Development of a 0.15 µ m GaAs pHEMT Process Design Kit for Low-Noise Applications

: This work presents a process design kit (PDK) for a 0.15 µ m GaAs pHEMT process for low-noise MMIC applications developed for AWR Microwave Ofﬁce (MWO). A complete set of basic elements is proposed, such as TaN thin ﬁlm resistors and mesa-resistors, capacitors, inductors, and transistors. The developed PDK can be used in technology transfer or education.


Introduction
At present, monolithic microwave integrated circuits (MMICs) are designed and produced using electronic design automation (EDA) tools ( Figure 1) [1][2][3]. Large factories offering MMIC production services (called foundries) must provide their customers with a process design kit, so that the customers can design MMICs for their own applications. The Institute of Nanoengineering in Electronics, Spintronics and Photonics of MEPhI University has developed a semiconductor process for the production of different types of circuits and semiconductor devices, including RF transistors and MMICs. There is a need to develop a PDK for the available processes for its own and collaborating design centers (by analogy with foundries).
As the researchers and engineers of MEPhI developed a 0.15 µm GaAs pHEMT process for low-noise MMICs, we decided to make a PDK for this process first. This research was partially funded by JSC ICC Milandr (Russia).

Materials and Methods
MMIC consists of different elements: active (transistor, diode) and passive ones (resistor, capacitor, inductor, transmission line, contact pad, via hole, and others); see Figure 2. Typically, a PDK consists of electrical models of active and passive elements, topological element cells, material parameters (for electromagnetic analysis), topological tolerances, topology check rules, special objects and symbols, and support information for the user.
In this paper, the PDK was developed through the following steps: 1.
Detailed study of the process and topological design rules; 2.
Design of topology templates for elements; 3.
Development of test structures for characterization of elements; 4.
Development of circuit fragments for initial verification of element models; 5.
Fabrication of test structures and circuit fragments; 6.
Measurement of test structures and circuit fragments, mathematical processing of measurement results; 7.
Development of electrical and topological models of elements; 8.
Initial verification of element models; 9.
Development of library structure of elements, setting up electromagnetic analysis and topology verification tools; 10. Preparation of reference information; 11. Release of the first version of PDK; 12. Design of test microwave devices for validation of the first version of PDK.
After the first release of the PDK, this cycle is repeated several times to increase the accuracy of the models, add new elements, actualize the process changes, etc.

Results
The first PDK development stage included a detailed investigation of the technology. The MEPhI technology has the following features: AlGaAs/InGaAs pHEMT structure, 3-inch substrate diameter, metalized via holes to the backside of the substrate, backside metallization, substrate thinning up to 100 microns, a depletion-mode transistor with a gate length of 0.15 µm, three levels of metallization, MIM capacitors (250 pF/mm 2 ), semiconductor resistors (170 ohm/square), and thin-film resistors (50 ohm/square). The main parameters of the transistor are listed in Table 1. In the next step, we developed topology templates of basic MMIC elements. These were used to design test structures to characterize active and passive elements, as well as fragments of matching and correcting networks to verify passive element models at the first iteration of PDK development.
Then, GaAs pHEMT wafers were produced and processed. Each wafer consists of repeated frames ( Figure 3)  After production, on-wafer probe measurements of the test structures and circuit fragments were carried out. S-parameters were measured for passive test structures. For active test structures, we measured the following parameters: The measurement results for each element were processed by: The next step was to construct electrical models of active elements for amplifier applications: • Small-signal noise models of transistors ( Figure 4); • Non-linear models of transistors ( Figure 5).   The measured and simulated small-signal S-parameters and 50-ohm noise figures for a 4 × 50 µm transistor are shown in Figure 6. Equivalent circuit models were constructed for the passive elements, e.g., a spiral inductor equivalent circuit shown in Figure 7. The calculated parameters of models of several inductors are given in Table 2. The methods used to extract the parameters of active and passive element models are described in detail in [23][24][25].
To stimulate the microstrip transmission lines, we set the following substrate parameters and inhomogeneities: relative dielectric constant 12.9, substrate thickness 100 µm, conductor thickness 3 µm, and loss tangent of dielectric 0.001.
After constructing the electrical models of active and passive elements, we developed the topology models with fixed and scalable geometry. Some examples of element topological models (shown in AWR Microwave Office) are presented in Figure 8. At this stage, the final settings of electromagnetic analysis tools were adjusted. The final stages were the compilation of support information for the user and the release of the first version of PDK. Figure 9 illustrates an example of an AWR Microwave Office project with the developed PDK. For validation of the first version of PDK, three low-noise amplifiers (LNA) were designed: • Single-stage LNA; • Two-stage LNA; • Three-stage LNA.
As an example, the electrical scheme, designed topology, and simulated S-parameters of the three-stage LNA are shown in Figures 10-12, respectively. The main parameters of MMIC, obtained through simulation, are presented in Table 3.    The three developed LNAs were included in the production frame along with the line fragments, test structures, and process supplement symbols ( Figure 13). Then, this frame was multiplied on the wafer. Several wafers are now in production.

Discussion
As was shown in the previous section, the parameters and curves that were simulated for the PDK elements showed good agreement with the experimental data. The developed PDK contains models and information that are enough for the development and production of low-noise amplifiers using the MEPhI GaAs 0. 15  The developed PDK can be used in well-known and widespread EDA tools from AWR. The PDK will simplify the technology transfer to other production sites, if this is needed in the future. It can also be used to make a complete design-production-testing cycle to educate MMIC design engineers. The obtained results showed that the methods used in this paper are suitable for PDK development and can be applied in other, similar technologies at other production or research sites.

Acknowledgments:
The authors express their gratitude to the AVK Design Team and its leading engineer Alexey V. Kondratenko for technical support during the development and testing of PDK.

Conflicts of Interest:
The authors declare no conflict of interest.