Etch Endpoint Research Articles

Level set simulations of the anisotropic wet etching process for device fabrication in nanotechnologies

Chemical etching is employed as micromachining manufacturing process to produce micron-size components. As a semiconductor wafer is extremely expensive due to many processing steps involved in the making thereof, the need to critically control the etching end point in an etching process is highly desirable. It was found that not only the etchant and temperature determine the exact anisotropy of etched silicon. The angle between the silicon surface and the mask was also shown to play an important role. In this paper, angular dependence of the etching rate is calculated on the base of the silicon symmetry properties, by means of the interpolation technique using experimentally obtained values of the principal <100>, <110>, <111> directions in KOH solutions. The calculations are performed using an extension of the sparse field method for solving three dimensional (3D) level set equations that describe the morphological surface evolution during etching process. The analysis of the obtained results confirm that regardless of the initial shape the profile evolution ends with the crystal form composed of the fastest etching planes, {110} in our model.

Controllable optical emission spectroscopy diagnostic system for analysis of process chemistries

Optical emission spectroscopy (OES) diagnostics have been employed for many years in plasma etch end point detection schemes. Unfortunately some newer process systems have much lower optical emission or limited optical access. To overcome such limitations, an OES diagnostic system making use of variable e-beam has been developed. That system is described and initial experimental results are presented. A strong correlation is observed between the optical emission intensity and e-beam current, a measurable electrical parameter. This correlation offers means to normalize optical signal and to be used as a feedback input to the electronics that control the plasma source. In addition there is a measurable response from the different lines due to energy of the electrons, indicating a new degree of freedom in the diagnostic that can be tapped for more precise analysis of end point.

Real-Time End-Point Detection Using Modified Principal Component Analysis for Small Open Area SiO2 Plasma Etching

Principal component analysis (PCA) was modified for real-time applications and applied to the end-point detection of small open area SiO2 plasma etching. Typically, the end point of plasma etching is determined from a few manually selected wavelengths. Determining the end point of the plasma etching using this approach is quite a challenge when the exposed open area is less than several percent. To increase the sensitivity, information was extracted from the entire spectra of 2755 signals in the range of 200−1100 nm, using a PCA algorithm. In this study, the PCA algorithm was modified to allow real-time applications of end-point detection. The loading vector was determined from the model wafer, and the score vector was determined using the real-time data of the target wafer to reduce the processing time. This algorithm was tested for the small open area of SiO2 etching of a 200 ms sampling period, using the entire optical emission spectra, through a comparison with a defined signal-to-noise ratio. The results were compared with the conventional single wavelength signals of SiF (440.2 nm), CO (482.5 nm), and Si (505.6 nm). The end-point detection of 0.4%−0.8% SiO2 open area was achieved using the suggested algorithm, while the single wavelength showed limitations in the open areas above a few percent. The sensitivity was also increased by a factor of 2.15, compared to the signal-to-noise ratio of the single wavelength method.

Back-etch method for plan view transmission electron microscopy sample preparation of optically opaque films

Back-etch methods have been widely used to prepare plan view transmission electron microscopy (TEM) samples of thin films on membranes by removal of the Si substrate below the membrane by backside etching. The conventional means to determine when to stop the etch process is to observe the color of the light transmitted through the sample, which is sensitive to the remaining Si thickness. However, most metallic films thicker than 75 nm are opaque, and there is no detectable color change prior to film perforation. In this paper, a back-etch method based on the observation of an abrupt change of optical reflection contrast is introduced as a means to determine the etch endpoint to prepare TEM samples for these films. As the acid etchant removes the Si substrate material a rough interface is generated. This interface becomes a relatively smooth and featureless region when the etchant reaches the membrane (film/SiO2). This featureless region is caused by the mirror reflection of the film plane (film/SiO2 interface) through the optically transparent SiO2 layer. The lower etch rate of SiO2 (compared with Si) gives the operator enough time to stop the etching without perforating the film. A clear view of the morphology and control of Si roughness during etching are critical to this method, which are discussed in detail. The procedures of mounting wax removal and sample rinsing are also described in detail, as during these steps damage to the membrane may easily occur without appropriate consideration. As examples, the preparation of 100-nm-thick Fe-based amorphous alloy thin film and 160-nm-thick Cu-thin film samples for TEM imaging is described.

