The NEW LEP400 Plasma Etch Depth Monitor

Real-time, in-situ plasma etch depth monitoring
   and end point detection
      plus co-linear wafer vision system

Applications

The LEP400 Series is a flexible product suitable for application in both R&D and volume manufacturing environments.

It is also suitable across a wide range of materials and industries, including;

Application Examples

• Interferometry & Reflectometry Modes
• GaAs/AlGaAs Quantum Cascade Laser Etch
• Failure Analysis Etch: SiO2 on Aluminium
• Metal Etching
• Selective Low Damage Plasma Etch Processes
• Silicon Etching for MEMS using the 'Bosch' ICP Process

Interferometry & Reflectometry Modes

Interferometry	Reflectometry

The LEP400 and its associated modelling software is ideally suited to operate in both Interferometry and Reflectometry Modes.

In Interferometry Mode, the user locates the laser spot over an open area of material to be etched. The reflected signal is a combination of signals from each layer within the sample. This is therefore ideally suited to etch rate monitoring and end point detection of samples with two or more layers. Both patterned and unpatterned wafers can be used. Many examples are given in the sections below.

In Reflectometry Mode, the user locates the laser spot so that it reflects from both the masked surface as well as the etching surface. This is ideally suited to the etching of bulk layers. Examples include quartz etching for diffractive optical elements.

The LEP400's advanced modelling capability also handles situations where both Interferometry and Reflectometry are occuring at the same time, such as the etching of fine grating structures into III-Vs.

GaAs/AlGaAs Quantum Cascade Laser Etch
Courtesy of Dr Geoff Hill, Sheffield University, UK

EtchDirector© Model	Process Data

Over 200 layers were modelled using EtchDirector's modelling capability as shown in the left-hand window.

The right-hand window shows the results from the actual etch run. The LEP400 reproduced the modelled data to a high degree and detected each turning point during the run enabling the progress of the etch to be followed and for the process to endpoint with a high degree of accuracy.

The LEP400's combination of modelling and monitoring capability removed the need for 4-5 'vent-&-measure' steps or performance of numerous sacrificial calibration runs, saving significant time and expensive epi-wafers.

The level of certainty achievable with the LEP400 becomes even more critical for devices requiring multiple etch stages.

The achievable etch stop accuracy in this case was around 5nm.

Failure Analysis Etch: SiO2 on Aluminium

As the graph above indicates, the LEP400 can be used to monitor the etch back of dielectric on metal for a number of applications including failure analysis.

The LEP400 monitors a small (approx 30 micron diameter) exposed area and is therefore ideal for monitoring one small chip in an entire chamber - unlike optical emission spectroscopy which would struggle to detect such dilute species.

EtchDirector utilises an advanced end point algorithm to detect the 'flat-line' at the end of the oxide etch. In field tests, EtchDirector reliably detects the endpoint well before a skilled operator thereby avoiding unwanted removal, damage or contamination of the underlying Al layer. The endpoint algorithm also allows the user to enter an overetch time to enable full clearout of the etch.

A major advantage of this algorithm is that reliable endpointing does not depend upon the starting oxide thickness. This is especially important in a manufacturing environment where premeasurement of the oxide thickness is prohibitive in terms of time and cost.

The LEP400 is also ideal for monitoring the etch of SiN on metal or even combinations of SiN on SiO2 or on Si.

Metal Etching

Metals are not transparent until very thin and therefore you cannot obtain interference fringes and monitor etch depth & rate through the bulk of a metal layer.

However, the LEP400 is ideal for picking out interfaces, indicated by a step level change in reflectivity. The above example shows titanium being etched from a LiNbO3 substrate. Whilst monitoring at 670nm, the large drop in signal occurs over an etch depth of less than 30nm. EtchDirector comes with an endpoint algorithm specifically designed to identify this step level change and enables the operator to choose whether to stop at the top, middle or bottom of the curve. Again, an overetch capability is also included to enable a clearout etch to be achieved.

A major advantage of this algorithm is that reliable endpointing does not depend upon the starting metal thickness. This is especially important in a manufacturing environment where premeasurement of the metal thickness is prohibitive in terms of time and cost.

This process works even at high etch rates and has been proven to be faster and more accurate than a skilled operator. The process works equally well for other metals including NiCr, Ni, Au, Tg, Pt, etc, and works for a wide range of substrates.

Selective Low Damage Plasma Etch Processes

Selective low damage plasma etch processes are used in a number of applications including III-V etching of InP & GaAs HEMTs and for active III-V optoelectronic devices including lasers, modulators and detectors. Often these processes experience an induction period at the beginning of the etch process during which the native oxide, and/or residues from the previous process stage, inhibit the etch. Once this layer has been removed the etch proceeds as normal.

The problem is that the induction time is variable and may become a significant proportion of the expected etch duration. Without in-situ monitoring this can lead to large uncertainties in the etch depth.

The LEP400 enables the operator to actually 'see' the induction period, as shown in the graph above, and still obtain a highly accurate and repeatable etch process.

Silicon Etching for MEMS using the 'Bosch' ICP Process

The 'Bosch' ICP etch process is widely used to achieve extremely high etch rate (> 20 microns per minute) high aspect ratio (> 100:1) etching of silicon microstructures used throughout the MEMS industry. It is a switched process characterised by alternate stages of silicon etch, polymer deposition, polymer etchback, and silicon etch again. This process is cycled until the required etch depth is achieved.

Although the process is extremely successful, it does have some issues. The first of these is that the etch rate is significantly dependant upon the lateral feature sizes as well as the mask-to-open-area ratio. The second of these is the fact that the polymer etchback time is variable as it depends upon the precise etch/dep parameters as well as the mask geometry and is therefore essentially outwith the control of the operator. This is compounded by the fact that the precise etch parameters are often 'tweaked' to achieve the verticality and smoothness for a particular mask design.

This means that every time a new mask design is used, or if the etch parameters are tweaked, then time consuming full-wafer dummy etches need to be undertaken to measure the etch depth and therefore calibrate the etch rate.

Use of the LEP400 completely avoids these stages and even avoids the need to measure the etch depth post etch using a profilometer.

EtchDirector incorporates patented high speed shape recognition algorithms that analyse the shape of the reflected signal thereby tracking the silicon etch and rejecting the polymer deposition and etchback stages. The LEP400 can precisely follow etch rates in excess of 20 microns per minute.

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