Critical Moves: Advanced Logic Devices And CIS Benefit From Applications Using IRCD Metrology

As 3D NAND continues to scale vertically — all in the name of increasing capacity and speed and reducing inefficiency and cost — maintaining channel hole critical dimension (CD) and shape uniformity becomes even more challenging. Faced with rising high-aspect ratios, addressing these challenges requires new inline non-destructive metrology to provide real-time process control. Infrared critical dimension metrology (IRCD) is one solution.

But while IRCD can be used to measure high-aspect ratio structures like the amorphous carbon hardmask and channel hole profile in 3D NAND, the mid-IR wavelength range can be used to measure non-memory devices like logic and CIS. In particular, IRCD can be a powerful metrology resource when it comes to detecting fluorinated polymer residue after cleans in advanced logic devices and measuring vertical doping concentration profiles after plasma doping in CIS.

How IRCD measurements work

Using a mid-IR ellipsometer with a high brightness light source that operates from roughly 5µm to 11µm, the mid-IR wavelength range exploits inherent absorption bands in the common dielectric materials used in the manufacturing of 3D NAND to attain ultra-high Z dimension fidelity (angstrom-level uncertainty) of etched holes. These etched holes are on the order of 100nm in diameter and have aspect ratios greater than 60:1.

Figure 1 shows how the absorption bands of SiO2 and SiN modulate the penetration depth of the mid-IR light using a finite-difference time-domain (FDTD) simulation of the electric field interaction with a channel hole structure at various wavelengths and compares them to the interactions of visible and near infrared (NIR) wavelengths with the channel hole structure.

Fig. 1: Comparison of electric field in 3D NAND channel hole structures at different wavelengths in relation to the extinction coefficient of SiO2 and SiN.

Detecting fluorinated polymer residue in advanced logic structures

Fluorinated polymers are a byproduct of the dry etching of common semiconductor materials, like Si and SiO₂, with fluorine-based chemistries. These polymers are purposefully used for controlling the anisotropy and selectivity of an etch. After the etching process, the polymer layer needs to be removed in a process called cleaning so that the material being deposited can contact the entire surface. As scaling in advanced logic continues, lateral dimensions shrink. This leads to higher aspect ratio etches in structures like the replacement metal gate (RMG) loop and gate contact, which makes the cleaning process more challenging.

Figure 2 shows a simulation of an IRCD measurement of a 5nm node SRAM cell after gate contact etch, with fluoropolymer residue on the sidewalls. An isometric and top-down view of the structure is shown on the left of the figure. Note that the pattern density of gate contact is roughly 2.5% of the SRAM cell. The middle image shows the extinction coefficients of common materials used in the manufacturing of advanced logic devices in the mid-IR region; these extinction coefficients determine the absorption of materials.

Fig. 2: SRAM cell after gate contact etch and cleans with fluoropolymer sidewall residue (left). Extinction coefficient comparison between common semiconductor materials used in advanced logic compared to fluoropolymer (middle) and spectral sensitivity in the mid-IR to fluoropolymer (right).

The fluoropolymer has a unique and strong absorption band near 8µm that is due to carbon-fluorine (C-F) bond stretching. The simulation on the right side of the figure shows a differential spectral simulation indicating the sensitivity of 1nm and 2nm of fluoropolymer residue on the sidewall of the contact etch structure. Due to the strength of the C-F bond absorption, IRCD has enough sensitivity — relative to the noise floor of the tool — to detect a 1nm residue at an extremely low pattern density.

Measuring CIS sidewall dopant concentration

Like advanced logic devices and memory, CIS devices are also scaling laterally in response to market demand for improved image resolution in smartphone cameras and other mobile products. Because of this, the active area of the image sensor pixel must be increased in the Z dimension to maintain performance, requiring a high-aspect ratio deep-trench isolation structure (DTI). To suppress dark current and crosstalk, a passivation layer is needed.

Plasma doping has been put forward as a method to implant HAR surfaces with minimal damage and low-penetration depth, making it ideal for scaled CIS devices. Secondary ion mass spectrometry (SIMS) is the most common method to measure the dopant concentration profile, but it is time-consuming and destructive.

Currently, there are no non-destructive inline methods to measure the sidewall dopant concentration profile. IRCD metrology has the potential to measure the dopant concentration profile in a DTI structure because of the free carrier absorption of doped semiconductors in the mid-IR.

Fig. 3. Extinction coefficient of boron-doped Si in the mid-IR from 1E16cm^-3 to 1E18cm^-3.

Figure 3 shows the extinction coefficient of boron-doped Si at concentrations from 1E16cm^-3 to 1E18cm^-3. To show that IRCD has unique sensitivity to the vertical dopant profile, we simulated seven different dopant concentration profiles with the same average concentration using AI-guided OCD modeling and analysis software in an example CIS structure (figure 4). Figure 5 shows the differential spectral response of each simulated dopant profile and the noise level of the measurement. All the simulated responses are above the noise level and are differentiated enough to be captured by IRCD metrology.

Fig. 4: Seven sidewall dopant concentration profiles with the same average concentration.

Fig. 5: Differential spectral response of the seven dopant profiles shown in figure 4 relative to noise.

Conclusion

IRCD metrology was originally targeted for HAR memory devices due to its unique capability to utilize dielectric absorption bands in the mid-IR wavelength to modulate the penetration depth of the electric field. But IRCD metrology also can be used for two other applications that utilize the absorption bands to measure key parameters in challenging layers. The first application of IRCD metrology detects fluoropolymer residue in advanced logic structures after the cleaning process by utilizing the strong C-F bond in the mid-IR wavelength, even when the pattern density is extremely sparse. The second application involves measuring the sidewall dopant profile in CIS structures by measuring the free carrier absorption bands in doped semiconductors in the mid-IR region. The differentiated spectral responses to varying profiles with the same average dopant concentration suggest IRCD is capable of such a measurement.