Near-infrared diode laser hydrogen fluoride monitor for dielectric etch

A hydrogen fluoride (HF) monitor, using a tunable diode laser, is designed and used to detect the etch endpoints for dielectric film etching in a commercial plasma reactor. The reactor plasma contains HF, a reaction product of feedstock gas CF4 and the hydrogen-containing films (photoresist, SiOCH) on the substrate. A near-infrared diode laser is used to scan the P(3) transition in the first overtone of HF near 1.31μm to monitor changes in the level of HF concentration in the plasma. Using 200ms averaging and a signal modulation technique, we estimate a minimum detectable HF absorbance of 6×10−5 in the etch plasma, corresponding to an HF partial pressure of 0.03mTorr. The sensor could indicate, in situ, the SiOCH over tetraethoxysilane oxide (TEOS) trench endpoint, which was not readily discerned by optical emission. These measurements demonstrate the feasibility of a real-time diode laser-based sensor for etch endpoint monitoring and a potential for process control.

Cr photomask etch

Theoretical and experimental Cr photomask etch studies are carried out using different resists [ZEP, chemically amplified resists (CAR), and optical resists] and different brand etch tools. The effects of chrome loading are analyzed, and theoretical equations are developed for etch time calculations and endpoint determinations of extremely low Cr load photomasks. It was found that these equations agreed well with experimental data. Etch critical dimension (CD) movement data are analyzed and calculated, showing agreement with experimental data. Metrology measurement and characterization tools include a profilometer, an optical film measurement system, and SEM and optical CD measurement systems. Significant etch performance differences are noted across etch tools, irrespective of the resist type used. An etch property number method is proposed, which is found to accurately describe the etch process analysis and the extent to which etch performance can be expected to be improved. Etch properties are focused on etch CD movement, isolated/dense etch CD bias, radial CD etch contribution, and Cr load effects.

Determination of SF 6 reactive ion etching end point of the SiO 2/Si system by plasma impedance monitoring

Plasma impedance monitoring is successfully used to determine the end point of reactive ion etching of a SiO 2 layer lying on a Si substrate in SF 6 plasma. The usefulness of this technique is demonstrated using a commercial Plasma Impedance Monitoring (PIM) system. The end point conditions are tested by monitoring changes in the fundamental and the first four harmonic components of the RF current, RF voltage, phase between RF voltage and current, RF discharge power and RF impedance. The best monitoring parameter found in this work is modelled as a polynomial equation of RF input power, chamber pressure and gas flow rate, from which the end point can be predicted with good precision and easily detected by the PIM. The end point conditions are confirmed by both the Fourier Transform Infrared Spectroscopy (FTIR) measurements and via observation of the plasma colour.

Reduction of microtrenching and island formation in oxide plasma etching by employing electron beam charge neutralization

During plasma etching of oxide thin-film patterns, nonuniform charge buildup within etching features results in formation of microtrenches. Near the etch endpoint, the underlying film layer adjacent to the feature edges is exposed first, leaving an oxide island in the feature center and potentially causing underlayer damage before the endpoint is reached. Herein, a directional electron flux is added to the plasma ion flux incident on the etching substrate with the goal of minimizing microtrenching and oxide island formation. Scanning electron microscopic images of patterns etched with added electron irradiation show a reduction in microtrenching and oxide island formation as compared to patterns etched under identical conditions without electron irradiation. A computer simulation shows that the added electron irradiation reduces microtrenching by allowing more uniform ion flux to reach the feature bottom.

Use of plasma impedance monitoring for the determination of SF6 reactive ion etch process end points in a SiO2/Si system

It is shown here that plasma impedance monitoring can be used successfully to determine the end point of reactive ion etching of a SiO2 layer lying on a Si substrate in SF6 plasma. The usefulness of this technique is demonstrated using a commercial Plasma Impedance Monitoring (PIM) system. The end point conditions are tested by monitoring changes in the fundamental and the first four harmonic components of the RF current, RF voltage, phase between RF voltage and current, RF discharge power and RF impedance. The best process monitoring parameter found in this work is modeled as a polynomial equation of RF input power, chamber pressure and gas flow rate, from which the end point can be predicted with good precision and easily detected by the PIM. The end point conditions are confirmed by both Fourier Transform Infrared Spectroscopy (FTIR) measurements and via observation of plasma color changes.

Etch end-point detection of GaN-based devices using optical emission spectroscopy

In this study, optical emission spectroscopy (OES) was performed to detect etch end-point during the etching of GaN/AlGaN-based devices. In-situ OES experiments during the etching were carried out to measure the etch product signals of AlGaN/GaN heterostructures as well as single III-nitrides. A possibility of detecting etch end-point by OES was identified for both optoelectronic devices and electronic devices using etch product emissions such as Al (396.1 nm) and Ga (417.2 nm).

