Resistor Trimming Research Articles

Influence of current-induced recrystallization on piezoresistive properties of polysilicon nanosized thin films deposited at different temperatures

To improve the accuracy and stability of piezoresistive sensors based on polysilicon nanosized thin films (PNTFs), 80nm-thick PNTFs were deposited on Si substrates by LPCVD at different temperatures and fabricated into cantilever beams. The electrical trimming characteristics and dependences of gauge factor, TCR and TCGF on resistor trim were measured. Based on interstitialvacancy model, the electrical trimming is due to the mobility increase caused by current-induced recrystallization of grain boundaries (GBs). The changes in GF, TCR and TCGF with resistor trim are due to the GB state variation (including scattering center, GB width, tunneling current, localized and extended state conductions).

Electrical Trimming of Boron-Doped Polysilicon Nanofilm Resistors Deposited at Different Temperatures

The previous research showed that highly doped polysilicon nanofilms (PSNFs) have a high gauge factor and stable temperature characteristics, promoting their applications in piezoresistive sensors. For correcting the resistance deviation and improving the resistance matching of sensing elements, the electrical trimming (ET) characteristics of PSNFs deposited by LPCVD at different temperatures were studied. By the interstitial-vacancy (IV) model, the resistance falloff in trimming phenomenon is considered as a result of the decrease of scattering centers caused by the recombination of IV pairs. By modeling grain boundaries (GBs) as the accumulation of IV pairs (basic defects), it is believed that IV pairs can recombine layer-by-layer under the energy excitation of joule heat generated by high current conduction. The experiment results show that the threshold current density for ET of PSNFs is an order of magnitude lower than that of polysilicon common films, while the trimming accuracy and stability of directly deposited PSNFs are superior to the recrystallized ones. Finally, it is gained that reducing amorphous phases at GBs by optimizing deposition temperature can improve ET characteristics of PSNFs.

A CMOS threshold voltage reference source for very-low-voltage applications

This paper describes a CMOS voltage reference that makes use of weak inversion CMOS transistors and linear resistors, without the need for bipolar transistors. Its operation is analogous to the bandgap reference voltage, but the reference voltage is based on the threshold voltage of an nMOS transistor. The circuit implemented using 0.35μm n-well CMOS TSMC process generates a reference of 741mV under just 390nW for a power supply of only 950mV. The circuit presented a variation of 39ppm/°C (after individual resistor trimming) for the −20 to +80°C temperature range, and produced a line regulation of 25mV/V for a power supply of up to 3V.

An assessment of the applicability of embedded resistor trimming and rework

The successful use of embedded resistors in many applications will require that the fabricated resistors be trimmed prior to lamination into printed circuit boards to attain required design tolerances. Depending on the application, the economic value of the board being fabricated, and the process used to create the embedded resistors, it may also be prudent to consider reworking resistors that are incorrectly trimmed or with initial values that are too large (untrimmable resistors). This paper uses a model of the resistor/board yield coupled with a cost model of the trim and rework processes to identify conditions under which applications should neither trim nor rework, trim but not rework, or perform both trimming and rework of embedded resistors, as a function of the design tolerance for the resistors and the accuracy with which the embedded resistors can be fabricated. Example results are presented for several applications ranging from small boards with a high density of embedded resistors to large boards with a low density of embedded resistors. Distinct regions of trimming and rework applicability that are nearly application independent can be identified as a function of design tolerance, printing/plating/etching variation, and the characteristics of the trimming process.

BEM simulation of laser trimmed hybrid IC resistors

The capability of hybrid Integrated Circuits (IC) depends on the precision of its own resistors. This is nowadays achieved by laser trimming of lm resistors. This process can be highly supported by a robust method of computational elec- tromagnetics. The paper shows how the Boundary Element Method (BEM) is used to simulate important quantities of the process.

A CMOS Subbandgap Reference Circuit With 1-V Power Supply Voltage

A CMOS subbandgap reference circuit with 1-V supply voltage is described. To obtain subbandgap reference voltages with a 1-V supply voltage, threshold voltage reduction and subthreshold operation techniques are used. Large /spl Delta/V/sub BE/ (100 mV) as well as a 90-dB operational amplifier are used to circumvent the amplifier offset. A power-on-reset (POR) circuit is used as startup. This circuit has been implemented using a standard 0.5-/spl mu/m CMOS process, and its size is 940 /spl mu/m/spl times/1160/spl mu/m. The temperature coefficient is 17 ppm from -40/spl deg/C to 125/spl deg/C after resistor trimming and the minimum power supply voltage is 0.95 V. The measured total current consumption is below 10 /spl mu/A and the measured output voltage is 0.631 V at room temperature.

