What is Aluminum Mat Etching?

Aluminum and aluminum alloys, as the connecting materials for chips, have been widely used in the manufacture of copper interconnect as the logical backend process. The aluminum pad is usually thicker, above 1μm, or even up to 6μm, and the thickness of the photoresist on the upper layer is generally 1~1.5 times that of aluminum, and the size is larger, and the etching is relatively simple. The pre- and post-etch coating structure of the aluminum pad includes the photoresist, aluminum layer, and underlying material, which involves removing the aluminum layer and creating the desired pattern.

Etching process steps and parameters

Aluminum pad etching is typically performed within the LAM-2300-Versys-Metal chamber, and standard etching gases include BCl₃ and polymer gas CH₄. The etching process is mainly divided into main etching (ME) and over-etching (OE), and the time of the main etching step is controlled by the end mode of detecting the aluminum signal. Scanning electron microscopy (SEM) is used to monitor the shape of the aluminum lines and the sidewalls of the aluminum pads.

In addition, aluminum pad etching can be subdivided into hard mask open step (BT), main etching step (ME), over-etch first step (OE1), and over-etching second step (OE2). The source power, total gas flow and process pressure of each step are increased. The BT step uses a large bias power and a higher proportion of BCl₃ to bombard the natural oxide layer (Al₂O₃) on the surface of etched aluminum. The ME step mainly increases the etch rate by increasing the process pressure, total gas flow rate and source power. The OE1 step is used to etch the residual aluminum and its lower TiN layer; In the OE2 step, the bias power and BCl₃ flow ratio are increased to bombard the lower layer of silicon oxide.

Challenges and load effects in etching

In the process development of 65nm/90nm node logic technology, the difference in pattern density poses a challenge to the etching process, mainly from macro and micro etch loads. The macroscopic loading is related to the different transmittance (TR) corrosion windows of the photoresist in the post-aluminum pad etching, while the microscopic loading is related to the morphological load between the aluminum wire (dense) and the aluminum pad (sparse). Low transmittance creates more polymer in etching, protecting the aluminum sidewall but exacerbating microloading effects, resulting in inconsistent connection resistance.

Transmittance has a strong linear dependence on the etching end time, and the higher the transmittance, the longer the etching end time, and the more serious the corrosion defect. There are no corrosion defects when the transmittance is below 70%, while the CH₄ flow rate needs to be optimized to compensate for the lack of polymer in the case of high transmittance.

 

Process optimization and gas selection

To balance macro and micro load effects, the combination of transmittance and CH₄ flow rate needs to be optimized. Increasing the CH₄ flow rate compensates for the missing polymer at high transmittance, but too high a flow rate can lead to too much sidewall polymer, adsorbing chloride and absorbing moisture, causing corrosion defects. Experiments show that the CH₄ flow rate T is sufficient for the transmittance less than 70%. For the case of 96.2% transmittance, the CH₄ flow rate is optimized to 2.5T.

In the micro-loading effect of aluminum wire and aluminum pad, there are more polymers in the aluminum wire area, and the side walls are more tapered. The sidewalls of the aluminum pads are prone to corrosion due to the lack of polymer protection. By adjusting the bias power and BCl₃ gas ratio, polymer deposition conditions can be optimized, resulting in steeper and straighter aluminum wire sidewalls and reduced residue.

The comparison of different shielding gases showed that the sidewalls were rough, defective and easy to corrode when N₂ and CHF₃ were used. When using CH₄, the corrosion morphology is better, and there are fewer defects and corrosion.

Common problems and solutions

Common problems with aluminum pad etching include rough aluminum sidewalls and abnormal grassy morphology at the bottom after etching. The roughness of the sidewall is mainly caused by the unclean removal of the sidewall polymer or the uneven polymer accumulation during the etching process, which can be solved by adjusting the sidewall polymer generation environment or reducing the polymer, such as adding He to dilution during the etching process, or increasing the Cl₂ flow rate. The grass-like morphology at the bottom is mostly due to the upper alumina not being etched clean, which plays a role in mask protection in the aluminum etching process, and the solution is generally to increase the intensity and time of BT step etching to completely etch the natural oxide layer on the surface.

Aluminum pad etching technology requires comprehensive regulation of transmittance, gas flow rate, power parameters, and step timing to cope with the load challenges caused by changes in pattern density, ensuring sidewall protection and etch quality. By optimizing process conditions and gas selection, defects can be effectively reduced and the reliability and consistency of chip manufacturing can be improved.