Fischione Model 1064 ChipMill

The ChipMill reduces time and costs associated with research and development, manufacturing, quality assurance, and failure analysis by yielding unsurpassed results. As the demand for semiconductor devices grows, device sizes decrease, and architecture becomes more complex, the ability to perform controlled delayering during all phases of the product life cycle is essential.

ChipMill components
The ChipMill contains an ion source to mill the sample, an optical camera to monitor milling progress, a scanning electron microscope (SEM) column working in conjunction with a secondary electron detector (SED) and a backscattered electron (BSE) detector for sample imaging. The electron beam also generates X-rays that are analyzed by the energy dispersive X-ray spectrometer (EDS) detector. A secondary ion mass spectrometer (SIMS) yields surface information. 

Detector technology
When the sample surface is excited by the electron beam, corresponding secondary electrons are collected by the SED. Electron images are displayed on one of the ChipMill’s monitors.  Backscattered electrons are produced by the elastic scattering of the primary electron beam with atomic nuclei. Because information is generated by the nucleus of the atom, an indication of elemental composition becomes known. By varying the electron beam accelerating voltage, the electron penetration depth changes. The ChipMill alters electron beam operating parameters to distinguish between the surface layer and the layer below. EDS detects X-rays emitted from the sample surface, providing the elemental composition of the delayered device. A SIMS system identifies the elemental composition of the top device layer.

The role of electron microscopy in semiconductors
Electron microscopy is used to support every phase of the semiconductor life cycle: research and development, manufacturing, quality control, and failure analysis.

During:

• research and development, circuit designs are developed and verified;
• manufacturing, processes are improved to enhance performance and yield;
• quality control, chip reliability is evaluated; and
• failure analysis, defects are identified and analyzed.

Chips are comprised of multiple layers, with each layer having a thickness ranging from a few atomic planes to several microns and node sizes of 3 nm or less. The need for precise, controlled delayering has never been more critical in supporting electron microscopy’s role of understanding material properties and chemistry at the atomic scale. 

Additional techniques enabled by precise delayering are electrical probing to test individual circuits; metrology to measure feature sizes to ensure compliance with design specifications; post-mortem, single-defect failure analysis to remove material until the fault location is revealed; and reverse engineering to delayer a chip in a serial manner for the subsequent analysis of individual layers.

Delayering workflow

Depackaging
Prior to ChipMillingSM, standard depackaging techniques apply. These methods typically employ mechanical or chemical processes that are better suited to removing significant amounts of material as compared to the precise thinning afforded by an ion beam.

ChipMilling
Once the depackaged chip is mounted onto a standard SEM stub, it is transferred through the ChipMill’s load lock and positioned on the sample stage within the vacuum chamber. Following the initial set-up procedure, the chip is ion milled until the predefined end point is achieved. In many cases, the end point is an individual metal layer. After ChipMilling, the chip can be transferred to any one of several analytical devices. 

Preparing TEM specimens
The ChipMilled sample can be placed into a focused ion beam (FIB) system for the site-specific extraction of transmission electron microscopy (TEM) lamellae. 

Following post-FIB polishing in either a Fischione Instruments’ Model 1040 NanoMill® TEM specimen preparation system or Model 1080 PicoMill® TEM specimen preparation system, the specimen’s atomic-level structural and chemical information can be ascertained via aberration-corrected TEM imaging and analysis.

The ChipMilling process
All system components are included within a chamber that is evacuated by an oil-free vacuum system. A dedicated sample stage featuring X, Y, Z, and rotational movements provides proper sample positioning in relation to various system components. Automated sample height detection places the sample surface at the optimal milling plane. The ChipMill chamber design allows the ion beam to project across the sample surface at a glancing angle. The milling angle is established by the combination of sample height control and the amount of vertical deflection applied to the ion beam. The milling angle range is 0 to 10°. While milling, the sample is continuously rotated to minimize non-uniform milling of various elements within the chip that sputter at different rates. Typically, harder materials mill more slowly, while softer materials mill more rapidly. This results in curtaining. Sample rotation normalizes milling and results in a planar surface.

Feedback from the sample surface controls ion beam scanning
During milling, the ion beam scanning pattern is controlled by an advanced algorithm which processes the output of various detectors: 

• An optical camera captures interference fringes to determine the material removal pattern.
• Electron beam sample interaction yields secondary electrons that are captured by the Everhart-Thornley SED.
• Backscattered electrons result from the elastic scattering of electrons from the primary beam interaction with the sample surface; the backscattered electrons assist in distinguishing different layers within the device.
• X-rays emitted from the sample during bombardment by the incident electron beam are detected by the EDS system, which yields elemental composition information.
  Ions emitted during the milling process are analyzed by the SIMS, which is a surface-sensitive technique for determining which layer is exposed. It is useful in establishing if a layer is a metal, an oxide, or a nitride.

ChipMill’s artificial intelligence process
The ChipMill features an artificial intelligence (AI) process that incorporates a feedback control loop to adjust the ion beam milling parameters in real time. The ChipMill receives and analyzes data from the various detectors to quantify both surface flatness and chemical composition of individual device layers. 

Depth profiles are calculated by the ion beam control algorithm, which automatically adjusts the ion beam raster pattern in terms of both dwell time and current density per point while moving across the delayered area. This method of continuous feedback yields a highly planar sample surface.

System programing
The ChipMill is programmable through a standard computer interface. System operating conditions are programmed and viewed on a dedicated monitor. An on-board touchscreen, positioned next to the load lock, facilitates sample exchange. User controls can be placed either adjacent to the ChipMill or in a separate room.

End-pointing
There are various methods of end-point detection, one of which is time-based. However, for time-based end-pointing to be effective, both layer thickness and milling rate need to be very well understood. 

The ChipMill’s pioneering end-pointing technique relies upon the acquisition of images and analytical data:

• Ion milling exposes hierarchical circuit information from each layer.
• The SEM is operated at a given accelerating voltage to yield information about the surface of the layer being milled.
• The accelerating voltage of the SEM electron beam is increased to yield information relative to the layer below the surface.
• The data is correlated with the chemical composition generated by the EDS and SIMS systems to yield precise end point control.

Maintenance and service
The ChipMill is covered by one of Fischione Instruments’ service contract options. To expedite service, the ChipMill has the capability of remote diagnostics. When connected to the Internet, the ChipMill can be accessed by Fischione Instruments Service for rapid troubleshooting and diagnostics support.