The effect of pulse voltage trimming on current noise of polymer thick film resistors

This paper deals with the investigations carried out on the variation of current noise (1/f noise) in polymer thick film resistors, when they are subjected to pulse voltage trimming. The current noise is measured in terms of noise index (micro volts of noise per volt of DC applied, in a decade of frequency) using Noise Meter Model 315 C manufactured by Quan‐Tech of New Jersey. It has been found that current noise decreases as the resistors are trimmed to lower values. An attempt has been made to explain the decrease in current noise with the trimming of resistors.

Dielectrophoretic mechanism for downward trimming of polymer resistors

The effect of high voltage pulses on the resistance of polymer thick film resistors (PTFR) is studied. It is found that the resistance decreases with both the amplitude and duration of the high voltage pulses and with the number of pulses. The change in resistance is attributed to the dielectrophoretic motion of graphite granules in the PVC medium when high voltage pulses are applied to PTFR. A model has been proposed through which the percentage change in resistance can be estimated in terms of the number of pulses, duration of pulses etc. Based on these investigations, we suggest a downward trimming method for PTFR through the application of high voltage pulses. This method of trimming is a clean process and trimmed resistors are free from hot spots.

Determination of surface and interior cracks from electrostatic measurements using Schwarz-Christoffel transformations

A crack detection algorithm using the Schwarz-Christoffel transformation is presented. The method uses practically feasible electrostatic measurements. After describing how a relevant direct boundary value problem can be efficiently solved with conformal maps, the idea of our numerical method for crack detection is explained in terms of a resistor trimming problem. Various surface and interior crack detection problems are then formulated as generalized Schwarz-Christoffel parameter problems. A number of computed examples are presented.

Theory and application of polysilicon resistor trimming

The resistivity of polysilicon films are dramatically altered when subjected to high current densities via a technique known as pulse current trimming[1,2,8,10]. A new physical model for the mechanisms governing the observed resistance reduction and recovery is presented. The model views the grain boundary as a defect-laden transition region between grains of different crystal orientation. Severe temperatures induced in the polysilicon film during pulse current trimming lead to non-reversible restructuring of the grain boundary. This produces a more coherent transition between grains and a reduction of scattering centers leading to increased carrier mobility without a significant increase in free carrier concentration. The effect is aided by “liquid phase” segregation of dopant atoms at the grain boundaries which provides a smaller, reversible component to the resistance change. Results of the first comprehensive reliability study show the trimmed resistors to be exceptionally robust. Finally, circuit design considerations for manufacturability and ESD tolerance are presented.

Pulse current trimming of polysilicon resistors

The practical application of pulse current trimming of polysilicon resistors has been investigated and successfully implemented in large scale integrated circuit production. In-package pulse current trimming of heavily doped polysilicon resistors allows precise control of the final resistor value and can effectively compensate for process variation in polysilicon sheet resistance. The technique requires no additional process complexity, is layout efficient, remarkably accurate, and is quick and inexpensive from a test perspective. Resistance reduction occurring during the trim process is shown to be reversible to a small, but usable, extent for n-type polysilicon. Thermal modeling of the resistor trim process shows that the peak temperature reached within the polysilicon film must exceed the highest temperature encountered during wafer fabrication before any permanent resistance change occurs. As the resistor is further trimmed, the film temperature approaches the melting point of silicon. >

Lightly trimming the hybrids

Laser resistor trimming, an effective tool for producing accurate and reliable electrical elements is discussed. Laser trimming has minimum effect on power handling capability; element value variation with time, temperature, and voltage; and electrical noise. The principles of laser trimming, laser types used, trimming techniques, and technology trends are described. Laser cutting of ceramics in such applications as electronic packaging and the use of lasers to pattern thin films are also discussed. >

Downward laser trimming of thick film resistors : T. Badri Narayana, K. Ramkumar and M. Satyam. IEEE Trans. Compon. Hybrids mfg Technol.14(4), 894 (1991)

Downward laser trimming of thick film resistors : T. Badri Narayana, K. Ramkumar and M. Satyam. IEEE Trans. Compon. Hybrids mfg Technol.14(4), 894 (1991)

Mechanism for downward trimming of polymer resistors

The effect of infrared radiation on the resistivity of polymer-based thick film resistors was studied. It was found that the resistivity decreases with both the density and duration of the infrared and with the number of exposures. The change in resistivity was attributed to the viscous flow of the polymer at the temperature attained by the film when exposed to infrared radiation. A model has been proposed through which the percentage change in resistance can be estimated in terms of the number of exposures, duration of exposures etc. Based on these studies it was possible to suggest a downward trimming method for polymer film resistors in which the polymer film is exposed to intermittent infrared radiation pulses of the duration of the order of a few seconds. This method of trimming appears to be a clean process, with the resistors being free from hot spots.

