This website uses cookies to implement certain functions. If you use this website you agree to our Privacy Policy.
News and Information about the Test of Electronics in Research & Design, Production, Maintenance, and Installation.  

Newsletter

Register to our newsletter
Every two weeks -
all news at a glance
captcha 

Latest Test and Measurement News

Background: Using On-Wafer Broadband Calibration Substrates

Figure 3 Anritsu July2024On-wafer measurements require a calibration in advance to ensure consistent, repeatable and reliable results. A suitable solution is the use of an on-wafer TRL (Through Reflect Line) calibration substrate for wide-bandwidth applications. This article explores the use of such a calibration substrate for wide-bandwidth applications with a VectorStar ME7838G VNA from Anritsu. In addition, the boundaries of the standard multiline TRL calibration substrate (calsubstrate) are presented along with scenarios that cannot be solved through the use of TRL. The article also touches on buying a ready-made calsubstrate versus implementing a custom design.

Having chosen the broadband Vectorstar VNA, built-up the probing table and set all the parameters up correctly, we can proceed to the next step, which would be the measurement starting with the calibration (figure 1). The question to answer here is whether to purchase an on-wafer calibration substrate or to design one.

 

Figure 1. ME7838G system comprising a VectorStar MS4640B VNA (background) and two MA25400A golden extender modules with broadband probes (front) mounted on a probing table.

 

A calsubstrate is an on-wafer calkit and is defined with combinations of different standards such as Open, Short, Load or Match, Through and Line. The advantages of buying one include less issues with a ready to use calsubstrate and the availability of very accurate Short Open Load Through (SOLT) or TRL calsubstrates. A TRL calibration is a 2+ port calibration which consists of three standards, Through (T), Reflect (R), and Line (L). A TRL calsubstrate with three Lines is shown in Figure  2.

 

Figure 2. Example of a three Line TRL calsubstrat.

 

It has some variants such as Through Reflect Match (TRM), Line Reflect Line (LRL), Line Reflect Match (LRM), whereby Reflect can be either an Open or a Short (resulting in a 180 degree phase shift between both), but the principles behind them all are the same. The main use of TRL is for noncoaxial environments such as on-wafer measurements and test fixturing. SOLT inherently provides a broadband calibration, essentially from DC to the upper frequency limit of the connector type being used. TRL calibration was developed for making accurate measurements of non- coaxial devices at microwave and millimeter-wave frequencies.

Though purchasing an on-wafer calsubstrate kit is the easier option, there are certain instances where custom designs are required or desired. For example, when designing microwave and high frequency circuits custom on-wafer calsubstrates can be tailored to specific requirements such as very high accuracies, implementing novel measurement techniques or tailoring the design to specific automated testing set ups or scenarios involving shared or multi project wafers. The custom design also lends itself to the optimisation of chip real-estate via a more efficient implementation specific to wafer and process being used.

Further, the self-designed calsubstrate can be on the same wafer as the circuits to measure, implemented as a Monolithic Microwave Integrated Circuit (MMIC) or better named Monolithic Microwave Integrated Calsubstrate (MMICalsubstrate).

Designing a SOLT calsubstrate sometimes creates problems because the Load or vias are not accurate feasible on some wafers. Therefore, TRL is the most commonly chosen method for self-designed calsubstrates. As the Line is dependent on the wavelength, does this introduce a bandwidth limit for TRL calsubstrates?

Here we show that the TRL calsubstrate can be used for wide bandwidth broadband applications, specifically over a part of of the 70 kHz to 220 GHz measurement range of the Anritsu ME7838G VNA, and how to specify the dimensions of a TRL calsubstrate on a wafer, particularly for broadband measurements or if one is required to design calsubstrate for the measurement of a different frequency on a shared wafer.

 

Figure 3. Two 220 GHz broadband probes over a wafer — mounted on MA25400A golden extender modules within the ME7838G system.

 

What is the length of the Line?

There is always a little space between two probes (Fig. 3) to measure a Through standard and therefore there is a line with a free chosen length. The half of this line is the Trough length. The Line and Through standards establish the reference impedance for the measurement after the calibration is completed. The Line length is defined as the additional length of the Line to the Trough. The length of the Line should not be less than the 20° phase length at the lowest frequency of the measured band, and not more than the 160° phase length at the highest frequency of the band (known as the 20°/160° rule) to guarantee an unambiguous calibration. The length is often chosen to be in the middle of the 20° to 160° range, which is 90° (quarter wavelength = λ/4) at the middle frequency of a defined frequency band. The bandwidth of this self-chosen band has to be the range that it complies with the 20°/160°rule. Consequently in wide-bandwidth applications, the full band to be measured and therefore calibrated should be divided into different bands, so that the different bands each comply with the 20°/160°rule. The exact length of the Line will depend on the specific frequency range; on the line type, for example, microstrip or coplanar microstrip; the substrate dimensions and properties; and the application [A, B].

