Floating gate trimming saves backend testing costs

Floating gate trimming saves backend testing costs
Floating gate trimming of semiconductors, particularly linear analog circuits have matured to the point where the technology is used extensively in many manufacturing and final test processes. Compatible with today's computer automated assembly and test lines, the obvious benefits of floating gate trimming or "e-trim" boils down to pure and simple cost-savings and higher manufacturing throughputs by automating backend manual test/measurement effort.

Trimming semiconductors and electronic systems with floating gate technology is a state-of-the-art replacement for older and more widely accepted mechanical potentiometer trimming and in some cases, laser trimming. While trimmer potentiometers are simple to implement and relatively inexpensive, they suffer from mechanical limitations and vulnerabilities, and the labor to properly adjust them to meet certain specifications requires technician level skills making automation even more costly and sometimes even impractical in high volume production applications. Except for the very high initial investment costs in dedicated machines and training special personnel, laser trimming of thin film resistors offers a practical solution for fine-tuning precision analog circuit elements such as operational amplifiers for example.

Floating gate trimming offers a practical all solid-state alternative to current aforementioned methods especially where the circuit elements are physically inaccessible for final test and calibration. This particularly applies to operational amplifiers where additional in-circuit trimming of packaged components is almost always required.

Technology

Floating gate trimming utilizes a CMOS transistor or EPAD (Electrically Programmable Analog Device) with a floating gate of polysilicon embedded in the device gate oxide. In linear components such as op-amps, the EPAD is part of the op-amp chip and adds virtually no additional cost or complexity to manufacture. Trimming the op-amp input offset voltage (Vos) for example is accomplished by injecting a charge of "hot" electrons with sufficient energy to enter the oxide and on into the floating gate structure. Once inside the floating gate, the electrons are trapped and stored indefinitely as a nonvolatile charge storage, which remains even after the power is removed.

Figure 1: Programmable thresholds enable EPADs to serve as a variable resistor.

Each EPAD can be programmed by delivering a sequence of rapid voltage pulses, each successively charging the floating gate to a higher level until the proper threshold voltage is established for the application. The threshold may be increased in millivolt or 0.1-millivolt incremental steps over a 2.0- volt range. For applications requiring bi-directional trimming, 2-EPAD circuits are used in a push-pull or differential mode thereby allowing positive and negative adjustment.

Trimming

While the floating gate silicon comes virtually for free, threshold trimming (programming) requires an EPAD Electronic Programmer unit that consists of a precision A/D converter, programming pulse counter, switch matrix, and precision voltage measurement circuitry. A PC (personal computer) is also required to control the EPAD Programmer, which is connected via the parallel printer port. Software driven, the EPAD Programmer can be custom tailored to trim EPAD devices in the standalone mode or in the in-circuit mode.

Stand-alone programming is applicable to large quantities of EPAD's where the threshold parameters are known in advance-independent of other circuit variables that tend to alter the overall system performance. In-circuit programming, perhaps the more popular method of trimming, is a more complete means of adjusting passive component differences and input unit-to-unit variables typically induced by a variety of opto, temperature, and other miscellaneous instrumentation sensors. The most significant benefit of in-circuit etrim is the ability to perform final test and calibration on a fully assembled circuit board (after soldering) without further affects of thermal processing- caused parametric drift.

Applications in building op-amps

One of the most common applications of floating gate trimming is used in the manufacture of precision operational amplifiers. Input offset voltage (Vos) is one parameter that generally plays a significant role in most designs and is dealt with by using laser trimming or by using external trimming. Op-amp manufacturers typically specify (Vos) in millivolts for general-purpose op-amps and in microvolts for precision amplifiers. Unless specified otherwise, most manufacturers will guarantee this value only when operated at a specified power supply voltage and temperature-typically +/- 5Vdc and 25C. Operation at any other voltage or temperature generally produces an imbalance between the inverting and non-inverting inputs resulting in an error at the output.

One of the basic characteristics of the EPAD is that its MOSFET output on-resistance can be controlled with a reference bias voltage. When the EPAD is biased off, as when the reference bias voltage (the input gate voltage) is set to more than 0.5V below Vt, the output resistance is greater than 10 Gigaohm. When the bias reference voltage is increased to the threshold voltage (Vt), the EPAD is biased on, and its on-resistance is 1 Mohm. As the reference bias voltage is set to above Vt, the output resistance decreases. Thus, the device can functions as a voltage-controlled resistor.