In situ characterization of CMOS post-process micromachining

We have developed and demonstrated a new methodology for in situ monitoring and characterization of CMOS post-process micromachining utilizing integrated circuits and micromachine test-structures. In our demonstration, the circuits provide automated readout of N-well resistors surrounding each of the 140 test pit structures at up to 14,000 samples per second per device during the post-process silicon etch and, thus, also provide etch progress and end point determination without extra analytical equipment. We use this technique to examine the effect of pit size, surrounding thin film layers, and topology in a 2 μm CMOS process with a XeF 2 post-process step, although the technique and results are of use to EDP, TMAH, and plasma post-processing.

Plasma etching endpoint detection using multiple wavelengths for small open-area wafers

This article proposes a new approach for etch endpoint detection of small open area wafers. The traditional endpoint detection technique uses a few manually selected wavelengths, which are adequate for large open areas. As the integrated circuit devices continue to shrink in geometry and increase in device density, detecting the endpoint for small open areas presents a serious challenge to process engineers. In this work, a high-resolution optical emission spectroscopy (OES) system is used to provide the necessary sensitivity for detecting subtle endpoint signals. Principal component analysis is used to analyze the OES data and extract key components that capture the endpoint signal. Data analysis from many wafers shows that the endpoint pattern in the principal components is repeatable. Two methods are used to select wavelengths so as to improve reliability and reduce susceptibility to noise. The first method is to remove those spectrum windows that contain no endpoint information. The second method is to use a “sphere” criterion to select the most relevant wavelengths. The final endpoint algorithm using a much-reduced number of wavelengths shows more distinguishable and reliable endpoint features.

Modeling and algorithm development for automated optical endpointing of an HBT emitter etch

This paper discusses the development of a high-accuracy endpointing algorithm for the emitter etch of a heterojunction bipolar transistor (HBT). Fabrication of high-performance HBTs using self-aligned base-emitter processes requires etching through the emitter layer and stopping with very high accuracy on the base layer. The lack of selectivity in dry etching coupled with the high etch rates possible in high density plasmas render the use of a standard timed overetch impractical, especially as device layers continue to become thinner. The etch process under study requires the complete removal of an AlInAs emitter while etching no more than 5 nm of the underlying GaInAs base layer. Etch products are monitored using optical emission spectroscopy (OES) to determine etch endpoint. The process under study relies on the intensity of the 417.2 nm Ga emission line. The detection of the Ga line indicates that the etch has reached the GaInAs layer. However, the presence of a time-varying Ga baseline signal before endpoint and significant noise in the OES signal necessitate more than a simple threshold scheme for critical endpoint detection. The algorithm presented here is based on a generalized likelihood ratio with a signature function. This algorithm is robust to variance in the optical gains of the measurement equipment and is applicable to other etch processes. Experimental results of automated endpointing using this algorithm are presented in the form of pre- and post-etch ex situ film thickness measurements.

A survey on the reactive ion etching of silicon in microtechnology

This article is a brief review of dry etching as applied to pattern transfer, primarily in silicon technology. It focuses on concepts and topics for etching materials of interest in micromechanics. The basis of plasma-assisted etching, the main dry etching technique, is explained and plasma system configurations are described such as reactive ion etching (RIE). An important feature of RIE is its ability to achieve etch directionality. The mechanism behind this directionality and various plasma chemistries to fulfil this task will be explained. Multi-step plasma chemistries are found to be useful to etch, release and passivate micromechanical structures in one run successfully. Plasma etching is extremely sensitive to many variables, making etch results inconsistent and irreproducible. Therefore, important plasma parameters, mask materials and their influences will be treated. Moreover, RIE has its own specific problems, and solutions will be formulated. The result of an RIE process depends in a non-linear way on a great number of parameters. Therefore, a careful data acquisition is necessary. Also, plasma monitoring is needed for the determination of the etch end point for a given process. This review is ended with some promising current trends in plasma etching.

Application of neural networks to plasma etch end point detection

A popular technique for ending a plasma etch process is to monitor the optical emissions from reaction species for gross changes. This at first appears to be a simple real-time edge detection task. However, differences in plasma chemistry across runs, reactor chambers, and wafer patterns combine to make this control strategy quite problematic. Moreover, as exposed wafer areas continue to shrink, distinct end points become increasingly harder to identify. This new approach is simple, robust, and flexible. The application user trains elementary pattern detectors, or neurons, on a few representative process end point shapes. Software algorithms process the training end point data to find a range of resolutions for pattern recognition. The application then automatically builds a pattern detection network that tolerates signal noise, adapts to changing end point shape, and precisely registers process end points. The neural network end point detector has been successfully tested on a wide variety of data gathered over a period of many months. The application is currently calling end point in real time on several oxide etch processes at a major wafer fabrication facility.

