What is the best method to store Calibration Wafer Standards produced with particle sizes of less than 100 nm? Cleanrooms normally operate at 70F, about 21C, and typically around 40% humidity.
When using a Calibration Wafer Standard to calibrate your Wafer Inspections Systems in the lab, the particle sizes deposited on the wafer standard under 100 nm are negatively affected by the surface roughness of the silicon wafer. Surface roughness is created by the natural polish of the wafer, as well as the natural growth of an oxide layer on the on the wafer surface over time. The polish level is a fixed element and does not change; but the oxide layer inherently grows on the wafer surface, and it affects particle detection sensitivity by a Wafer Inspection System when scanning the wafer for size calibration. The air we breathe has about 21% oxygen content. That same air contacts the silicon surface of the Calibration Wafer Standard every time it is used for calibration. The wafer normally sits in that same air pocket when enclosed in a wafer carrier, which is filled with that same air/oxygen/humidity content. When oxygen and humidity contact a non-organic surface, such as the surface of silicon wafer, the oxygen and humidity begin to form an oxide layer bonded to the silicon surface. Over time the oxide layer gets thicker and thicker, and eventually makes it difficult to detect small particles when scanning the wafer with a Wafer Inspection System, also referred to as an SSIS tool. If a wafer standard is produced with 30 nm to 80 nm polystyrene or silica nanoparticles, the wafer standard is often stored in an air/oxygen environment. The oxidation at the silicon wafer surface would naturally form an oxide layer over the entire surface of the wafer, over time. Gradually, the nanoparticles may disappear in the noise background, or become much harder to detect, as the wafer is scanned by a typical Wafer Inspection System. What causes this decrease of particle signal sensitivity by the optical detection system of a Wafer Inspection System?
When a laser beam scans a wafer surface, the optical detector detects two signals, a DC electrical signal, and an AC electrical signal. As the laser scans the silicon surface, the amplitude of the DC signal represents the surface roughness and polish of the silicon wafer. The amplitude of the AC signals represents the size diameter of each detected particle on the silicon wafer surface. A 40 nm particle detected by a laser would have a very small AC amplitude signal, while a 1 um particle would have a higher AC amplitude signal, as detected by the optical detection circuit. When scanning the Calibration Wafer Standard, the DC signal increases and decreases in milli-volts according to the level of surface roughness detected as the laser scans back and forth across the wafer or around the wafer, depending on the specific technology of each type of wafer inspection tool. If the surface roughness is high, the DC signal level increases, and vice versa. The DC signal, as detected by the optical laser during each moment of time, forms a noise boundary due to the laser scatter from the silicon surface. Increasing and decreasing, typically measured in milli-volts by the optical detector and displayed as the base line of the particle distribution, which is imaged on the display screen of the Wafer Inspection System. The physical polish of the surface is a constant value, and as technology has improved, 300mm wafers tend to have a much better polish that the older 150mm wafers. Thus, a 300mm wafer would allow for smaller particles to be deposited on the surface since the surface polish is much better with a corresponding lower level of DC signal, as detected by the Optical detector during a wafer scan.
An oxide layer begins to form on all silicon surfaces encountering an air/oxygen/humidity environment, no matter how well polished. It continues to grow over time. As the oxide layer grows over a 1 or 2 year period, a detected DC laser signal on the surface of the wafer would increase in DC signal amplitude over time due to an increase in surface roughness detected by the laser. Since a 30nm or 60nm particle has a very low AC amplitude signal; a particle’s AC signal, as detected by the Optical collector, gets overtaken by the DC noise signal level generated by the laser as it scans the surface of the wafer. The particles are deposited on the surface, but if the scanned silicon surface scatters a high DC signal noise amplitude during the laser scan, representing a rough surface; the DC signal noise can easily hide small particles deposited on the wafer surface. The particles are there, but the ever-growing layer of oxide on the wafer surface produces an ever-increasing DC signal noise, which hides the AC signal of the 30 nm particles, and can increase enough over time to hide 40 nm, then 50 nm particles, etc. Each use of the Calibration Wafer Standard adds unwanted particles to the surface of the Calibration Wafer Standard, and the oxide growth continues to increase in thickness on the surface, and after several years, the wafer standard must be replaced due to surface defects caused during normal handling, as well as oxide growth on the wafer surface.
For this reason, it is a good idea to store any Calibration Wafer Standards produced with particle sizes deposited under 125nm in a Nitrogen Storage Cabinet. This helps to reduce oxide growth on the wafer surface during wafer standard storage and helps to increase the life of the Calibration Wafer Standard with particles deposited under 100 nm on the wafer standard surface. Particles deposited larger than 100nm on a wafer standard would normally not be affected by surface oxide growth; and the calibration of a Wafer Inspection System, SSIS, would not normally be affected using particle sizes greater than 100 nm.
John Turner, Applied Physics Applications, 1 November 2023
