Simultaneously measure multi-layer metal thickness of repeating materials in complex reverse side metallization stacks.
In recent years, the applications of power semiconductors have expanded from industrial and consumer electronics to renewable energy and electric vehicles. In the future, the most promising power semiconductor devices will be insulated gate bipolar transistor (IGBT) and power metal oxide semiconductor field effect transistor (MOSFET) modules.
During the manufacturing process of these devices, metal films are deposited onto the chips of MOSFET and IGBT power devices. These thin-film layers serve two main functions: they connect the basic blocks that make up the power chip to the source (power MOSFET) or emitter (IGBT), and they also allow wire bonding or soldering on the chip to facilitate thermal conduction. Because power devices operate at high currents and high operating temperatures, the electrical properties and thickness of the metal layers must be properly controlled to improve thermal conductivity.
In addition, power devices are moving from 6-inch to 8-inch plates; this occurs at the same time that the process window narrows. Therefore, it has become increasingly important to accurately measure the thickness of the multilayer metal and characterize the uniformity of metal film deposition at the edge of the wafer. For example, a thicker layer of metal, usually an aluminum alloy, 5 microns or more thick, is required on the front side of the plate. Uniform coating of aluminum to conduct high currents across the plate is key to device performance and reliability.
Power devices also require back side metallization (BSM) to ensure good electrical and thermal contact. Its quality significantly increases the operational reliability and service life of the equipment. Additionally, these devices use different combinations of metal films, each with a specific purpose in the stack. Missing layers or uneven films can affect performance. With all this in mind, reliable and reliable measurements are critical, especially when taking measurements on product wafers.
In this Interface and BSM blog, we will discuss the application of picosecond ultrasonic metrology to measure metal films on IGBT and MOSFET power devices.
Picosecond ultrasound technology has been well studied and described in detail in the literature. It has been the workhorse of semiconductor foundries for over 20 years (Figure 1). It is a non-contact, non-destructive, first-principles technology that relies on a pump beam to generate acoustic waves that bounce off various interfaces, and uses a time-delayed probe beam to monitor changes in surface reflectivity to reflect the echoes.
Thickness is extracted from this acoustic signature using the echo arrival time and the longitudinal speed of sound in the material. Metallized films used in power applications are often rough both during deposition and after etching. In some niche applications they also tend to be very thick (eg 30 micron copper).
Recent improvements in picosecond ultrasonic equipment have expanded the upper limit of thickness while simultaneously improving the signal-to-noise ratio (SNR) when measuring rough surfaces. This allows equipment manufacturers to quickly display intra-wafer uniformity and gain the necessary feedback to optimize their processes. In addition, the technology’s smaller spot size allows for measurements with edge exclusions of up to 0.5 mm in small areas of the product wafer. Time-resolved signals obtained from multilayer stacks of power devices can distinguish individual layers in a stack of multilayer metal films, even if they include repeated metal layers of the same material. Competing metrology cannot resolve these layers, and optical methods or profilometers cannot measure these complex stacks.
In this blog, we present two selected examples to highlight the capabilities of picosecond ultrasound technology.
For power devices, gate plating often requires multiple layers of metal. Using picosecond ultrasonic technology, this instrument is widely used by device manufacturers to monitor gate/source metallization.
As mentioned earlier, one of the biggest advantages of picosecond ultrasonic technology is the ability to simultaneously measure the thickness of multiple layers of metal on repeating materials. For example, in a layer stack consisting of Ti/TiN/AlCu/TiN, we can separately distinguish the top TiN layer from the bottom TiN layer. Altmetric tools cannot recognize this difference. Therefore, they are useless for actually monitoring devices.
Picosecond ultrasonic technology demonstrates excellent repeatability of static and dynamic measurements of Ti/TiN/AlCu/TiN multilayer stacks with 3-sigma accuracy of less than 0.5% for the thickness of all three top layers. (Table 1) 3 sigma of the bottom Ti layer is less than 1%.
Table 1. Typical static and dynamic repeatability performance of picosecond ultrasonic measurements on stacks of Ti/TiN/AlCu/TiN multilayer metal films commonly used in power applications.
BSM uses multiple layers of metal. They have good adhesion and electrical properties, as well as long-term reliability. The most commonly used BSM packages are Ti/NiV/Ag or Al/Ti/NiV/Ag. Typically, the back surface of the metal has a rougher grain structure compared to the front side.
Thanks to hardware improvements, we have been able to demonstrate excellent static and dynamic repeatability of thin film measurements.
Table 2. Static and dynamic repeatability of picosecond ultrasonic technology when measuring multilayer Ti/NiV/Ag thin films commonly used in power applications.
Picosecond ultrasound technology is widely used in process monitoring of semiconductor power devices due to its fundamental principles, non-contact, non-destructive nature and ability to simultaneously measure multiple layers of thin films, even if those layers are made of the same material. With ongoing improvements, picosecond ultrasound technology now offers additional capabilities for more complex BSM packages such as thin films with rough surfaces.
As electric vehicles continue to evolve and demand for power semiconductors continues to grow, metrology systems using picosecond ultrasonic technology will help power device manufacturers better address the unique manufacturing challenges in this field.
Post time: Nov-02-2023