Laser assisted device alteration
Encyclopedia
Laser Assisted Device Alteration (LADA) is a laser-based timing analysis technique used in the failure analysis
of semiconductor devices. The laser is used to temporarily alter the operating characteristics of transistors on the device.
at specific device transistors. The laser is typically of a short wavelength variety on the order of 1064 nm. This allows the laser to generate photo carriers in the silicon without resulting in localized heating of the device. The LADA technique is somewhat similar in execution to the Soft Defect Localization (SDL) technique, except that SDL utilizes a longer wavelength laser (1340 nm) in order to induce localized heating rather than generate photo carriers. Both techniques require the device to be scanned with a laser while it is under active stimulation by the tester.
The device being tested is electrically stimulated and the device output is monitored. This technique is applied to the back side of the semiconductor
device, thereby allowing direct access of the laser to the device active diffusion
regions. The effect of the laser on the active transistor region is to generate a localized photocurrent
. This photocurrent is a temporary effect and only occurs during the time that the laser is stimulating the target region. The creation of this photocurrent alters the transistor
operating parameters, which may be observed as a change in function of the device. The effect of this change in parameters may be to speed up or slow down the operation of the device. This makes LADA a suitable technique for determining critical timing paths within a semiconductor circuit.
The laser has differing effects on NMOS and PMOS transistors. In the case of NMOS, the transistor will turn on. For PMOS, however, the effect is to lower the transistor threshold voltage. The effect on the PMOS transistor becomes proportionately stronger as the laser power is increased. The effect is to either increase or decrease the speed of the device being tested.
Setup for a LADA analysis involves connecting the device to a test stimulus. The test parameters for operating voltage and device speed are then adjusted in order to place the device into a state which borders on a Pass/Fail or Fail/Pass transition. It is useful to use a tester Shmoo plot
in order to select the appropriate operating conditions. The effect of scanning the laser over sensitive regions is to trip the device from a Pass into a Fail condition, or from a Fail into a Pass condition.
LADA has been used to analyze failures in Domino logic
, state elements in memories and leakage.
Failure analysis
Failure analysis is the process of collecting and analyzing data to determine the cause of a failure. It is an important discipline in many branches of manufacturing industry, such as the electronics industry, where it is a vital tool used in the development of new products and for the improvement...
of semiconductor devices. The laser is used to temporarily alter the operating characteristics of transistors on the device.
Theory of operation
The LADA technique targets a variable power continuous wave (CW) laserLaser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
at specific device transistors. The laser is typically of a short wavelength variety on the order of 1064 nm. This allows the laser to generate photo carriers in the silicon without resulting in localized heating of the device. The LADA technique is somewhat similar in execution to the Soft Defect Localization (SDL) technique, except that SDL utilizes a longer wavelength laser (1340 nm) in order to induce localized heating rather than generate photo carriers. Both techniques require the device to be scanned with a laser while it is under active stimulation by the tester.
The device being tested is electrically stimulated and the device output is monitored. This technique is applied to the back side of the semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
device, thereby allowing direct access of the laser to the device active diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
regions. The effect of the laser on the active transistor region is to generate a localized photocurrent
Photocurrent
Photocurrent is the current that flows through a photosensitive device, such as a photodiode, as the result of exposure to radiant power.The photocurrent may occur as a result of the photoelectric, photoemissive, or photovoltaic effect....
. This photocurrent is a temporary effect and only occurs during the time that the laser is stimulating the target region. The creation of this photocurrent alters the transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
operating parameters, which may be observed as a change in function of the device. The effect of this change in parameters may be to speed up or slow down the operation of the device. This makes LADA a suitable technique for determining critical timing paths within a semiconductor circuit.
The laser has differing effects on NMOS and PMOS transistors. In the case of NMOS, the transistor will turn on. For PMOS, however, the effect is to lower the transistor threshold voltage. The effect on the PMOS transistor becomes proportionately stronger as the laser power is increased. The effect is to either increase or decrease the speed of the device being tested.
Setup for a LADA analysis involves connecting the device to a test stimulus. The test parameters for operating voltage and device speed are then adjusted in order to place the device into a state which borders on a Pass/Fail or Fail/Pass transition. It is useful to use a tester Shmoo plot
Shmoo plot
In electrical engineering, a shmoo plot is a graphical display of the response of a component or system varying over a range of conditions and inputs. Often used to represent the results of the testing of complex electronic systems such as computers, ASICs or microprocessors...
in order to select the appropriate operating conditions. The effect of scanning the laser over sensitive regions is to trip the device from a Pass into a Fail condition, or from a Fail into a Pass condition.
Applications
LADA is useful for confirming or disproving an existing theory for the cause of failure. It may be used to confirm suspected transistor leakage or bus noise. It has also found wide use in localizing process defects as the LADA effect easily modulates transistor characteristics in the same path as the process defect.LADA has been used to analyze failures in Domino logic
Domino logic
Domino logic is a CMOS-based evolution of the dynamic logic techniques which were based on either PMOS or NMOS transistors. It allows a rail-to-rail logic swing. It was developed to speed up circuits....
, state elements in memories and leakage.