Laser trimming of thick film metal resistors on aluminum nitride substrates : Yasutoshi Kuriharaet al. IEEE Trans. Compon. Hybrids mfg Technol.14(1), 204 (1991)

Laser trimming of thick film metal resistors on aluminum nitride substrates : Yasutoshi Kuriharaet al. IEEE Trans. Compon. Hybrids mfg Technol.14(1), 204 (1991)

Laser trimming of thick film metal resistors on aluminum nitride substrates

Problems and their countermeasures in laser trimming of thick-film metal resistors on aluminum nitride (AIN) substrates are studied. Trimming was made possible by selection and combination of suitable Q-switching frequency, laser power, and number of irradiations. For example, under the conditions of 1 kHz, 2.0 W, and double irradiation, resistance values could be controlled to within +or-0.3% of target values. Resistance changes of trimmed metal resistors were less than +5.5% and comparable to those on alumina after thermal cycle tests (-55-150 degrees C, 1000 times), high-temperature storage tests (150 degrees C, 1000 h), and high-temperature and humidity storage tests (80 degrees C, 90% RH, 1000 h). Since the insulation characteristics of an irradiated portion strongly depended on the irradiation conditions, suitable conditions were selected for practical applications. The degree of insulation degradation after the laser irradiation was different for the two types of AlN substrates studied. This is attributed to the different amounts of free Al formed on the AlN surfaces. Further, AlN has a higher absorption factor and smaller reflectivity than alumina for a YAG laser beam. Thermal and electrical damage of AlN are affected by these properties. >

Downward laser trimming of thick film resistors

A method of laser trimming thick-film resistors to lower resistance is presented. It involves heating of certain regions of the resistor film in the form of tracks (to a temperature where a certain transformation in the structure of the film takes place, like segregation of the constituents of the resistor material, but not high enough to evaporate the material) by the use of a laser beam of appropriate power density. The heated regions are found to become regions of very high electrical conductivity. It has been found that trimming tracks made perpendicular the current flow direction are not effective in reducing the resistance. Trimming tracks made along the direction of their current flow are found to decrease the value of the resistance. It has been found that the resistance decreases with an increase in length and width of the track. The authors provide data from computer simulations and measurements made on analog networks that enable design of trimming tracks.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Polysilicon resistor trimming by laser link making

A technique for laser trimming of polysilicon resistors has been developed. In this scheme an undoped polysilicon film is patterned lithographically, and then photoresist patterns are used to prevent doping of certain narrow areas of each resistor during the impurity implant; thus, the protected areas remain insulating after wafer processing. Subsequent laser scanning of the undoped regions can produce precise reductions in resistance by lateral diffusion of dopant impurities. Trimming can be accomplished by changing either the laser power or the position. Q-switched Nd:YAG (1.06- mu m wavelength) and second-harmonic (0.53- mu m wavelength) radiations have produced successful results. The process was applied to passivated resistors with negligible (perhaps zero) post-trim drift, and the temperature coefficients of resistivity (TCRs) of trimmed resistors can be matched to the TCRs of untrimmed resistors in the same film.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Laser trimming of thick film resistors on aluminum nitride substrates

Problems, and their countermeasures encountered in laser trimming of thick film resistors on aluminum nitride (AlN) substrates were studied. Trimming was done in air by the selection of a suitable laser power (up to 1 W of YAG laser at the interface between the resistor and AlN). Under these conditions, resistance values could be controlled to within +or-1% of their target values. Resistance changes of trimmed resistors were less than +or-0.4% after a thermal cycle test (-55 to 150 degrees C, 1000 cycles) and a high temperature storage test (150 degrees C, 1000 h). Under the conditions generally used for trimming of alumina hybrid integrated circuits (YAG, 2 W, in air), resistance control was possible, but the insulation characteristics at the irradiated regions were degraded. When AlN substrates were directly irradiated under these conditions, the surfaces were also damaged. Damaged regions were fully covered with a fluid substance consisting mainly of Al/sub 2/O/sub 3/. Insulation degradation was attributed to formation of free Al, along with the Al/sub 2/O/sub 3/. However, under irradiation at low power, no insulation degradation was found, because the formation of free Al and Al/sub 2/O/sub 3/ was suppressed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Soft Guard—A Method of Trimming Resistors in Closed Loops

The automatic laser trimming of resistors is highly developed, but some areas of application still allow improvements. The method described is a new development for trimming resistor networks. When resistors within a network have to be trimmed, the influence from the other resistors has to be compensated in order to obtain a correct measurement (e.g, π‐network). Normally this is achieved with an Active Guard circuit, but new generation laser trimmers also offer an entirely mathematical compensation called Soft Guard.