晶体硅结构的选择性湿蚀刻工艺优化

ABSTRACT 

Lattice-selective etching of silicon is used in a number of applications, but it is particularly  valuable in those for which the lattice-defined sidewall angle can be beneficial to the functional  goals. A relatively small but important niche application is the fabrication of tip characterization  standards for critical dimension atomic force microscopes (CD-AFMs).  

CD-AFMs are commonly used as reference tools for linewidth metrology in semiconductor  manufacturing. Accurate linewidth metrology using CD-AFM, however, is critically dependent  upon calibration of the tip width. Two national metrology institutes (NMIs) and at least two  commercial vendors have explored the development of tip calibration standards using latticeselective etching of crystalline silicon.  

The National Institute of Standards and Technology (NIST) standard of this type is called  the single crystal critical dimension reference material (SCCDRM). These specimens, which are  fabricated using a lattice-plane-selective etch on (110) silicon, exhibit near vertical sidewalls and  high uniformity and can be used to calibrate CD-AFM tip width to a standard uncertainty of less  than 1 nm.  During the different generations of this project, we evaluated variations of the starting  material and process conditions. Some of our starting materials required a large etch bias to achieve  the desired linewidths. During the optimization experiment described in this paper, we found that  for KOH etching of the silicon features, it was possible to independently tune the target linewidth  and minimize the linewidth non-uniformity. Consequently, this process is particularly well-suited  for small-batch fabrication of CD-AFM linewidth standards.

INTRODUCTION   

Anisotropic, lattice-selective etching of silicon has been known and used in semiconductor  manufacturing since the 1960s.[1] It has been used for a number of applications, but it is  particularly valuable in those for which the lattice-defined sidewall angle can be beneficial to the  functional goals.[2-5] The potential for deep etching of near-vertical sidewalls has also made it  appropriate for micro-electromechanical systems (MEMS).[6-9] During the last 15 years a  relatively small but important niche application in metrology has emerged: the fabrication of tip  characterization standards for critical dimension atomic force microscopes (CD-AFMs).

CD-AFMs use flared tips and two-axis surface sensing and tip control in order to image  features with vertical and slightly reentrant sidewalls.[14-15] These instruments are commonly  used as reference tools for linewidth metrology in semiconductor manufacturing. Accurate  linewidth metrology using CD-AFM, however, is critically dependent upon calibration of the tip  width.[16-17] Since the primary effect of CD-AFM tip-sample dilation is an additive bias of the  measured width, the most effective method of tip calibration is to measure the apparent width of a  known structure to determine the effective tip width.

Historically, many CD-AFM users have developed in-house reference standards for tip  width calibration – often based on scanning electron microscope (SEM) or transmission electron  microscope (TEM) cross sections. But the resultant uncertainty of such standards can be  significant. Tip characterizer samples - which have a sharp ridge that can be used to calibrate tip  width - are commercially available. However, scanning such samples can result in tip damage, and  the standard uncertainty of tip calibrations based on this typically exceeds 5 nm.

Since the early 2000s, two national metrology institutes (NMIs), the National Institute of  Standards and Technology (NIST) in the United States and the Physikalisch-Technische  5 Bundesanstalt (PTB) in Germany, and at least two commercial vendors have explored the  development of standards suitable for CD-AFM tip calibration.[10-13,18] In most of these cases,  lattice-selective etching of crystalline silicon was involved in the fabrication process.  The NIST standard of this type is called the single crystal critical dimension reference  material (SCCDRM). These specimens, which are fabricated using a lattice-plane-selective wet etch  on (110) silicon, exhibit near vertical sidewalls and high uniformity and can be used to calibrate  CD-AFM tip width to a standard uncertainty of less than 1 nm.

During the different generations of this project, we experimented with different variations on  the starting material and process conditions – including the etchant.[11,19] Both potassium  hydroxide (KOH) and tetramethyl ammonium hydroxide (TMAH) are commonly used etchants.  Some of our starting materials required a large etch bias to achieve the desired linewidths. During  the optimization experiment described in this paper, we found that for KOH etching of the silicon  features, it was possible to independently tune the target linewidth and minimize the linewidth non-uniformity. Consequently, this process is particularly well-suited for small-batch fabrication of  CD-AFM tip calibration standards.

SCCDRM ETCH PROCESS OPTIMIZATION EXPERIMENT  

The background and overall methodology of the SCCDRM project has been described  elsewhere.[10-11] While we have experimented with differences in the starting material and  processing, the use of selective etching on (110) Si has always been a key element. For some  generations of the project, we used starting material with a buried oxide that served both as an etch  stop and as an electrical isolation layer. In this paper, we are specifically dealing with etch  6 optimization experiments using silicon-on-insulator (SOI) starting material that itself was fabricated  using an implantation process – such as separation by implantation of oxygen (SIMOX).

In figure 1, we illustrate some of the major characteristics of the current starting material  and process. The substrate and device layer – in which the features are patterned – are both (110)  crystalline silicon and are separated by a buried oxide. The device layer was approximately 160 nm  thick and the buried oxide was about 390 nm. This same type of starting material was also used in  prior generations of the SCCDRM [11,19] – including the first one to combine the use of CD-AFM  and HRTEM for the calibration.

For small-batch fabrication projects like the SCCDRMs, access to suitable lithography at  affordable costs can be a challenge. The trade-offs between electron beam direct write lithography  and optical lithography present challenges either way. Electron beam lithography offers very small  resolution, avoids the cost of mask making, and does not require large batches or even whole  wafers. However, the cost is generally proportional to write time and thus essentially proportional  to the patterned area. This makes it most suitable for samples on which the patterned region is a  relatively small percentage of the surface area.