Spectral data, a rich information carrier intertwined with the dual characteristics of wavelength and light intensity, is gradually penetrating and profoundly influencing many frontier fields, including industrial manufacturing, medical diagnosis, and modern agriculture, becoming a key engine driving technological innovation and industrial upgrading. On the stage of spectral detection technology, detectors play a key role. According to the differences in the layout of their photosensitive elements, they are divided into front-illuminated and back-illuminated. Front-illuminated detectors are known for their classic structure and stable performance, showing excellent adaptability in a variety of application scenarios; The innovative design of back-illuminated detectors, which directly illuminates light from the back to the photosensitive layer, significantly improves the quantum efficiency and signal-to-noise ratio, providing a strong support for the pursuit of higher sensitivity and lower noise detection tasks.
1.Differences between front-illuminated and back-illuminated detectors
Front-illuminated detectors:
Light is incident from the front of the wafer, with the circuit layer on top and the light-receiving surface on the bottom, while the circuit layer has a certain degree of shielding from the light. As shown in the figure above: the incident light is required to traverse the circuit layer deposited on the detector's surface, enabling photons to be absorbed by silicon and subsequently converted into charge signals. The circuit layer structure is composed of a gate oxide film, a polysilicon electrode, and BPSG (surface protection film). Due to the obstruction of this circuit layer, the incident light is absorbed, reflected, and scattered before reaching the semiconductor area, resulting in the loss of some energy and it cannot reach the light-receiving surface completely. Therefore, the quantum efficiency of the front-illuminated detector is generally between 50% and 60%, and it is not sensitive to weak light detection.
Back-illuminated detector:
In a back-illuminated photodetector, the circuit is still located on the front side of wafer, but the light is incident from the back side of the wafer and directly illuminates the photosensitive element (such as a photodiode). This design avoids the need for light to pass through multi-layer structures before reaching the photosensitive layer, which not only reduces light loss and improves spectral response, but also theoretically increases the pixel duty cycle to 100%. In fact, some test data shows that the quantum efficiency of a back-illuminated detector in a certain wavelength range is as high as 95% (as shown in the figure below).
Back-illuminated detectors are generally thin-backed. Since the manufacture of back-illuminated detectors must collect the charge signal generated near the light input surface (back) into the potential well-formed near the front surface, in order to achieve this function, the substrate must undergo a thinning and accumulation process. Usually, the substrate is thinned to a thickness of about 20um, so that the resolution will not be affected when the generated charge diffuses into the potential well. The accumulation process ensures that an internal electric field is formed from the light input surface (back) to the front, preventing the interface formed on the light input surface from recombination, and the thin-backed detector can efficiently detect these photons. However, due to the thinning of the substrate, the back-illuminated type causes the long-wavelength light to be reflected on the silicon oxide surface after passing through the photosensitive area, and interferes with the light reflected from the surface of the photosensitive area to produce an etaloning fringe effect (as shown in the figure below), which is superimposed on the collected spectrum and affects signal recognition.
2. Advantages and disadvantages of front-illuminated and back-illuminated detectors
Quantum efficiency: Since the light of the back-illuminated detector can directly illuminate the photosensitive layer, avoiding the obstruction and interference of the multi-layer structure, the quantum efficiency is significantly improved, usually at least 30% higher than that of the front-illuminated detector.
Weak light sensitivity: Back-illuminated detectors are very sensitive to weak light and can achieve better imaging quality in weak light environments.
Signal-to-noise ratio: Back-illuminated sensors are sensitive to weak light and often have a high signal-to-noise ratio, making them suitable for applications in weak light and that require a quick response.
Manufacturing cost: Back-illuminated wafers are relatively expensive because they require a special process to place the photosensitive element on the back of the wafer.
Spectral response range: The front-illuminated type covers visible light and part of the near-infrared light band; while the back-illuminated type has higher quantum efficiency and signal-to-noise ratio in the ultraviolet, visible and near-infrared regions.
Etaloning fringes: The front-illuminated type is not affected by the interference fringe phenomenon and is suitable for applications where the fluorescence effect is superimposed and the signal has a certain intensity; the back-illuminated type will produce a fringe effect when collecting long-wavelength light, which is superimposed on the spectrum and affects signal recognition.
3. Product recommendations
For some users of Raman spectroscopy applications, the following table recommends some spectrometer detector types.
| Application | Back-illuminated | Front-lit |
| 532Raman | High signal-to-noise ratio | Low signal-to-noise ratio |
| 785Raman | Long waves will appear fringes | No streak effect |
| Rapid acquisition of Raman spectra | High quantum efficiency | Weak signal, long integration time |
| Biological samples (requires long wavelength laser) | Affected by fluorescence and interference superposition | Long integration time boosts signal |
JINSP COMPANY LIMITED provides fiber optic spectrometers based on front-illuminated and back-illuminated detectors. The detector type is replaceable. Users can contact sales for trial or purchase existing models as needed. At the same time, we also accept customized requirements and provide professional and fast customization services. For more information, please visit the following link: High Performance Back-illuminated Fiber Spectrometer - JINSP Company Limited (jinsptech.com)
4. Summary
The main difference between front-illuminated and back-illuminated photodetectors in their working principles lies in the incident path of light and the layout of the photosensitive elements. In front-illuminated detectors, light needs to pass through multiple layers to reach the photosensitive layer, resulting in light loss and reduced quantum efficiency; while back-illuminated detectors directly irradiate light onto the photosensitive layer, reducing light loss and improving quantum efficiency and weak light detection capabilities. These differences make back-illuminated photodetectors have a wider range of application prospects in fields such as high-performance imaging and spectral analysis, but at the same time, etaloning will occur during long-wave acquisition. We will make professional selections and judgments based on the actual situation of user applications and provide suitable spectral equipment.
Post time: Jul-12-2024