For the ALD1108E/ALD1110E devices, for example, the output on-resistance decreases from 1 Mohm to approximately 1.5 Kohm at a reference voltage of 5 Volts. This represents many orders of magnitude of change in on-resistance. For a given reference bias voltage, the on-resistance of an EPAD is adjustable within a certain range. This may be useful in applications where a fixed reference voltage is available, and where many different on-resistance values are desired. In Figure 1, with the addition of a fixed resistor, the circuit becomes a voltage divider. The output voltage varies according to the resistance ratios between the fixed resistor and the EPAD on-resistance. As the on-resistance of the EPAD is adjusted, the output voltage is changed, producing the desired Vout permanently.

A basic current source with output Io is shown in Figure 2A. The value of resistor R is selected such that current through R is approximately 68A for low temperature effects. Resistor Rset is biased with the same voltage registered across the EPAD. This voltage is set by bias resistor value R and the EPAD programmed threshold voltage. The temperature coefficient (tempco) of output current Io is directly proportional to the tempco of Rset. This tempco can be minimized by selection of an appropriate resistor type. In this circuit, the output current can be set by programming the EPAD threshold voltage.

The EPAD threshold voltage (Vt) can be scaled, ratioed and shifted with operational amplifier circuits. Many of these circuits are common operational amplifier circuits. (Figure 2B, for example, provides a means of Amplifying and Shifting the EPAD offset Adjustment range with an operational amplifier.) In general, a relatively large EPAD Vt adjustment range should be used. A recommended adjustment range would be from a 1V range such as Vt =1.000V to 2.000V range. This minimizes error budget due to programming voltage errors and temperature effects causing a shift to Vt.

Figure 2A: Current source with EPAD trimming. The temperature coefficient of output current Io is directly proportional to the tempco of Rset.

Figure 2B: An additional op amp provides a means of Amplifying and Shifting the EPAD offset adjustment range.

EPAD op-amps offer analog circuit designers a cost-effective solid-state alternative, which is easily implemented and can be remotely trimmed under PC control. Op-amps with embedded EPAD functions allow the user to trim Vos over a range of +/- 5 to 10 mV with a step resolution of 10V. EPAD op-amps cost no more than conventional devices and may ultimately offer improved reliability and considerable cost savings by eliminating mechanical trim pots and manual trimming.

Backend test and measurement

Total Input Offset Voltage (Vost), is defined as the sum total of all the equivalent input offset voltage errors, i.e., (Vos), equivalent (Vos) due to PSRR, CMRR, ambient temperature (TA), and, equivalent input noise voltage (due to noise voltages and noise currents). For applications where source impedance is high, or where a variation of external circuit equivalent (Vos) is greater than the op-amp internal offset voltages, the (Vost) can be significantly different from that of the sum of the equivalent (Vos), which is how op-amps have been traditionally specified.

External circuit equivalent (Vos) error is traditionally not accounted for in a conventional op-amp specification. Externally, the user generally trims equivalent (Vos) errors, such as that resulting from an external sensor, or from other circuit components, after the circuit has been built. This system level equivalent (Vos) error can be compensated for in an EPAD op-amp by trimming the opamp at in-system levels. At that point, a specific set of sensor and circuit board components has been paired with a specific EPAD op-amp. Therefore any equivalent (Vos) error caused by the aggregate component to component variation can be minimized or eliminated by user programming.

At the backend test and measurement station, the outputs of an assembled circuit board can be tweaked (trimmed) to meet a full range of system output limits automatically under test software, by e-trimming the EPAD op-amp Vos. This function exactly duplicates the trim-and-forget function of a trimmer pot, meeting or exceeding the mechanical and electrical requirements, with the added benefits of reduced component count (eliminating the trimmer pot), free e-trim function (built-in to the EPAD op-amp), and fully automatic trimming (integrated into the final test software routine). All these added benefits result in saving backend test and measurement cost and effort. For high volume circuit board assemblies, or circuits requiring numerous trimming channels, these savings can add up in today's competitive environment.

Distribution of input offset voltage

Most conventional op-amp manufacturing lots processed without special trimming will result in the standard bell curve distribution shown in Figure 3A. More than 50% of the parts typically fall outside the range of +/- 500Volts (Vos)-a value deemed acceptable for near precision classification. These so-called premium grade devices are generally priced at a premium, whereas the balance requires a relaxed specification standard classification.

Figure 3A (top): Total input offset voltage before and after standard e-trim; Figure 3B (bottom): Example of a backend test and measurement stage e-trim with a different target Vos of --750 uV.

Utilizing floating gate e-trim, higher precision classification yields are possible with better control of inventory parts levels. Figure 3A illustrates the classical distribution curve of (Vos) over a full range of +/- 2500-Volts. Example "A" further illustrates the distribution of parts that fall into the target range after e-trim: 0.0V +/- 25Volts-considered a precision category. Example "3B" on the other hand illustrates an after e-trim distribution with an adjusted target (Vos) of -750V +/- 25V- also considered a precision device, but with application- specific error compensation. ■