Sensor integration into plasma etch reactors of a developmental pilot line

During the course of the Microelectronics Manufacturing Science and Technology (MMST) program, a number of sensors have been integrated into various test-bed plasma etch reactors with the goal of monitoring, diagnosing, and controlling these processes. These sensors include single wavelength and spectral ellipsometers for real-time in situ etch rate and endpoint determination; a standard monochromator for etch rate and nonuniformity measurements; an eddy current sensor for incoming metal thickness control; a rf monitor for rf current and voltage diagnostics; and a scatterometry-based critical dimension sensor for linewidth measurements. The full integration of these sensors turned out to be a complex and time-consuming task including hardware, optical, software, material, and processing issues. Once integrated into the reactor, and the appropriate process modeling completed, these sensors enabled the monitoring, diagnosis, and model-based control of these processes. Besides maintaining specific process observables at their target values, running under process control has highlighted phenomena such as equipment aging and first-wafer effects. This work has clearly shown that the full implementation of sensors into commercial manufacturing equipment is essential for the model-based control and diagnosis of semiconductor processes.

Sensitive plasma etching endpoint detection using tunable diode laser absorption spectroscopy

We report the use of a tunable diode laser locked to a molecular vibrational absorption line as a sensitive plasma etching endpoint detector. Measurements were made on multilayer silicon wafers etched in a SF6 plasma discharge. We show that polycrystalline silicon to silicon dioxide endpoint transitions on wafers with exposed area as small as 33 mm2 should be observable by detecting the etch end product SiF4. The method shows considerable potential as an endpoint detection technique for applications such as contact hole etching wherein very small areas are being etched.

Etch-stop behaviour of depletion layers

The authors demonstrated that the thickness of a diaphragm using the electrochemical etch-stop technique can be monitored with the applied reverse bias voltage on individual pn-junctions. It is thus possible to produce diaphragms of differing thickness on one wafer using different reverse bias voltage values. This effect is attributed to an etch-rate reduction of approximately a factor of ten when the etch front reaches the depletion layer of the pn-junction. Since the depletion-layer thickness is voltage dependent different diaphragm thicknesses can be obtained. For a metallurgical junction depth of 4.4 mu m diaphragm thicknesses of 5.5 mu m, 6.5 mu m and 9.5 mu m were obtained for respective reverse bias values of 2 V, 8 V and 20 V. Since depletion layers can also be obtained with metal oxide silicon semiconductor (MOS) structures, MOS-capacitors with n+ source diffusions were biased with 10 V gate to substrate and 3 V source to substrate voltages. Electrochemical etching yielded membranes of typically 3 mu m thickness. They are lightly p-doped in this case (3*1015 cm-3) and completely stress free. For MOS-structures no final etch-stop can be obtained, however, etch-through monitors are used as etch end-point detectors. Very thin, single-crystal diaphragms with thicknesses below 2 mu m are of importance in several applications. Such thin diaphragms were made with shallow implanted pn-junctions of 1.2 mu m junction depth and an electrical activation anneal. The resulting diaphragm thickness obtained was 2 mu m by sufficient overetching.

Active neutral network control of wafer attributes in a plasma etch process

We have investigated the use of a neural network to compute plasma etch end point times based on in situ monitoring of the optical-emission trace. The network was trained using data from a complementary metal–oxide semiconductor production facility. In some cases, even though an end-point system is used, the overetch that follows has to be adjusted continuously to reflect machine conditions as it ages after a clean. This can be done manually by measuring wafers after the etch and adjusting the overetch time for the next run. At its present level of training, the neural network shows equivalent performance without the need for such intervention. As the network’s learning database grows, we can expect its performance to improve.

Real‐Time Monitoring and Analysis of Chemical Wet Etching of III‐V Compound Semiconductors

We report the use of optical interferometry for in situ real‐time monitoring and analysis of chemical wet etching processes in and structures with or without masked regions. Interference between reflections from the etching surface and buried heterointerfaces in the wafer allows determination of the etch depth and any degradation of surface quality, identification of heterointerfaces, and detection of changes in etch rate and end point in real‐time. We have used a sensor consisting of a 0.95‐μm light‐emitting diode and four detectors enclosed in a 12.5‐mm‐diameter transparent Teflon tube which is kept in front of the substrate being etched. The etching process can be stopped as soon as the desired etched depth is achieved. The technique can be useful particularly in regrowth requiring etching of base epitaxial structures, fabrication of lasers, and many other devices, and development of an etching process.