance requirements of the camera. No moving parts also means no vibration, which is perfectly ideal for high magnification SWIR microscopy. TE cooled cameras are considered to be suitable for industrial process control or any other applications indicating long cycles of operation due to their long lifetime and reliability.
Their small size is perfect to manufacture compact sensors that can be effortlessly installed in either industrial environments or academic laboratories.
A continuous flow of cold water in the camera is not needed by an air-cooled system. This greatly facilitates its integration in different environments.
Low Dark Current
The small bandgap of InGaAs (~0.75 eV at room temperature) and even smaller bandgap of HgCdTe (~0.15-0.43 eV) indicate the possibility of electrons to reach the conduction band and contribute to the dark current. For this reason, sensors based on HgCdTe or InGaAs possess a high intrinsic dark current at room temperature. For instance, the dark current of InGaAs-based sensors approximately triples with every 10 °C increase. Cooling these sensors is considered to be important in order to obtain higher sensitivity and a good dynamic range.
Thus, Photon etc. has incorporated a four stage Peltier module into their cameras reaching -80 °C, a temperature which considerably lowers the dark current of the camera. The ZephIR 1.7 dark current is usually 300 e-/pix/sec.
TE4 AIR-Cooled Vs the Competition
Figure 3 presents a comparison between Photon etc’s ZephIR 1.7 InGaAs camera furnished with a TE4 air-cooled system and other sensors available on the market. The figure presents the dark current per pixel of varied SWIR sensors as a function of their respective cooling temperatures. The heat dissipating methods (water or air cooled) and the order of magnitude of the price are also indicated. The ZephIR 1.7, when compared to its competitors, offers the best price-performance ratio, besides being easier to install and operate because of its air-cooled system.
The suitable cooling method strongly depends on the application. TE4 air-cooled system is ideal for industrial application and also for demanding scientific imaging. This cooling system is simpler, more reliable and less expensive when compared to other available cooling technologies. The TE4 cooled ZephIR InGaAs and HgCdTe are cameras suitable for advanced scientific research and different industrial applications. They are perfectly suited for fast broadband imaging, hyperspectral microscopy, line-scanning systems, quality control and industrial sorting, thus widening Photon etc’s scientific solutions to the SWIR spectral window.
* ΔT is the difference of temperature between the hot and the cold side of the Peltier stage.
This information has been sourced, reviewed and adapted from materials provided by Photon Etc.
For more information on this source, please visit Photon Etc.