How many Lines do I need for an unambiguous calibration?

Firstly, let us calculate the number of required Lines NLines of a TRL calsubstrate for a certain wanted bandwidth ratio BRwanted. In this case, it's much more convenient to think of frequency coverage in terms of the ratio of the upper to lower frequency. Hence 20° to 160° is an 8:1 bandwidth, so a Line standard that is 20° at 1 GHz will work to 8 GHz [A;B]. This give us:

Formula 1

The covered bandwidth ratio BRcovered has to be larger than the wanted bandwith ratio BRwanted so that the result has to be rounded up.

Formula 2                                   

How do I design a TRL calsubstrate in general and as example applied on our 70 kHz to 220 GHz system (with a uniqueness range working for all systems)?

At first the number of Lines needed for the frequency band from 70 kHz to 220 GHz have to be calculated. From the equations above we get:

Formula 3 4

Eight line standards are needed! However, as many engineers might notice, eight lines will result in extremely long lines related to the upper frequency limit. The longest line would be meters long for a standard. This is not realizable on thinfilm substrate or silicon chip.

As a result a healthy trade off has to be made. A way to keep the maximum frequency is to change the starting frequency. Therefore, we could consider the frequency range from 1 GHz to 220 GHz, which should be enough for the most applications. In this case, only 3 Lines are necessary.

At first the crossover points need to be calculated and with this the line lengths of the quarter wave lines in the center between the crossover points are calculated.

The two crossover points FTn are calculated as in the following [B]:

Formula 5

Fstart = 1 *109 Hz is the start frequency; Fend = 220 * 109 Hz is the stop frequency;

Formula 6

n = is the index of the crossover point and N = 3 is the number of necessary lines.

Formula 7

The Line lengths are now calculated for 90° (quarter wavelength) at the center between each crossover point (including start and end point). These center points are:

Formula 8

We now need the Line lengths for 90° at FC1, FC2, FC3.

A real scenario is better than just air waves and air lines. Therefore, let us use as an example the thinfilm parameters and dimensions described in [C], which are for a polyimide substrate with the following relevant properties: thickness = 18.5 µm; 
εr = 3.5; tan δ = 0.0027.

The 90° microstrip Line lengths, obtained with the calculator from [D] with Z0 = 50 Ω and width W ≈ 46.6 µm are L1 = 13.71 mm; L2 = 2,27 mm; L3 = 0,376 mm (as shown in Fig. 2).

 

Conclusion

The first insight we get in this article is wide bandwidth in this broadband use case and that the Anritsu VNA with its measurement range from 70 kHz to 220 GHz is exceptional. The second is that a normal TRL calibration with multi Lines is not feasible for this frequency range and 1 GHz to 220 GHz can be covered by a three Lines TRL calibration. Lastly, it is potentially better to buy a commercial substrate for the full frequency range instead of a TRL calsubstrate on the wafer.

An example of how to use a substrate to get a calibration from 70 kHz to 220 GHz is a combination of TRL with SOLT as presented in [E]. In addition, in this article [E] another broadband calibration method is presented in combination with the Distance To Fault (DTF) function of the VNA. A combined coaxial and waveguide calibration was done and the probes were de-embedded with the DTF function.

 

References:

[A] Müller, D., 2018. RF Probe-Induced On-Wafer Measurement Errors in the Millimeter-Wave Frequency Range (Vol. 89). KIT Scientific Publishing.

[B] Microwaves101, ‘TRL calibration,’ [Online]. Available: https://www.microwaves101.com/encyclopedias/trl-calibration. [Accessed: Feb. 29, 2024].

[C] Sterzl, G., Dey, U. and Hesselbarth, J., 2021. Subnanoliter sensing of dielectric properties of liquid-in-flow at 190 GHz. IEEE Microwave and Wireless Components Letters31(6), pp.808-811.

[D] em: talk, ‘Electromagnetics and Microwave Engineering,’ [Online]. Available: https://www.emtalk.com. [Accessed: Feb. 29, 2024]

[E] Rumiantsev, A., Martens, J. and Reyes, S., 2020, August. Calibration, Repeatability and Related Characteristics of On-wafer, Broadband 70 kHz–220 GHz Single-Sweep Measurements. In 2020 95th ARFTG Microwave Measurement Conference (ARFTG) (pp. 1-4). IEEE.

 

Author

Georg Sterzl, RF & Microwave Field Application Engineer, Anritsu

www.anritsu.com/



Related Articles:

Upcoming Events

electronica 2024
Munich/Germany
12 to 15 November
LOPEC 2024
Munich/Germany
25 to 27 February
Mobile World Congress 2025
Barcelona (Spain)
03 to 06 March

  More events...
  See our Trade Show Calendar
  Click here

 

Advertising
Advertising